Integrated Approach of Using Biostimulants for Improving Growth, Physiological Traits, and Tolerance to Abiotic Stressors in Rice and Soybean

Abstract

Abiotic stressors such as drought, salinity, waterlogging, and high and low temperatures significantly reduce the growth and productivity of rice (Oryza sativa) and soybean (Glycine max), which are vital for global food and nutritional security. These stressors disrupt physiological, biochemical, and molecular processes, resulting in decreased yield and quality. Biostimulants represent promising sustainable solutions to alleviate stress-induced damage and improve crop performance under stressful conditions. This review provides a comprehensive analysis of the role of biostimulants in enhancing rice and soybean resilience under abiotic stress. Both microbial and non-microbial biostimulants including phytohormones such as salicylic acid; melatonin; humic and fulvic substances; seaweed extracts; nanoparticles; and beneficial microbes have been discussed. Biostimulants enhance antioxidant defenses, improve photosynthesis and nutrient uptake, regulate hormones, and activate stress-responsive genes, thereby supporting growth and yield. Moreover, biostimulants regulate molecular pathways such as ABA- and ROS-mediated signaling and activate key transcription factors (e.g., WRKY, DREB, NAC), linking molecular responses with physiological and phenotypic resilience. The effectiveness of biostimulants depends on crop species, growth stage, stress severity and application method. This review summarizes recent findings on the role of biostimulants in enhancing the mechanisms underlying growth, yield, and stress tolerance of rice and soybean under abiotic stress. Additionally, the incorporation of biostimulants into sustainable farming practices to increase productivity in the context of climate-related challenges has been discussed. Furthermore, the necessity for additional research to elucidate the underlying mechanisms, refine application methods, and verify their effectiveness in field conditions has been highlighted.

1. Introduction

Abiotic stressors significantly affect plant growth, development, and production quality, which potentially leads to substantial yield losses during the most vulnerable phenological plant stages [1]. Stressors such as drought, soil salinity, and high or low temperatures are key factors that limit the quality and quantity of crop production [2]. Quality is indicated by agronomic characteristics such as fruit size, yield, resistance to bacterial and fungal infections; sensory attributes such as color, shape, and firmness; and nutritional and vitamin content [3]. Moreover, abiotic and biotic stressors interact within the ecosystem, and environmental conditions often directly affect plant pest dynamics by reducing plant tolerance or increasing the likelihood of pathogen infection [4,5]. Climate change and related environmental changes exacerbate these stresses, often resulting in sub-optimal growth and yield [6]. Unfortunately, most land areas worldwide have already been subjected to stressful conditions, which has complicated the efforts to meet the rising demand for food [7]. For example, Boyer [8] has estimated that abiotic stress would reduce yields by >60% on average, whereas more recent estimates suggest that yield losses from abiotic stress range from 51 to 82% [9]. Additionally, climate extremes account for ≤50% of global variability in crop yields [10].

Global demand for food is projected to approximately double by 2050 because of increase in global population [11]. Global warming and climate fluctuations reduce the yield of essential food grains such as rice by directly affecting agricultural productivity worldwide [12]. Rice (Oryza sativa) is a staple food for half the global population [13]. In recent years, its production has declined due to both biotic and abiotic stressors, largely driven by climate change and the low stress resistance of modern cultivars [14]. Notably, abiotic stressors alone have been estimated to reduce global rice production by 32% or approximately three million tons annually [15]. Consequently, researchers have increasingly focused on enhancing rice yield and quality under stressful conditions [16]. Similarly, soybean (Glycine max (L.)) is an important crop worldwide because of its widespread use for feed, food, and industrial purposes [17]. The significant expansion of soybean cultivation during the late 20th and early 21st centuries was driven by the demand for edible oils and feed proteins [18]. However, abiotic stressors have greatly impacted soybean production by contributing to an average yield loss of >50% [19].

In addition to reducing the yield, abiotic stress compromises product quality, which leads to morphological, physiological, and biochemical alterations that may change the appearance and nutraceutical value of the food product and potentially make them unsuitable for the market [20]. Plants adapt to challenging environmental conditions through intricate mechanisms governed by signaling molecules, transcription factors, genes, and defense systems that help alleviate stress effects. Each type of abiotic stress triggers unique and shared responses to other stresses that may be synergistic or antagonistic [21]. A significant physiological effect of abiotic stress is the disruption of redox homeostasis due to the excessive generation of reactive oxygen species (ROS), resulting in oxidative stress [22], which affects enzymatic activity and gene regulation and poses a threat to plant survival. Oxidative stress is a common outcome of various adverse conditions that occur as a result of the stress-induced disruption of the balance between ROS production and ROS quenching activity [23]. ROS include radicals (such as the superoxide anion (O2⁻), peroxyl (RCOO), hydroxyl (OH), and alkoxyl (RO)) and non-radicals that initiate chain reactions, which target biomolecules such as DNA, pigments, lipids, and proteins [24,25]. The maintenance of redox balance is influenced by factors such as the timing, intensity, and duration of stress exposure. Notably, moderate or controlled stress enhances the quality of various crops [26]. For example, water deprivation might be a beneficial crop management strategy to improve the nutritive and health-promoting value and taste of lettuce and fleshy fruits by stimulating secondary metabolism and increasing the concentration of phytochemicals such as α-tocopherol, β-carotene, and flavonoids [27,28]. In addition to producing ROS-scavenging compounds, plants enhance the biosynthesis and accumulation of compatible solutes such as sugars and proline to perform osmoprotective roles under this condition.

Therefore, a novel and environmentally friendly strategy in cultivation is to use natural plant biostimulants that enhance plant growth, flowering, fruit set, crop yield, and nutrient use efficiency to additionally improve tolerance to various abiotic stressors [29]. Biostimulants are natural formulations that improve nutrient utilization efficiency, stress tolerance, and crop quality [30], they include organic and inorganic substances as well as microorganisms [31]. The external application of these compounds can positively influence plant growth and development, and their natural origins and ability to enhance crop physiological traits, such as photosynthesis and nutrient uptake, make them advantageous for improving crop stress tolerance [32]. Recently, the significance of biostimulants has increased amidst the advent of new technologies, and their economic importance is expected to increase further [Transparency Market Research]. Biostimulants often fit well with the circular economy model because the end products of processes involve the collection, transformation, and reuse of various natural, urban, industrial, and agricultural waste materials [33]. Plant biostimulants were initially used in organic farming and are now used in various cropping systems including conventional and integrated crop production [34]. Both microbial and non-microbial plant biostimulants are commonly applied to open-field and greenhouse crops such as fruit trees, berry crops, grapevines, vegetables, ornamentals, cereals, and turfs [35]. Additionally, organic farmers favor the use of natural stimulants to enhance crop quality [36,37].

Unlike the development of varieties that are tolerant to multiple stressors, the use of biostimulants provides a sustainable solution for enhancing crop health under abiotic stress and helps manage climate-resilient farms [37]. Moreover, growing consumer awareness of healthy foods has further increased the importance of organic farming [38]. Subsequently, organic farmers favor the use of natural stimulants to improve the quality of their produce [36] as it enables them to act as stewards of the rural environment, add value to their by-products, and adopt eco-friendly farming practices [39]. Hence, new technologies and strategies need to be critically evaluated to improve crop yields under both optimal and suboptimal conditions and enhance the efficiency of utilization of resources such as water and fertilizers to ensure food security, preserve soil health, and create business opportunities for farmers [40]. The objective of this review is to provide a comprehensive overview of the roles and mechanisms of biostimulants in enhancing abiotic-stress tolerance in rice and soybean. This review categorizes the different types of microbial and non-microbial biostimulants, summarizes their effects on plant growth, yield, and stress tolerance mechanisms under abiotic stress conditions, and explores their underlying physiological, biochemical, and molecular mechanisms. Additionally, the current knowledge gaps have been identified, and practical applications have been highlighted with suggestions for future research directions to improve the sustainable use of biostimulants in climate-resilient agriculture.

A comprehensive literature review was conducted utilizing Web of Science, Google scholar, Scopus, and PubMed to find articles published from 1982 to 2025. Keywords included “biostimulant,” “rice,” “soybean,” and “abiotic stress.” Studies were selected based on relevance, experimental focus, and English language. Information regarding the type of biostimulant, method of application, type of stress, and physiological or molecular responses was collected and summarized.

2. Definition and Concept of Biostimulants

Biostimulants are substances or microorganisms that, when applied to plants, enhance nutrition efficiency, abiotic stress tolerance, or crop quality traits, without being fertilizers or pesticides, according to the European Biostimulant Industry Council definition [31,41,42]. Biostimulant products have been recently recognized as innovative tools in agronomy, as evidenced by the increase in the number of scientific studies on biostimulants and their growing global market [43]. They are generally categorized into two major groups: nonmicrobial and microbial biostimulants. Nonmicrobial biostimulants include plant and seaweed extracts, humic substances, protein hydrolysates, and substances with plant growth regulator-like effects (which may overlap functionally with plant growth regulators but differ in regulatory classification), such as phytohormones and hormone-like compounds (e.g., melatonin) derived from organic waste, vermicompost, and other natural sources. Furthermore, other natural polymers such as chitin, chitosan, and chitosan oligosaccharides and inorganic compounds such as nitric oxide (NO) and hydrogen sulfide (H₂S) are considered biostimulants because they improve plant performance, although they do not directly provide essential nutrients. Microbial biostimulants include mycorrhizal and non-mycorrhizal fungi, Trichoderma, and plant growth-promoting rhizobacteria (PGPR) [44]. A summary of biostimulants types and their function in plants is shown in Figure 1.

Among non-microbial biostimulants, seaweed extracts (macroalgae) are extensively used in both conventional and organic farming systems for a variety of crops [45]. These extracts contain phytohormones such as cytokinins and auxins and hormone-like substances that promote plant growth [46]. Seaweeds are abundant in bioactive compounds including minerals and complex polysaccharides such as laminarin, fucoidan, and alginates, which promote plant development [47]. Recently, microalgae have been identified as promising sources of biostimulants because of their ability to enhance plant performance under stressful conditions [48,49,50]. Similarly, vermicompost extracts have the potential to increase antioxidant enzyme activity and scavenge ROS, particularly under salt and drought stress conditions [51]. However, the precise underlying mechanisms of action of these extracts remain unclear, primarily because of the variability in source materials and extraction methods [32].

Humic substances include humic acids (HA), fulvic acids (FA), and humin. They represent a major component of non-microbial biostimulants and comprise ≤ 60% of the soil organic matter [52]. HA are soluble in alkaline solutions but not in acidic solutions, whereas FA dissolve under both alkaline and acidic conditions. Both HA and FA play vital roles in enhancing soil organic carbon and alleviating environmental stresses by modulating the physio-biochemical properties of the soil and plants [53,54,55]. Specifically, they promote water and nutrient uptake, improve the plant water status, enhance antioxidant activity, and increase endogenous cytokinin levels [1,56]. Furthermore, humic substances help reduce toxic sodium (Na) buildup and improve the uptake of vital nutrients such as nitrogen (N), potassium (K), calcium (Ca), magnesium (Mg), phosphorus (P), iron (Fe), sulfur (S), manganese (Mn), and copper (Cu), all of which are essential for salt-stress resistance [57]. These positive effects are associated with the upregulation of stress-responsive gene [58]. Additionally, the application of humic acids enhances nutrient absorption by increasing cell membrane permeability and improving nutrient availability through cation chelation [59,60]. The long-term use of HA and FA in agriculture has been well documented to show their beneficial effects on plant growth, microbial activity, and nutrient cycling [61,62,63].

Another major category of biostimulants is protein hydrolysate (PH), which enhances antioxidant capacity, promotes ROS scavenging, and facilitates metal chelation [64]. Hydrolyzed proteins that contain amino acids, peptides, polypeptides, and denatured proteins can be derived from both plant and animal sources through chemical, enzymatic, and thermal hydrolysis or a combination of these methods [65,66]. These compounds are vital for enhancing plant defense mechanisms under stress, and their application increases crop yield [67,68,69]. For example, Botta et al. [70] found that lettuce treated with animal-based PH showed significantly higher fresh and dry biomass than those of the untreated controls. Furthermore, PHs activate plant defense responses and improve tolerance to various abiotic stressors [71,72,73].

Microbial inoculants are increasingly used as biostimulants because they promote sustainable and eco-friendly agriculture. The most common types include PGPR, arbuscular mycorrhizal fungi (AMF), and Trichoderma spp. [65,74]. These microbial biostimulants, particularly PGPR, are crucial for the mobilization and solubilization of both micro- and macronutrients in the soil rhizosphere [75]. Microbial biostimulants help narrow the yield gap by enhancing plant growth through improved biological N₂ fixation, mineral and nutrient solubilization, and plant access to soil nutrients [34], even under challenging environmental conditions.

In addition to the conventional categories, new biostimulants encompass certain advantageous elements like aluminum (Al), silicon (Si), sodium (Na), selenium (Se), and cobalt (Co). Although these elements are not generally necessary for plant species, they are crucial for the growth and development of specific plants. Typically, these elements are found in plants and soils as inorganic salts such as chlorides, carbonates, silicates, phosphates, and phosphites [76,77]. They play a vital role in enhancing tolerance to abiotic stress by aiding processes such as osmoregulation, strengthening of cell walls, thermal regulation, nutrient absorption, antioxidant defense, hormone production, and heavy metals [77]. Additionally, these inorganic salts modulate stress responses by affecting hormonal signaling, osmotic balance, redox stability, pH levels, and enzymatic activity of peroxidases [41]. Despite their known use as fertilizers, they need to be studied as biostimulants.

A new suggested category of biostimulants includes a novel class of biostimulant products, nanoparticles, and nanomaterials [78]. These particles range from 1 to 100 nm in size. They may be used as foliar sprays or nutrient solution additives, and they affect plant physiological processes by altering ion transport, receptor activity, and cellular energy states to improve stress tolerance and production quality [79,80,81]. Their biostimulant properties are largely attributed to their distinctive structures and interactions with plant surfaces, which enhance ion and metabolite transport and modify energy dynamics and charge distribution [1]. In summary, plant biostimulants include a wide range of substances and microorganisms that foster plant growth, enhance nutrient efficiency, and increase resilience to abiotic stressors. In the context of growing global focus on sustainable agriculture, biostimulants are an environmentally friendly alternative to conventional agrochemicals. Further research is crucial to understand the mechanisms of action of new biostimulant categories such as waste-derived products and nano-biostimulants.

3. Biostimulants Enhancing Growth, Yield, and Stress Tolerance in Rice and Soybean Under High-Temperature Stress

Global warming is characterized by rising temperatures, which negatively impact agriculture [82,83]. High-temperature stress (HTS) poses a significant threat to global food systems [84]. Since the 19th century, global temperature has increased by 0.9 °C because of human activities primarily associated with greenhouse gas (GHG) emissions [85]. Additionally, the air temperature has increased by 0.5 °C in the 20th century owing to global warming. The projections for the 21st century suggest an increase of 1.5–4.5 °C [86] with higher temperatures becoming more frequent. A rapidly warming climate results in HTS that exceeds the threshold for longer than a tolerable duration, causing morphological and physiological irreversible damage to plant development [87,88]. These effects include significant impact on seed germination; plant growth and development; product yield and quality [1]. Furthermore, HTS directly affects plant vegetative stages, resource allocation, and reproductive processes, which potentially lead to significant yield reduction [89]. During the vegetative phase, HTS reduces the photosynthesis rate, which leads to a decrease in biomass [90]. High temperatures adversely affect essential physiological processes such as stomatal opening, photosynthesis, growth, and grain yield [91]. Rise in temperature beyond the tolerance level induces severe stress in plants by directly affecting their functioning [92]. Karwa et al. [93] and Zhang et al. [94] found that the gas exchange properties and chlorophyll fluorescence parameters of flag leaves (prior to the reproductive stage) of rice cultivars decreased under HTS conditions. Each crop exhibited ideal temperature ranges for the efficient functioning of physiological processes such as growth, development, and reproduction. Furthermore, to protect against physiological and biochemical decay caused by oxidative stress at high temperatures, plants have developed different tolerance mechanisms such as protective solute formation, enzyme activation, and gene expression [95]. Extremely high temperatures result in rapid cellular injury and cell death [96]. Although a temperature range of 30–45 °C is optimal for structural integrity and enzymatic activity, these proteins are irreversibly denatured when the temperature exceeds 60 °C [1]. Furthermore, severe HTS leads to the accumulation of ROS, which causes oxidative damage to the cells. ROS negatively affects plant growth by interacting with various cellular components [97]. However, biostimulants that enhance tolerance in cereal crops may increase the water absorption capacity and stimulate the accumulation of protective substances in cell membranes. In contrast to developing various stress-resistant varieties, utilizing biostimulants offers a sustainable approach to improving crop health under abiotic stress and aids in the management of climate-resilient agriculture [98]. Recently, the use of growth regulators or biostimulants has been assessed as an agronomic management strategy that could mitigate the adverse effects of environmental stress [91,99,100]. Biostimulant treatments against HTS protect cell membranes by increasing their stability and reducing or preventing ROS accumulation. As summarized in (Table 1 and Figure 2), the examples discussed in this review show that the external application of biostimulants such as phytohormones, beneficial microbes, mineral nutrients, and plant extracts significantly improves the high-temperature resistance of rice and soybean by enhancing their growth, yield, and various physiological, biochemical, and molecular processes.

3.1. Biostimulants and Rice Tolerance to High-Temperature Stress

The exogenous application of abscisic acid (ABA) to rice before exposure to HTS reduced pollen sterility by enhancing sugar metabolism, heat shock protein expression, antioxidant activity, and energy balance, thereby improving high-temperature tolerance [101]. High temperatures induce a rapid increase in endogenous ABA levels, which enhances plant heat tolerance by activating antioxidant defense mechanisms [102,103]. Kosakivska et al. [104] found that applying external cytokinins improved stomatal conductance, photosynthesis, and various photosynthetic pigments in cereals under HTS conditions. Similarly, Pantoja-Benavides et al. [105] noted that foliar application of cytokinins and brassinosteroids (BR) enhanced photosynthesis and chlorophyll content and reduced oxidative stress during the reproductive stage at temperatures up to 40 °C. BR spray significantly improved stress tolerance indicators, spikelet fertility, and grain yield, whereas GA₃ treatment notably increased plant height and panicle length [106]. As BR are recognized as growth regulators, they have been used to mitigate various abiotic stresses including heat stress [107]. For example, Thussagunpanit et al. [108] have shown that BR application during the reproductive phase of rice plants increased the photosynthetic rate and stomatal conductance and reduced photoinhibition, thereby promoting HTS tolerance. Additionally, biostimulant treatments of 1 mM spermidine and 10⁻⁵ M indole-3-acetic acid (IAA) significantly enhanced rice high-temperature tolerance by improving seedling vigor, chlorophyll content, proline levels, and yield and reducing the expression of oxidative damage markers such as malondialdehyde (MDA) and H₂O₂ [109]. Similarly, Mostofa et al. [110] have reported that rice seeds primed with spermidine at high temperatures exhibited an increased seedling vigor index. Furthermore, Guangwu and Xuwen [111] have observed that exogenous application of 10⁻⁴ M and 10⁻⁵ M IAA significantly improved seed germination in Chinese red pine (Pinus massoniana Lamb).

Melatonin acts similarly to auxin (IAA). It effectively neutralizes free radicals in plants, promotes growth and development, and enhances resistance to abiotic and biotic stressors [112]. For example, Barman et al. [113] found that foliar application of 200 µM melatonin during anthesis significantly increased chlorophyll content and photosynthetic rate in heat-stressed rice, particularly in thermo-sensitive genotypes, which indicates its role in enhancing antioxidant activity and stress tolerance at 40.6 °C. Seed soaking with 100 μM melatonin significantly improved rice germination potential, shoot and root growth, and antioxidant enzyme activities under HTS and reduced MDA content, thereby enhancing heat tolerance during seed germination [114]. Furthermore, the application of exogenous melatonin during the grain-filling stage improved leaf photosynthesis, preserved starch structure (notably amylopectin), and enhanced both the yield and quality of japonica rice Nipponbare under HTS conditions [115]. The role of melatonin in increasing tolerance to high temperatures is attributed to its capacity to directly neutralize ROS, improve antioxidant enzyme activity and photosynthetic efficiency, and modulate the transcription of HTS-related genes [116,117]. Li et al. [118] observed that melatonin positively affected tea growth and quality by modulating photosynthesis and biosynthesis of polyphenols, amino acids, and caffeine in tea leaves at moderately high temperatures.

Furthermore, foliar application of phytohormones and nutrients enhanced physiological and biochemical parameters under HTS conditions. Salicylic acid (SA) is a phytohormone that enhances stress tolerance. Akasha et al. [119] found that applying 100 mg L⁻¹ SA to 15-day-old rice seedlings exposed to 45 ± 2 °C HTS increased seedling growth, fresh and dry biomass, and organic and inorganic solute concentrations. Similarly, Feng et al. [120] found that foliar application of SA (0.01–50 mM) on rice exposed to 40 °C HTS for 10 days before flowering reduced ROS in anthers, prevented tapetum cell death, and enhanced pollen viability through gene regulation and increased H₂O₂. These findings have been validated by other studies that have shown that foliar spraying of SA effectively mitigated the negative effects of high temperatures [121,122]. For example, Dat et al. [123] observed that high-temperature hardening at 45 °C for 1 h in the dark significantly increased the SA content in the stem of mustard seedling; additionally, spraying SA at a low concentration (100 μM) on the leaves enhanced antioxidant enzyme activity and heat tolerance. Lv et al. [124] have reported that SA primarily improved heat tolerance by influencing the photosynthetic system and redox levels in rice seedlings. These results indicate that external application of various phytohormones and biostimulants, including ABA, cytokinins, BRs, IAA, spermidine, melatonin, and SA, greatly enhances the tolerance of rice to HTS by integrating molecular and physiological responses. These compounds specifically affect gene expression and signaling pathways, activate antioxidant defenses, maintain photosynthesis, support reproductive success, and boost seedling vigor, thereby linking molecular mechanisms to observable phenotypic outcomes.

3.2. Biostimulants and Soybean Tolerance to High-Temperature Stress

The application of 100 µM melatonin enhanced soybean resilience to temperatures between 24 and 42 °C by alleviating oxidative stress, increasing antioxidant activity, modulating stress-related hormones, and promoting the accumulation of protective metabolites and expression of stress-responsive genes [125]. Melatonin pretreatment triggered plant defense systems by enhancing the activity of antioxidant enzymes that neutralize radicals and minimize oxidative damage under stressful conditions [126,127,128,129]. Similarly, melatonin pretreatment promoted plant growth and elevated the levels of photosynthetic pigments such as Chl a and Chl b; additionally, it reduced oxidative stress by neutralizing hydrogen peroxide and superoxide, which prevented MDA and electrolyte leakage in soybean plants [125]. However, the effects of melatonin on seed germination, early seedling growth, and other physiological traits vary depending on the type of abiotic stress, plant species, growth stage, and stress duration [130]. Aftab et al. [131] treated soybean seedlings grown in a greenhouse with various concentrations of SA and observed that the application of 100 µmol L⁻¹ SA maintained an elevated photosynthetic rate for five days. Dinler et al. [132] found that applying FA (2.0 mg L⁻¹) to the foliage at the seedling stage improved soybean tolerance to combined salt (150 mM NaCl) and heat stress (35 °C) by increasing RWC and the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), and glutathione S-transferase; additionally, it reduced oxidative damage and H₂O₂ and MDA levels. FA reduced soil erosion, enhanced soil fertility, and facilitated the transfer of mineral nutrients from soil to plants. Similarly to the effects of other organic fertilizers, FA protected plants against stressful conditions by increasing soil efficiency [133].

Moreover, treatment of seeds with a biostimulant containing lignin derivatives, plant-derived amino acids, and molybdenum improved germination and seedling vigor and reduced oxidative stress in soybean subjected to 35 °C HTS [134]. Auxins and ABA appeared to inhibit soybean germination [135], whereas gibberellin and ethylene promoted germination [136]. Additionally, Repke et al. [137] have shown that applying Ascophyllum nodosum seaweed extract (1 L ha⁻¹) to the foliage 43 days after sowing enhanced soybean yield under 40 °C HTS conditions by lowering leaf temperature and elevating antioxidant enzyme activity, proline accumulation, and photosynthetic performance through increased CO₂ assimilation, stomatal conductance, transpiration, and carboxylation efficiency. In summary, these studies highlight a range of substances—such as melatonin, FA, SA, plant-derived amino acids, lignin derivatives, and seaweed extracts—that enhance soybean resilience to HTS by integrating molecular and physiological responses. They influence stress-responsive genes, signaling pathways, and hormone levels, activate antioxidant defenses, and regulate ROS, leading to improved photosynthesis, water regulation, metabolite accumulation, germination, seedling vigor, and reproductive success. The effectiveness depends on species, growth stage, stress type, and application method, emphasizing the importance of targeted, stage-specific application to maximize stress application. Overall, biostimulants connect molecular mechanisms to observable phenotypic outcomes, offering a promising strategy to enhance soybean tolerance under HTS.

Table 1. Effects of biostimulant on high-temperature stress tolerance in rice and soybean: application method, growth, yield, and mechanistic insights.

Crop	Stress Level	Bio Stimulants	Application Methods/ Concentrations	Growth /Yield Effect	Mechanisms	Research Gaps	References
Rice	30–41 °C	Abscisic acid	Foliar (0, 1, 10 and 100 μmol L−1)		Enhanced sugar metabolism, heat shock protein expression, antioxidant activity, and energy balance	Limited to controlled; no field-sale validation	[101]
Rice	24–40 °C	Cytokinin (CK), Brassinosteroids (BR)	Foliar (CK 1 × 10−5 M, BR 5 × 10−5 M)		Enhanced chlorophyll content, gas exchange, and photosynthetic efficiency; reduced oxidative damage and canopy temperature	Early growth stage focus; yield-related data lacking	[105]
Rice	40.8 °C	Brassinosteroid (BR)-, Boron (B), Calcium chloride (CaCl2), Salicylic acid (SA), Glycine betaine (GB), Pink-pigmented facultative methylotrophs (PPFM), 1-methyl cyclopropane (1-MCP), (GA3)	Foliar (BR- 5 ppm, B-100 ppm, CaCl2 −0.6%, SA-50 ppm, GB-20 ppm, PPFM-1%, (1-MCP)-50 ppm, GA3-50 ppm)	Pollen viability, spikelet fertility, and yield	Improved physiological traits such as CMSI, photosynthesis, stomatal conductance, and chlorophyll stability	Multi-treatment approach; difficult to isolate effect of individual biostimulants	[106]
Rice	42 °C	Spermidine, Indole-3-acetic acid, Brassinolide, and boron	Seed priming/foliar (Spermidine- 1 mM and 2 mM), (Indole-3-acetic acid 10−3 M, 10−5 M), (Brassinolide 1 mg L−1, 2 mg L−1) and (Boron 50 mg L−1, 100 mg L−1)	Seedling vigor and yield	Enhanced chlorophyll, proline and reduced oxidative damage markers	Needs field trials and reproductive stage validation	[109]
Rice	40.6 °C	Melatonin (MT)	Foliar (200 µM)		Improved chlorophyll content and photosynthesis, especially in thermo-sensitive rice, likely by enhancing antioxidant activity	Only tested in thermosensitive cultivar; broader genotypic validation required	[113]
Rice	38 °C for 1 day and 26 °C in the light incubator	Melatonin (MT)	Seed-Soaking (20, 100, 500 µM MT)	Germination, shoot and root growth	Increased antioxidant enzyme activity and reduced oxidative damage	Short duration stress; field application unknown	[114]
Rice	38 °C/28 °C days/night	Melatonin (MT)	Foliar (250 mL of 200 µmol L−1 MT)	Yield and grain quality	Increased photosynthetic performance	Yield improvements shown; trial detail missing	[115]
Rice	45 ± 2 °C	Salicylic acid (SA)	Foliar (100 mg L−1)	Growth, fresh and dry biomass	Enhanced organic and inorganic solute concentrations	Lack of detail molecular analysis; limited yield validation missing	[119]
Rice	40 °C	Salicylic acid (SA)	Foliar (0.01, 0.1, 1.0, 10, and 50 mM)		Enhanced pollen viability under heat stress by reducing ROS and tapetum PCD, with H2O2 as key mediator	Mostly physiological study; field and yield validation missing	[120]
Soybean	24–42 °C	Melatonin (MT)	Root-treated (100 µM MT)		Reduced oxidative stress, enhanced antioxidants, balanced stress hormones, and promoted protective metabolites and gene expression	Needs soil/field validation	[125]
Soybean	150 mM NaCl + 35 °C	Fulvic acid	Foliar (FA, 2.0 mg L−1)		Increased RWC and activity of SOD, APX and GST. Reduced oxidative damage, H2O2 and MDA content	Lacks growth/yield outcomes	[132]
Soybean	35 °C	Biostimulant based on lignin derivatives, plant- derived amino acids, and molybdenum	Seed treatment (20 mL biostimulants)	Germination percentage and seedling vigor	Reduced oxidative stress level	Tested only germination stage; lacks growth/yield outcomes	[134]
Soybean	40 °C	Ascophyllum nodosum (L.) seaweed	Foliar (0.25, 0.50, 0.75 and 1 L ha−1)	Yield, reduced leaf temperature	Enhanced antioxidant activity, proline levels, and photosynthetic through improved CO2 assimilation, stomatal conductance, transpiration rate, and carboxylation efficiency	Positive yield response shown, but long-term field consistency unclear	[137]

Table 1. Effects of biostimulant on high-temperature stress tolerance in rice and soybean: application method, growth, yield, and mechanistic insights.

Crop	Stress Level	Bio Stimulants	Application Methods/ Concentrations	Growth /Yield Effect	Mechanisms	Research Gaps	References
Rice	30–41 °C	Abscisic acid	Foliar (0, 1, 10 and 100 μmol L−1)		Enhanced sugar metabolism, heat shock protein expression, antioxidant activity, and energy balance	Limited to controlled; no field-sale validation	[101]
Rice	24–40 °C	Cytokinin (CK), Brassinosteroids (BR)	Foliar (CK 1 × 10−5 M, BR 5 × 10−5 M)		Enhanced chlorophyll content, gas exchange, and photosynthetic efficiency; reduced oxidative damage and canopy temperature	Early growth stage focus; yield-related data lacking	[105]
Rice	40.8 °C	Brassinosteroid (BR)-, Boron (B), Calcium chloride (CaCl2), Salicylic acid (SA), Glycine betaine (GB), Pink-pigmented facultative methylotrophs (PPFM), 1-methyl cyclopropane (1-MCP), (GA3)	Foliar (BR- 5 ppm, B-100 ppm, CaCl2 −0.6%, SA-50 ppm, GB-20 ppm, PPFM-1%, (1-MCP)-50 ppm, GA3-50 ppm)	Pollen viability, spikelet fertility, and yield	Improved physiological traits such as CMSI, photosynthesis, stomatal conductance, and chlorophyll stability	Multi-treatment approach; difficult to isolate effect of individual biostimulants	[106]
Rice	42 °C	Spermidine, Indole-3-acetic acid, Brassinolide, and boron	Seed priming/foliar (Spermidine- 1 mM and 2 mM), (Indole-3-acetic acid 10−3 M, 10−5 M), (Brassinolide 1 mg L−1, 2 mg L−1) and (Boron 50 mg L−1, 100 mg L−1)	Seedling vigor and yield	Enhanced chlorophyll, proline and reduced oxidative damage markers	Needs field trials and reproductive stage validation	[109]
Rice	40.6 °C	Melatonin (MT)	Foliar (200 µM)		Improved chlorophyll content and photosynthesis, especially in thermo-sensitive rice, likely by enhancing antioxidant activity	Only tested in thermosensitive cultivar; broader genotypic validation required	[113]
Rice	38 °C for 1 day and 26 °C in the light incubator	Melatonin (MT)	Seed-Soaking (20, 100, 500 µM MT)	Germination, shoot and root growth	Increased antioxidant enzyme activity and reduced oxidative damage	Short duration stress; field application unknown	[114]
Rice	38 °C/28 °C days/night	Melatonin (MT)	Foliar (250 mL of 200 µmol L−1 MT)	Yield and grain quality	Increased photosynthetic performance	Yield improvements shown; trial detail missing	[115]
Rice	45 ± 2 °C	Salicylic acid (SA)	Foliar (100 mg L−1)	Growth, fresh and dry biomass	Enhanced organic and inorganic solute concentrations	Lack of detail molecular analysis; limited yield validation missing	[119]
Rice	40 °C	Salicylic acid (SA)	Foliar (0.01, 0.1, 1.0, 10, and 50 mM)		Enhanced pollen viability under heat stress by reducing ROS and tapetum PCD, with H2O2 as key mediator	Mostly physiological study; field and yield validation missing	[120]
Soybean	24–42 °C	Melatonin (MT)	Root-treated (100 µM MT)		Reduced oxidative stress, enhanced antioxidants, balanced stress hormones, and promoted protective metabolites and gene expression	Needs soil/field validation	[125]
Soybean	150 mM NaCl + 35 °C	Fulvic acid	Foliar (FA, 2.0 mg L−1)		Increased RWC and activity of SOD, APX and GST. Reduced oxidative damage, H2O2 and MDA content	Lacks growth/yield outcomes	[132]
Soybean	35 °C	Biostimulant based on lignin derivatives, plant- derived amino acids, and molybdenum	Seed treatment (20 mL biostimulants)	Germination percentage and seedling vigor	Reduced oxidative stress level	Tested only germination stage; lacks growth/yield outcomes	[134]
Soybean	40 °C	Ascophyllum nodosum (L.) seaweed	Foliar (0.25, 0.50, 0.75 and 1 L ha−1)	Yield, reduced leaf temperature	Enhanced antioxidant activity, proline levels, and photosynthetic through improved CO2 assimilation, stomatal conductance, transpiration rate, and carboxylation efficiency	Positive yield response shown, but long-term field consistency unclear	[137]

4. Biostimulants Enhancing Growth, Yield, and Stress Tolerance in Rice and Soybean Under Low-Temperature Stress

Low temperatures are among the most severe abiotic stressors that significantly affect growth and development and limit crop yields. Low-temperature stress is divided into chilling (0–15 °C) and freezing (<0 °C) stresses [138]. Unfortunately, cold events are expected to persist up to or increase in severity by the end of the 21st century because of climate change [139]. Several cereal crops such as maize and rice are well adapted to tropical or subtropical climates; therefore, they exhibit high sensitivity to cold when cultivated in temperate climates [140]. Soybean growth is adversely affected by extreme temperatures, salinity, and drought [141], and rice yield is frequently impacted by low-temperature stress [142,143]. Low-temperature stress occurs during both vegetative and reproductive phases of cereal development [144], and it induces morphological, physiological, and cellular responses. Moreover, sudden temperature fluctuations, which become increasingly frequent with climate change, prevent gradual acclimation and increase the vulnerability to damage in many crops. Low temperatures are important environmental factors that limit plant growth and reduce crop productivity and quality [145]. Moreover, chilling and freezing stresses damage cell membranes, impede plant growth, and reduce the yield and quality of crops such as wheat and rice [146,147] by initiating changes in membrane fluidity and activating Ca²⁺ channels and receptor-like kinases in the plasma membrane [148]. Although no single protein functions as a dedicated cold receptor, several candidates, including COLD1, cyclic nucleotide-gated channels (CNGCs), glutamate receptors, and PHYB are involved in cold perception [149,150,151,152]. For example, in rice, COLD1 interacts with the G protein subunit OsRGA1 to modulate calcium signaling and mediate cold sensing [150]. Overall, low temperatures limit plant growth and productivity by disrupting membrane stability, metabolism, and oxidative balance. The application of biostimulants effectively reduces the effects of external stressors on crops, thereby enhancing plant adaptability and resistance to abiotic stressors [153]. Biostimulants such as AABA and BABA alleviate chilling injury directly by enhancing antioxidant enzymatic activities and indirectly by increasing the level of metabolites such as phenolic compounds (e.g., catechin, corilagin, epicatechin, and gallocatechin gallate in longan fruit), which additionally contribute to improved qualitative traits [154]. In organic agriculture, the application of bioactive compounds or symbiotic microbes as biostimulants improved plant performance under both optimal and stressful conditions [155]. Table 2 and Figure 3 outline the findings of studies that have shown that the external application of biostimulants such as phytohormones, beneficial microbes, mineral nutrients, and plant extracts significantly enhanced low-temperature tolerance in rice and soybean by improving their growth, yield, and various physiological, biochemical, and molecular responses.

4.1. Biostimulants and Rice Tolerance to Low-Temperature Stress

Priming rice seeds with 100% aqueous carrot root extract significantly improved germination speed, final germination, and seedling growth under low-temperature stress; particularly strong effects were observed in low-temperature-tolerant varieties such as Brilhante [156]. In fact, the Brilhante cultivar showed superior growth performance than those of the other tested cultivars [157,158]. The seedling vigor index effectively measures both germination and seedling length under low-temperature stress and was useful for selecting the most suitable carrot extract treatment for each cultivar and identifying the best kelp extract for tomato seedlings [159]. Furthermore, foliar application of zinc oxide nanoparticles (ZnO NPs) mitigated low-temperature stress in rice seedlings by promoting growth, enhancing antioxidant enzyme activity, and upregulating stress-responsive genes and transcription factors [160]. ZnO NPs have been previously shown to alleviate abiotic stresses such as Cd toxicity in plants [161], although their role in reducing low-temperature stress remains unclear. In colder regions, Zn application promoted earlier tiller germination and increased the number of effective panicles [162]. However, Zn accumulation in wheat, maize, and rice decreased at low temperatures [163]. Notably, Liu et al. [164] found that zinc application (0.08–0.31 μM ZnSO₄·7H₂O) enhanced rice tillering and tiller bud growth by increasing zinc and nitrogen accumulation and maintaining a favorable cytokinin-to-auxin (CTK/IAA) ratio, which supported nitrogen metabolism and improved low temperature tolerance. Biochar is frequently used as an additive to enhance the environmental quality and amend soil characteristics [165]. Such additions have improved plant growth characteristics [166]. Biochar leachates increased low-temperature tolerance of rice at 10 °C by increasing the ABA levels and activating ABA-related genes; furthermore, it has been identified as an ABA analog that binds to the ABA receptor OsPYL2 [167]. Furthermore, biochar addition positively affected low-temperature stress resistance in rice plants [168]. Under low-temperature conditions (16 ± 1 °C), brassinolide application (2 mg L⁻¹) to the foliage at the booting stage in rice improved dry matter accumulation, enhanced antioxidant activity and chlorophyll content, and reduced oxidative stress, as indicated by lower MDA levels [169].

Moreover, exogenous melatonin improved rice seed germination under low-temperature stress by activating antioxidant defenses, regulating hormone balance, and interacting with the ABI5-OsCAT2 signaling pathway to maintain ROS homeostasis [170]. Shi et al. [171] found that exogenous melatonin alleviated ROS accumulation and cold-induced oxidative damage by directly scavenging ROS and enhancing antioxidative enzymes in Bermuda grass. Han et al. [172] showed that melatonin application at 12 °C particularly via seed soaking and root immersion significantly enhanced cold tolerance in rice by reducing oxidative damage, enhancing photosynthetic efficiency, and improving antioxidant defense mechanisms. Seed germination under abiotic stress involves a complex network of phytohormones such as (ABA), gibberellin (GA), and melatonin [173,174]. The crosstalk between melatonin and these hormones along with other stress markers constitutes a sophisticated regulatory network [174]. Notably, melatonin regulates seed germination by improving the antioxidant systems and directly scavenging reactive oxygen species [175,176]. The effectiveness of exogenous application of melatonin for improving seed germination has been confirmed under various stress conditions [177,178,179]. These studies indicate that biostimulants, including carrot root extract, ZnO nanoparticles, biochar amendments, and phytohormone analogs such as brassinolide and melatonin, play a significant role in enhancing rice tolerance to low-temperature stress by integrating molecular and physiological responses. These biostimulants modulate stress-responsive genes, hormone signaling pathways (such as ABA, GA, and melatonin), and antioxidant defenses to maintain ROS balance, resulting in improved germination, seedling vigor, photosynthesis, chlorophyll levels, and biomass growth. Overall, these findings support biostimulants as promising tools to mitigate low-temperature stress and improve rice productivity in cooler climates.

4.2. Biostimulants and Soybean Tolerance to Low-Temperature Stress

Melatonin application in soybean alleviated low temperature-induced oxidative stress by enhancing antioxidant activity and modulating reactive oxygen species levels, as well as regulating the expression of 145 identified B3 genes. These genes responded to both low temperature and melatonin treatment and may improve stress tolerance by interacting with other transcription factors [180]. Raza et al. [130] found that exogenous melatonin enhanced plant tolerance to extreme temperatures by either directly neutralizing ROS molecules or indirectly enhancing photosynthetic efficiency, antioxidant enzyme activity, and metabolite levels in plants. Thus, melatonin effectively supports plant growth and development under stressful conditions [181]. Furthermore, Bawa et al. [182] observed that pretreatment of soybean seedlings with melatonin (5 µmol L⁻¹) under low-temperature stress reduced oxidative damage and ROS buildup by increasing mineral element concentrations and expression of antioxidant genes, which improved low-temperature stress tolerance. Similarly, melatonin pretreatment elevated plant mineral elements in Malus and cucumbers under different stress conditions [183,184]. Melatonin plays a significant role in combating various environmental stressors and diseases [185,186,187]. Furthermore, it is present in all plant parts, which enhances plant resilience to numerous stresses [188,189]. In addition to melatonin, foliar application of α-oxoglutarate enhanced GS activity, GDH activity, ammonium assimilation, and proline content at both the S1 and S2 stages in plants, thereby playing a vital role in stress responses [190]. Gai et al. [191] have reported that applying 5 mmol L⁻¹ of exogenous α-oxoglutarate improved low-temperature tolerance in soybean seedlings at 6 °C by enhancing key enzyme activity, increasing proline accumulation and photosynthesis, and reducing ammonium levels. These findings were corroborated by Yuan et al. [192] who examined the role of α-oxoglutarate in regulating carbon and nitrogen metabolism in rice under abiotic stress and found that exogenous α-oxoglutarate enhanced GS activity.

Additionally, Moradtalab et al. [193] have shown that silicon (Si) application maintained plant growth and biomass under low-temperature stress, possibly because of the formation of phytoliths and subcutaneous layers that reduce dehydration and help maintain water turgor. Moreover, Si application enhanced the growth characteristics of soybean plants under various biotic stressors such as heat stress [194], water scarcity, and low light conditions [195]. Additionally, temperature fluctuations affect the physio-biochemical parameters of plants. Rapid ROS formation immediately impairs cellular function, thereby inhibiting plant growth and development [196]. Recently, Ahmad et al. [197] have shown that foliar application of Si (1.0 mM) at 8–10 °C enhanced soybean seedling growth by alleviating oxidative stress, regulating stress-responsive genes, and promoting beneficial microbial activity, which increased low-temperature tolerance. Zhu et al. [198] observed that silicon reduced stomatal conductance by causing turgor loss in guard cells through its deposition and alteration in cell wall properties. Guntzer et al. [199] have highlighted the dynamic role of silicon in alleviating both abiotic and biotic stress, causing considerable interest among plant biologists. Overall, the external applying of biostimulants such as melatonin, α-oxoglutarate, and silicon enhance soybean tolerance to low-temperature stress by integrating molecular and physiological responses. Melatonin regulates ROS, activates antioxidant enzymes, and modulates stress responsive gene; α-oxoglutarate improves nitrogen and carbon metabolism, proline accumulation, and photosynthesis; and silicon strengthens cell structures, regulates stomatal conductance, and supports beneficial microbes. These applications result in improved germination, seedling vigor, photosynthesis, biomass, and osmotic balance. The effectiveness of these treatments depends on the type, concentration, application method, growth stage, and environmental condition, highlighting the need for precise interventions. Thus, biostimulants offer a promising strategy to mitigate low-temperature stress and maintain soybean yield.

Table 2. Effects of biostimulant on low-temperature stress tolerance in rice and soybean: application method, growth, yield, and mechanistic insights.

Crop	Stress Level	Bio Stimulants	Application Methods/ Concentrations	Growth /Yield Effect	Mechanisms	Research Gaps	References
Rice	15 °C	Carrot extract	Seed soaking (0, 25, 50, and 100%)	Germination speed and final germination percentage and the growth		Mechanistic basis not explored; only seedling stage tested	[156]
Rice	10 °C (16 h light/8 h dark)	ZnO NPs	Foliar (25, 50, and 100 mg/L ZnO NPs with 50 mg L−1 TX-10)	Plant growth	Reduced oxidative stress markers (H2O2, MDA, and proline) and enhanced antioxidant enzyme activity (SOD, CAT, and POD)	Lacks yield data	[160]
Rice	12 °C and 20 °C	Zinc (Zn)	Hydroponic (root application) (0.08, 0.15 and 0.31 µM)	Germination and growth	Improved nitrogen metabolism by enhancing Zn and N accumulation and maintaining a higher CTK/IAA ratio	Hydroponic only; field application lacking	[164]
Rice	10 °C	Biochar fast pyrolysis of rice husks, Abscisic acid (ABA)	Foliar (1, 3, 5, 7 and 10% Biochar, 0, 10, 20, and 30 mg of ABA)		Increased ABA and carotenoid levels, gene expression	Interaction effects unclear; long-term soil effects of biochar missing	[167]
Rice	(16 ± 1) °C	Brassinolide (BR)	Foilar (2 mg L−1 BR)		Enhanced antioxidant enzyme activities, nutrient content, chlorophyll and reduced MDA levels	Yield effects not evaluated; short-term response only	[169]
Rice	15–24 °C	Melatonin (MT)	Seed treatments (150 µmol L−1 MT)	Seed germination	Enhanced GA biosynthesis, reduced ABA and H2O2, and activated OsCAT2 through OsABI5 regulation	Gene-specific results; not validated across cultivars	[170]
Rice	12 °C	Melatonin (MT)	Soaking seed, immersing roots, and spraying (0, 20, or 100 µM MT)		Reduced ROS, MDA, and cell death, improved photosynthesis, and enhanced antioxidant defenses	Mostly physiological data; yield effects not assessed	[172]
Soybean	4 °C/3 days	Melatonin (MT)	Foliar (100 mmol L−1 MT)		Enhanced antioxidant and hormones levels and activated B3 stress-response genes	Agronomic feasibility unclear	[180]
Soybean	12 °C	Melatonin (MT)	Foliar (1, 5, 10 and 50 µmol L−1 MT)		Reduced oxidative damage and ROS accumulation and enhanced mineral uptake and antioxidant gene expression	Field trials absent	[182]
Soybean	6 °C	Exogenous α-oxoglutarate	Foliar (0, 2.5, 5.0 and 7.5 mmol L−1)		Improved soybean cold tolerance by improving key enzymes, proline, and photosynthesis and lowering ammonium	Early-stage study; yield response missing	[191]
Soybean	8–10 °C	Silicon (Si)	Soil drench (1.0 mM Si)	Plant growth	Reduced oxidative stress, regulated stress genes, and enhanced beneficial microbes	Pot experiments only; multi-season validation needed	[197]

Table 2. Effects of biostimulant on low-temperature stress tolerance in rice and soybean: application method, growth, yield, and mechanistic insights.

Crop	Stress Level	Bio Stimulants	Application Methods/ Concentrations	Growth /Yield Effect	Mechanisms	Research Gaps	References
Rice	15 °C	Carrot extract	Seed soaking (0, 25, 50, and 100%)	Germination speed and final germination percentage and the growth		Mechanistic basis not explored; only seedling stage tested	[156]
Rice	10 °C (16 h light/8 h dark)	ZnO NPs	Foliar (25, 50, and 100 mg/L ZnO NPs with 50 mg L−1 TX-10)	Plant growth	Reduced oxidative stress markers (H2O2, MDA, and proline) and enhanced antioxidant enzyme activity (SOD, CAT, and POD)	Lacks yield data	[160]
Rice	12 °C and 20 °C	Zinc (Zn)	Hydroponic (root application) (0.08, 0.15 and 0.31 µM)	Germination and growth	Improved nitrogen metabolism by enhancing Zn and N accumulation and maintaining a higher CTK/IAA ratio	Hydroponic only; field application lacking	[164]
Rice	10 °C	Biochar fast pyrolysis of rice husks, Abscisic acid (ABA)	Foliar (1, 3, 5, 7 and 10% Biochar, 0, 10, 20, and 30 mg of ABA)		Increased ABA and carotenoid levels, gene expression	Interaction effects unclear; long-term soil effects of biochar missing	[167]
Rice	(16 ± 1) °C	Brassinolide (BR)	Foilar (2 mg L−1 BR)		Enhanced antioxidant enzyme activities, nutrient content, chlorophyll and reduced MDA levels	Yield effects not evaluated; short-term response only	[169]
Rice	15–24 °C	Melatonin (MT)	Seed treatments (150 µmol L−1 MT)	Seed germination	Enhanced GA biosynthesis, reduced ABA and H2O2, and activated OsCAT2 through OsABI5 regulation	Gene-specific results; not validated across cultivars	[170]
Rice	12 °C	Melatonin (MT)	Soaking seed, immersing roots, and spraying (0, 20, or 100 µM MT)		Reduced ROS, MDA, and cell death, improved photosynthesis, and enhanced antioxidant defenses	Mostly physiological data; yield effects not assessed	[172]
Soybean	4 °C/3 days	Melatonin (MT)	Foliar (100 mmol L−1 MT)		Enhanced antioxidant and hormones levels and activated B3 stress-response genes	Agronomic feasibility unclear	[180]
Soybean	12 °C	Melatonin (MT)	Foliar (1, 5, 10 and 50 µmol L−1 MT)		Reduced oxidative damage and ROS accumulation and enhanced mineral uptake and antioxidant gene expression	Field trials absent	[182]
Soybean	6 °C	Exogenous α-oxoglutarate	Foliar (0, 2.5, 5.0 and 7.5 mmol L−1)		Improved soybean cold tolerance by improving key enzymes, proline, and photosynthesis and lowering ammonium	Early-stage study; yield response missing	[191]
Soybean	8–10 °C	Silicon (Si)	Soil drench (1.0 mM Si)	Plant growth	Reduced oxidative stress, regulated stress genes, and enhanced beneficial microbes	Pot experiments only; multi-season validation needed	[197]

5. Biostimulants Enhancing Growth, Yield, and Stress Tolerance in Rice and Soybean Under Drought Stress

Drought stress is a most severe environmental challenge that constrains global crop productivity [200] and the primary factor that limits yield. Drought stress is expected to persist as climate change intensifies evapotranspiration and increases water demand in agricultural systems [201,202]. The effects of climate change have exacerbated the frequency and intensity of droughts along with increasing temperature and atmospheric CO₂ levels and causing erratic weather patterns, which pose significant risks to both wild and cultivated plant species [203]. Currently, approximately 70% of the global freshwater resources are consumed for agriculture, and this consumption is expected to increase rapidly [204]. In fact, drought affects more than one-third of the agricultural lands worldwide, endangering global food security [205]. Water shortages may result in an average yield reduction of up to 50% worldwide [206]. Approximately 11% of current croplands, especially in regions in Africa, the Middle East, China, Europe, and Asia, are projected to face the risk of water scarcity [207]. Agricultural drought occurs after a prolonged lack of rainfall or water availability and is often exacerbated by high evapotranspiration rates that deplete soil moisture [208]. Drought stress interferes with essential plant physiological processes such as metabolism, stomatal conductance, and gas exchange, ultimately leading to stunted growth and poor agricultural performance [209]. As plant gas exchange activities including photosynthesis and transpiration are directly related to yield, drought stress significantly affects these processes [1]. In severe cases, drought leads to complete yield loss, as observed in barley grown under field conditions without irrigation [210]. At the physiological level, drought stress reduces plant growth, disrupts reproductive development, damages vital metabolic functions, which collectively diminish the yield [211]. Moreover, drought stress leads to excessive ROS accumulation in plant cells, which results in oxidative damage and cell death [212]. ROS include superoxide anions (O₂⁻), hydrogen peroxide (H₂O₂), hydroxyl radicals (OH⁻), ozone (O₃), and singlet oxygen (¹O₂). These contribute to lipid peroxidation, cellular oxidative damage, cellular homeostasis disruption, DNA mutations, and protein denaturation [213]. Although high ROS levels are harmful, a balanced process of ROS generation and detoxification is crucial for optimal plant functioning [214]. Plants possess both enzymatic [superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), guaiacol peroxidase (GPX), dehydroascorbate reductase (DHAR), and monodehydroascorbate reductase (MDHAR)] or ascorbate–glutathione cycle enzyme and non-enzymatic [glutathione (GSH) and ascorbate (AsA)] antioxidant defense mechanisms to remove excess ROS and maintain redox balance [215]. Increased activity of these antioxidant enzymes is associated with better protection against drought-induced oxidative damage [216]. Furthermore, melatonin is a multifunctional molecule that enhances drought tolerance in plants by delaying leaf aging, regulating water balance, promoting lateral root growth and seed germination, preserving chloroplast and leaf structures, and modulating nitro-oxidative homeostasis and proline metabolism [217,218,219,220]. Additionally, biostimulants, especially humic acid combined with micronutrients such as boron and zinc, have shown significant potential for enhancing plant growth, grain-filling rate, and yield components under drought conditions [221]. As water shortages often occur intermittently in many cropping systems, the external application of biostimulants has become a widely used strategy to improve plant performance under various abiotic stressors [1,58]. Therefore, biostimulants are emerging as promising tools to alleviate the negative effects of drought by enhancing stress tolerance mechanisms and sustaining crop productivity under challenging environmental conditions. Table 3 and Figure 4 outline the studies that have clearly shown that the application of biostimulants such as phytohormones, beneficial microbes, mineral nutrients, and plant extracts have significantly improved drought resistance in rice and soybean by enhancing growth, yield, and various physiological, biochemical, and molecular processes.

5.1. Biostimulants and Rice Tolerance to Drought Stress

Foliar application of 100 µM melatonin in rice effectively mitigated drought-induced damage by promoting growth, enhancing water retention, increasing chlorophyll content, enhancing antioxidant enzyme activities, and facilitating osmolyte accumulation and concurrently reducing oxidative stress marker levels [222]. Pretreatment of seedlings with 100 µM melatonin enhanced drought tolerance by improving growth, photosynthesis, root development, increasing both enzymatic antioxidants (SOD, CAT, POD, APX, GR, DHAR, MDHAR, GST) and non-enzymatic antioxidants (AsA, GSH), reducing ROS and Lipid peroxidation, and promoting osmolyte accumulation [223]. Similarly, exogenous application of 100 µM melatonin significantly improved rice seed germination and seedling vigor under drought conditions by reducing oxidative damage and enhancing physiological resilience [224]. The same melatonin concentration increased proline content and antioxidant activity in cotton, which resulted in decreased MDA and H₂O₂ levels during drought stress [225]. PGRs such as methyl jasmonate (MJ) and jasmonic acid (JA) are crucial signaling molecules that control seed germination and defence responses under stress [226]. In rice, seed priming with MJ significantly enhances drought tolerance by improving antioxidant activity, increasing phenolic and ABA content, modulating NADPH oxidase, and upregulating drought-responsive genes [227]. Similarly, JA signaling plays a crucial role in regulating plant growth and responses to environmental stressors [228]. SA is another key compound that alleviates stress under drought conditions by regulating various physiological processes such as photosynthesis, proline and nitrogen metabolism, glycine–betaine production, antioxidant defense, and water balance [229,230,231]. External application of 750 μMm⁻² SA significantly improved rice growth, yield components, and drought tolerance under both moderate and severe water stress conditions [232]. Moreover, SA foliar spray reduced oxidative damage, improved antioxidant capacity, and enhanced metabolic performance in drought-stressed rice with the drought-tolerant genotype HTT-138 showing higher resilience than that of HTT-39 [233].

According to Devarajan et al. [234], applying Bacillus endophyticus and Bacillus altitudinis to rice leaves during the flowering phase notably enhanced yield parameters. This improvement was achieved by enhancing RWS and increasing the levels of sugars, proteins, proline, phenolics, potassium, calcium, and essential hormones such as ABA and indole acetic acid (IAA). Additionally, spraying bacteria generated extracellular polymeric substances (EPS), which facilitated calcium storage in rice leaves by forming carbonate precipitates near the bacterial cell walls [235]. In plants, sugars and proteins are crucial for protection against abiotic stressors as they stabilize cell membranes and prevent oxidative damage [236,237,238]. An increase in protein and sugar levels has been observed in rice leaves following foliar application of plant growth-promoting Bacillus during moisture stress [239,240]. Among mineral nutrients, potassium acts as an inorganic osmolyte. Its application (120 kg K₂O ha⁻¹) during drought cycles enhanced grain yield, harvest index, photosystem II efficiency, and nutrient uptake in the Malaysian rice variety MR220 [241]. Naser et al. [242] found that potassium application reduced the decrease in chlorophyll a, b, and total chlorophyll levels during water stress. The application of selenium (Se) promoted growth and mitigated the effects of abiotic stressors (e.g., drought, salinity, high temperatures, and potentially toxic elements) [243,244,245,246]. Selenium application at low rates (≤0.5 mg kg⁻¹) enhanced rice plant growth, photosynthesis, antioxidant enzyme activity, and water deficit stress tolerance by lowering hydrogen peroxide concentration and improving water use efficiency [247]. An increase in this index in plants under both water-deficit conditions and Se treatment was linked to an increase in chlorophyll content, which is consistent with previously reported findings [248]. Under mild drought stress conditions, nitrogen application at both standard and high levels significantly improved root system adaptability and dry matter production in the CSSL50 genotype compared with that observed in Nipponbare; however, no notable genotypic differences were observed under well-watered or severe drought conditions [249]. Zinc aids in mitigating ROS-induced damage and enhancing the nutrient profile of cereals [250]. Priming seeds with 25 ppm of ZnO NPs effectively increased drought tolerance in rice by enhancing growth metrics, increasing antioxidant enzyme activities, reducing oxidative stress, and significantly improving yield characteristics under water scarcity [203]. Similar effects were noted after ZnO NP priming in maize [251] and rice [252], which could be attributed to improved enzyme biosynthesis related to nutrient uptake [253]. Additionally, Zn priming enhanced seedling growth and yield in mung bean [254].

Furthermore, the foliar application of moringa leaf extract (Moringa oleifera) significantly enhanced photosynthetic activity, pigment content, antioxidant enzyme activities, and overall growth, yield, and grain quality of rice under both normal and drought conditions, with the Faisalabad landrace exhibiting the highest biostimulant potential [255]. Moringa has attracted significant research interest because its leaves are rich in mineral nutrients, growth hormones, vitamins, and antioxidants [256]. Additionally, foliar application of moringa leaf extract enhanced seedling establishment, plant growth, and the final yield of field crops grown in both normal and stressful environments [256,257,258]. Overall, these studies indicate that foliar and seed treatments using natural and synthetic compounds including melatonin, MJ, SA, beneficial microbes (e.g., Bacillus spp.), mineral nutrients (potassium, selenium, and zinc), and moringa leaf extract consistently enhance drought tolerance in rice. They regulate genes stress respond gene, modulate hormone signaling, and strengthen antioxidant defenses to maintain ROS balance and promote osmolyte accumulation. These responses improve growth, photosynthesis, water use efficiency, nutrient uptake, and yield. In summary, these treatments provide sustainable approaches to mitigate drought stress and maintain rice productivity under water-limited conditions.

5.2. Biostimulants and Soybean Tolerance to Drought Stress

In soybean, the application of external melatonin particularly to the roots enhanced drought resistance by improving growth, photosynthesis, antioxidant activity, and hormonal balance and reducing oxidative damage and stress-related metabolites [259]. Chlorophyll levels declined significantly under stress conditions; however, melatonin application successfully prevented its breakdown and increased the chlorophyll level, thereby improving photosynthesis [260]. Li et al. [218] found that plants exhibited increased H₂O₂ and electrolyte leakage during drought stress, and melatonin pretreatment reduced these levels and enhanced antioxidant enzyme activity. Additionally, soaking seeds in 500 µM melatonin improved soybean germination under water stress by decreasing lipid peroxidation and improving the activities of antioxidant enzymes such as SOD, POD, CAT, and APX [261]. Foliar MJ application mitigated the adverse effects of drought stress in soybean by enhancing growth, photosynthetic pigments, and biochemical components. Furthermore, Giza 22 exhibited better drought resistance than that of Giza 35 [262]. MJ is a natural plant growth regulator that enhances plant resistance to various abiotic stressors [263,264]. It plays a crucial role in regulating secondary metabolism by promoting the accumulation of bioactive compounds such as alkaloids, flavonoids, phenols, and coumarins [265]. Anjum et al. [206] noted that exogenous MJ improved drought tolerance in soybean by increasing antioxidant enzyme activity, proline content, and RWC and reducing membrane lipid peroxidation, which improved yield under both drought and normal conditions. Additionally, SA showed drought-mitigating effects in soybean. Seed pretreatment with SA enhanced drought tolerance and yield stability by increasing energy production, carbon and nitrogen remobilization, redox balance, and antioxidant defense at various developmental stages [266]. Similar findings were reported by Sharafizad et al. [267] and Sharma et al. [268]. They observed improved yield traits under drought stress conditions. Similarly, Saruhan et al. [269] documented SA-induced increase in antioxidant enzyme activity and antioxidant levels. Moreover, SA upregulated the genes associated with the AsA-GSH cycle under drought conditions [270]. Razmi et al. [271] found that applying 0.4 mM SA significantly improved physiological traits and yield components in soybean, particularly in the Williams genotype, by enhancing antioxidant enzyme activity and reducing oxidative stress.

External application of glycine betaine (GB) effectively counteracted yield losses caused by drought. GB enhanced yield-related characteristics such as the number of branches, seeds per plant, and weight of 1000 seeds, although it did not influence plant height [272]. Foliar application of GB elevated its concentration in soybean plant by up to 60 μmol/g dry weight; consequently, it resulted in enhanced photosynthesis, nitrogen fixation, leaf area expansion, and seed yield in both well-watered and drought-affected soybean plants [273,274]. Iqbal et al. [275] reported that GB application during the vegetative phase in sunflower effectively alleviated drought stress and increased 100-achene weight, thereby improving yield. In contrast, Meek et al. [276] found that foliar GB application did not influence yield components or internal GB levels in cotton plants under drought conditions. Nevertheless, the beneficial effects of external GB have been widely documented in other crops such as tobacco, wheat, barley, sorghum, and common beans [277]. Seaweed extract is another biostimulant that mitigates the effects of drought, cold stress, and salinity by accumulating nonstructural carbohydrates and proline to improve energy storage, metabolism, and water regulation [278,279]. Furthermore, 10% red seaweed extracts (Gracilaria tenuistipitata var. liui) significantly enhanced growth and grain yield under both drought and normal conditions [280]. This improvement in drought tolerance has been linked to reduction in stressor-induced physiological stress and yield decline. Youssef et al. [281] have noted that seaweed extracts increase chlorophyll production (as indicated by the SPAD index) likely by improving the magnesium content. Additionally, Du Jardin and Geelen [282] have confirmed the role of seaweed-based biostimulants in enhancing nutrient use efficiency, stress tolerance, and crop quality.

Nitrogen (N) application is crucial in drought-stressed soybean. High N application improved physiological traits and yield under drought but decreased yield components under well-watered conditions, and 100-seed weight showed the strongest correlation with yield [283]. Additionally, nitrogen is essential for the vegetative growth of soybean to achieve optimal biomass [284]. Nitrogen deficiency leads to the remobilization of nutrients from leaves to seeds, thereby reducing photosynthesis and yield [285]. Several studies have confirmed that N fertilization enhances seed yield under drought conditions [285,286]. The application of ZnO significantly improved soybean seed germination, root growth, and seed reserve utilization under drought stress induced by PEG-6000 [287]. Zn contributes to membrane stability and cell elongation under stress [288], which improved root growth and biomass accumulation.

Biochar enhances drought resilience, and its application influences plant performance under drought conditions [289,290]. The pores of biochar may hold water, although not all of it may be accessible to plants because of its relatively high-water potential [291]. Despite this, biochar improved soybean biomass and water-use efficiency in clay soils, primarily by increasing nutrient availability—particularly that of nitrogen and potassium [292]. Biochar may enhance plant water uptake by enhancing soil water retention either by storing water in its pores or improving soil structure [293]. However, biochar increased the salt and low-molecular-weight organic compound levels in soil, which potentially worsened water stress [294,295]. Overall, the external applications of melatonin, MJ, SA, GB, seaweed extract, nano-zinc oxide, nitrogen fertilizers, and biochar play unique roles in alleviating the negative effects of water scarcity. These biostimulants improve drought tolerance by modulating stress-related genes, hormones, and antioxidants, maintaining ROS balance, and enhancing water, nutrient, and osmolyte levels. Such regulation minimizes oxidative damage while promoting growth, vitality, and yield. Future research should refine the dosage, timing of application, and combined biostimulant strategies to optimize their effectiveness under various environmental conditions.

Table 3. Effects of biostimulant on drought-stress tolerance in rice and soybean: application method, growth, yield, and mechanistic insights.

Crop	Stress Level	Bio Stimulants	Application Methods/ Concentration	Growth /Yield Effect	Mechanisms	Research Gaps	References
Rice	7 days water withholding	Melatonin (MT)	Foliar (MT-50, 100, 200, and 300 μM)	Growth	Increased RWC, chlorophyll content, antioxidant enzyme activities, and reduced electrolyte leakage, MDA, and H2O2	Controlled pot study only; field trials absent	[222]
Rice	PEG 6000	Melatonin (MT)	Seed soaking (MT-20, 100, and 500 μM)	Germination, seedling growth, agronomic traits	Enhanced antioxidant enzyme activity, soluble protein content and reduced MDA	Osmotic simulation, not real drought; lacks yield data	[224]
Rice	6-day irrigation withdrawal	Methyl Jasmonate (MJ), salicylic acid (SA), paclobutrazol (PBZ)	Seed-priming (100 μM)		Improved antioxidant activities, increased phenolic and abscisic acid content, modulated NADPH oxidase activity	Lack of field validation or yield-related assessments	[227]
Rice	Severe/moderate drought	Salicylic acid (SA)	Foliar (250, 500, 750 and 1000 µMm2)	Growth and yield components		Limited mechanistic data	[232]
Rice	100%, 80%, and 60% field capacity	Salicylic acid (SA)	Foliar (SA-100 mg L−1)	Yield	Increased antioxidant capacity and reduced oxidative stress	Lack of multi-location; only two genotypes tested	[233]
Rice	55–60% water holding capacity	Bacillus endophyticus PB3, Bacillus altitudinis PB46, and Bacillus megaterium PB50	Foliar (15–25 mL of inoculant per plant)	Yield parameters	Improved RWC, key biochemical compounds and stress-responsive gene expression	Pot-scale; needs large-scale agronomic testing	[234]
Rice	5, 10, and 15 days of irrigation interval	Potassium (K)	Soil basal (80,120, 160 kg K2O ha−1)	Yield and harvest index	Increased photosystem II efficiency	Limited mechanistic insight	[241]
Rice	50 kPa	Selenium	Soil (Se-0.5, 1.0 and 2.0 mg kg−1)	Plant height	Enhanced SOD and Reduced H2O2	Growth only; no yield evaluation	[247]
Rice	29.3–2.8% w/w of soil moisture	Nitrogen	Basal dressing (25, 75, 150 kg N/ha for 2009 and 60, 120, 180 kg N ha−1)	Root plasticity and dry matter production		Limited to short-term season; evaluated only a few genotypes	[249]
Rice	35%, 70% water holding capacity	ZnO NPs	24 h seed priming (5, 10, 15, 25, and 50 ppm)	Growth parameters	Increased antioxidant enzyme activities and reduced oxidative stress	Field applicability unknown	[203]
Rice	50, 75, and 100% field capacity	Moringa oleifera	Foliar/ (MLE 3% w/v)	Growth, yield, and grain quality	Increased photosynthesis, pigment content, and antioxidant enzyme activities	Limited to pot; needs field and multi-season trials	[255]
Soybean	30–35% field capacity	Melatonin (MT)	Foliar (MT-50 or 100 μM)	Growth	Improved photosynthesis, hormone balance, and antioxidant activity and reduced oxidative damage	Short-term study; no yield data	[259]
Soybean	40 and 80% field capacity	Methyl jasmonate (MJ)	Foliar (20 µM MJ)	Growth	Enhanced growth, photosynthetic pigments, and biochemical constituents	Growth only; yield untested	[263]
Soybean	35–75% field capacity	Methyl jasmonate (MJ)	Foliar/ (0.5 µM MJ)	Grain yield	Increased antioxidants enzymatic, RWC and reduced lipid peroxidation	Pot-based; Limited field applicability	[206]
Soybean	50%, 75% relative water content	Salicylic acid (SA)	Seed priming (0.5 mM SA)	Yield parameters	Improved energy production, carbon and nitrogen remobilization, redox homeostasis, and antioxidative defense mechanisms	Early-stage; field confirmation missing	[266]
Soybean	45, 65, and 85% field capacity	Salicylic acid (SA)	Foliar (SA, 0.4 and 0.8 mM)	Physiological traits and yield components	Enhanced antioxidant enzyme activity and reduced oxidative damage	Pot-level; needs field-scale testing	[271]
Soybean	30, 50, 70 and 100% field capacity	Exo-GB (glycine betaine)	Foliar (0, 2.5, 5 and 7.5 kg ha−1)	Yield components such as branch number, seed number per plant, and 1000-seed weight		Pod-level, single cultivar; lack of field variability	[272]
Soybean	40 and 80% field capacity	Red seaweed extract (Gracilaria tenuistipitata var. liui)	Foliar (0.0%, 5.0%, and 10.0% v/v)	Growth and yield		Mechanistic pathways unstudied	[280]
Soybean	Non-irrigated, half-irrigated, and fully irrigated	Nitrogen (N)	Soil (35, and 105 kg ha−1 N)	Physiological traits and yield		No molecular data	[283]
Soybean	PEG-6000	Nano zinc oxide	Added to Petri dish (0.5, and 1 g L−1)	Germination percentage and rate		Mechanistic pathways unstudied; lacks growth/yield validation	[287]
Soybean	40, 60 and 80% field capacity	Biochar (pyrolysed at ~400 °C for 5 h)	Soil amendment (0, 25, 50 and 100 t ha−1)	Crop growth rate, total biomass production, and seed yield	Increased water uses efficiency, soil available potassium, and K uptake	Mechanistic pathways unstudied	[292]

Table 3. Effects of biostimulant on drought-stress tolerance in rice and soybean: application method, growth, yield, and mechanistic insights.

Crop	Stress Level	Bio Stimulants	Application Methods/ Concentration	Growth /Yield Effect	Mechanisms	Research Gaps	References
Rice	7 days water withholding	Melatonin (MT)	Foliar (MT-50, 100, 200, and 300 μM)	Growth	Increased RWC, chlorophyll content, antioxidant enzyme activities, and reduced electrolyte leakage, MDA, and H2O2	Controlled pot study only; field trials absent	[222]
Rice	PEG 6000	Melatonin (MT)	Seed soaking (MT-20, 100, and 500 μM)	Germination, seedling growth, agronomic traits	Enhanced antioxidant enzyme activity, soluble protein content and reduced MDA	Osmotic simulation, not real drought; lacks yield data	[224]
Rice	6-day irrigation withdrawal	Methyl Jasmonate (MJ), salicylic acid (SA), paclobutrazol (PBZ)	Seed-priming (100 μM)		Improved antioxidant activities, increased phenolic and abscisic acid content, modulated NADPH oxidase activity	Lack of field validation or yield-related assessments	[227]
Rice	Severe/moderate drought	Salicylic acid (SA)	Foliar (250, 500, 750 and 1000 µMm2)	Growth and yield components		Limited mechanistic data	[232]
Rice	100%, 80%, and 60% field capacity	Salicylic acid (SA)	Foliar (SA-100 mg L−1)	Yield	Increased antioxidant capacity and reduced oxidative stress	Lack of multi-location; only two genotypes tested	[233]
Rice	55–60% water holding capacity	Bacillus endophyticus PB3, Bacillus altitudinis PB46, and Bacillus megaterium PB50	Foliar (15–25 mL of inoculant per plant)	Yield parameters	Improved RWC, key biochemical compounds and stress-responsive gene expression	Pot-scale; needs large-scale agronomic testing	[234]
Rice	5, 10, and 15 days of irrigation interval	Potassium (K)	Soil basal (80,120, 160 kg K2O ha−1)	Yield and harvest index	Increased photosystem II efficiency	Limited mechanistic insight	[241]
Rice	50 kPa	Selenium	Soil (Se-0.5, 1.0 and 2.0 mg kg−1)	Plant height	Enhanced SOD and Reduced H2O2	Growth only; no yield evaluation	[247]
Rice	29.3–2.8% w/w of soil moisture	Nitrogen	Basal dressing (25, 75, 150 kg N/ha for 2009 and 60, 120, 180 kg N ha−1)	Root plasticity and dry matter production		Limited to short-term season; evaluated only a few genotypes	[249]
Rice	35%, 70% water holding capacity	ZnO NPs	24 h seed priming (5, 10, 15, 25, and 50 ppm)	Growth parameters	Increased antioxidant enzyme activities and reduced oxidative stress	Field applicability unknown	[203]
Rice	50, 75, and 100% field capacity	Moringa oleifera	Foliar/ (MLE 3% w/v)	Growth, yield, and grain quality	Increased photosynthesis, pigment content, and antioxidant enzyme activities	Limited to pot; needs field and multi-season trials	[255]
Soybean	30–35% field capacity	Melatonin (MT)	Foliar (MT-50 or 100 μM)	Growth	Improved photosynthesis, hormone balance, and antioxidant activity and reduced oxidative damage	Short-term study; no yield data	[259]
Soybean	40 and 80% field capacity	Methyl jasmonate (MJ)	Foliar (20 µM MJ)	Growth	Enhanced growth, photosynthetic pigments, and biochemical constituents	Growth only; yield untested	[263]
Soybean	35–75% field capacity	Methyl jasmonate (MJ)	Foliar/ (0.5 µM MJ)	Grain yield	Increased antioxidants enzymatic, RWC and reduced lipid peroxidation	Pot-based; Limited field applicability	[206]
Soybean	50%, 75% relative water content	Salicylic acid (SA)	Seed priming (0.5 mM SA)	Yield parameters	Improved energy production, carbon and nitrogen remobilization, redox homeostasis, and antioxidative defense mechanisms	Early-stage; field confirmation missing	[266]
Soybean	45, 65, and 85% field capacity	Salicylic acid (SA)	Foliar (SA, 0.4 and 0.8 mM)	Physiological traits and yield components	Enhanced antioxidant enzyme activity and reduced oxidative damage	Pot-level; needs field-scale testing	[271]
Soybean	30, 50, 70 and 100% field capacity	Exo-GB (glycine betaine)	Foliar (0, 2.5, 5 and 7.5 kg ha−1)	Yield components such as branch number, seed number per plant, and 1000-seed weight		Pod-level, single cultivar; lack of field variability	[272]
Soybean	40 and 80% field capacity	Red seaweed extract (Gracilaria tenuistipitata var. liui)	Foliar (0.0%, 5.0%, and 10.0% v/v)	Growth and yield		Mechanistic pathways unstudied	[280]
Soybean	Non-irrigated, half-irrigated, and fully irrigated	Nitrogen (N)	Soil (35, and 105 kg ha−1 N)	Physiological traits and yield		No molecular data	[283]
Soybean	PEG-6000	Nano zinc oxide	Added to Petri dish (0.5, and 1 g L−1)	Germination percentage and rate		Mechanistic pathways unstudied; lacks growth/yield validation	[287]
Soybean	40, 60 and 80% field capacity	Biochar (pyrolysed at ~400 °C for 5 h)	Soil amendment (0, 25, 50 and 100 t ha−1)	Crop growth rate, total biomass production, and seed yield	Increased water uses efficiency, soil available potassium, and K uptake	Mechanistic pathways unstudied	[292]

6. Biostimulants Enhancing Growth, Yield, and Stress Tolerance in Rice and Soybean Under Salt Stress

Salinity is a significant abiotic stressor that greatly restricts the global crop yield [296]. Approximately 20% of arable lands and 33% of the irrigated farmlands worldwide are affected by high salinity. Climate forecasts suggest that ≥50% of the planet’s arable land will become salinized by 2050 [297,298]. According to available data, salt stress affects approximately 20% or 45 million hectares of irrigated arable lands and 1.26 billion hectares of total land worldwide [299,300]. Soil salinization increases annually by 10% owing to several factors including weathering of native rocks, inadequate agricultural practices, low rainfall, and use of saline water for irrigation [298]. Consequently, enhancing crop plant resilience to various abiotic stresses is a significant challenge to ensuring a sufficient food supply for the continuously growing global population [301]. In fact, the production of food crops such as rice, wheat, soybean, and corn will need to increase by 87% by 2050 to satisfy the demands of the rising global population [302]. Crops such as rice are particularly susceptible and show potential yield losses of ≤70% under severe salt stress [303]. Salinity is a critical abiotic stress factor that drastically hampers the growth and productivity of cereal crops by disrupting cellular functions and diminishing overall plant health and development [304]. Additionally, salinity is the most harmful among the various abiotic stressors to plant growth, physiology, and metabolism [305]. Salinity affects essential physiological processes such as ion balance, mineral nutrition, water status, stomatal behavior, and photosynthetic efficiency. Moreover, it causes oxidative damage by generating excessive ROS and altering antioxidant enzymes [306,307,308]. Furthermore, salt stress disturbs osmotic balance and ionic homeostasis, which leads to secondary stress responses that result in the accumulation of ROS like hydrogen peroxide (H₂O₂) and superoxide anions (O₂⁻) [309]. These reactive molecules are byproducts of vital plant processes such as photorespiration, photosynthesis, and respiration, and they harm crucial cellular components including membranes, lipids, proteins, DNA, and photosynthetic pigments [310]. To mitigate this damage, plants use protective mechanisms such as ROS scavenging, membrane stabilization, and maintenance of the ultrastructural integrity of organelles [311]. Nevertheless, the harmful effects of salinity are evident in many crops such as spinach [312], beans [313], and other crops [314] by the significant decrease in fresh weight and chlorophyll content.

Thus, enhancing plant oxidative stress response and maintaining ultrastructural integrity are critical to mitigating these effects. A promising strategy involves the use of biostimulants. These substances include hormonal, microbial, organic, inorganic, and nanoparticle-based formulations, which sustainably enhance salt tolerance [315]. Biostimulants trigger plant defense mechanisms and enhance physiological processes such as nutrient absorption, chlorophyll production, root growth, and water uptake [45]. Additionally, they facilitate the accumulation of compatible solutes such as proline, simple sugars, alcohols, and ABA, which help counteract the osmotic effects of salinity and neutralize ROS [45]. These findings are summarized in (Table 4 and Figure 5), and they clearly show that applying biostimulants such as phytohormones, beneficial microbes, mineral nutrients, and plant extracts significantly enhance salt tolerance by improving plant growth, yield, and key physiological, biochemical, and molecular responses.

6.1. Biostimulants and Rice Tolerance to Salt Stress

Melatonin (200 μM) application in rice through foliar spraying enhanced growth under salt stress by increasing antioxidant enzyme activity, improving nutrient uptake and ion balance, and minimizing salt-induced cellular damage [316]. Yan et al. [317] found that melatonin enhanced the antioxidant capacity of rice under salt stress by reducing ROS accumulation, enhancing membrane stability, and adjusting antioxidant enzyme activity and levels, although the antioxidant responses varied over time. Liu et al. [318] have shown that melatonin reduced K⁺ loss and ROS-induced damage by modulating signaling pathways and transporter gene expression, which aided in improved K⁺ retention during salt stress. Furthermore, melatonin enhanced Na⁺/K⁺ balance in both rice and maize and upregulated stress-related genes such as MdNHX1/MdAKT1 in apple and SOS2/NHX1 in rapeseed [319,320]. SA is another crucial biostimulant that mitigates salt stress in rice. Its application decreased Na⁺ and Cl⁻ accumulation, improved endogenous SA levels, and activated antioxidant enzymes, which enhanced plant growth and yield under salinity stress [321]. Khan et al. [322] observed that foliar SA treatment improved photosynthesis, protein content, and expression of stress-related genes including those involved in MAPK-1 signaling and autophagy. These effects helped reduce ROS levels and protected the cells. Additionally, SA increased chlorophyll a, chlorophyll b, and carotenoid content under both normal and stress conditions [323]. Pai et al. [324] have shown that foliar application of 0.5 mM SA improved water status, proline accumulation, photosynthetic efficiency, antioxidant defense, and ion balance in salt-stressed rice, whereas higher concentrations (2 mM) were inhibitory. Furthermore, SA caused variations in the physiological responses of salt-sensitive and salt-resistant cultivars of rice (C3 plants) [321] and maize (C4 plants) [325]. FA has shown promise as an agent that enhances plant resilience to salt stress. At 0.25 mL L⁻¹, FA improved rice seedling growth, increased phenolic compound levels (including SA and momilactones), and strengthened salt stress resistance [326]. Additionally, FA promoted the production of primary and secondary metabolites, thereby contributing to abiotic stress resistance in various crops [327,328]. Moreover, its beneficial effects under salt stress have been reported in soybean [132], almond rootstock [329], yarrow [330], and spinach [331].

GB potently mitigates stress. External GB application enhanced growth, survival, and stress tolerance in a broad range of accumulating and non-accumulating plants [332,333]. Demiral and Turkan [334] have shown that external GB application improved salt tolerance in rice seedlings by increasing RWC, altering antioxidant enzyme activities in both tolerant and sensitive varieties, and minimizing lipid peroxidation to protect against oxidative stress. Paclobutrazol (PBZ) exerted a positive effect on rice. Spraying PBZ on the leaves enhanced the growth, pigment levels, and antioxidant activity of rice seedlings under salt stress [335]. It mitigated salinity-induced damage by enhancing the antioxidant defense system through both enzymatic and non-enzymatic pathways [336,337,338]. Silicon (Si) has become popular as a cost-effective and environmentally friendly method for improving salt tolerance [339] and is considered a most beneficial element for plant activities with no harmful effects even at high concentrations [340]. It enhanced salt tolerance in various crops such as soybean [341], corn [342], and rice [343]. In rice, silicon improved salt tolerance by increasing polyamine production, decreasing polyamine breakdown, and adjusting gamma-aminobutyric acid (GABA) metabolism; in fact, the salt-sensitive cultivar MTU 1010 was most responsive to Si treatment [344]. Foliar application of nanosilicon alleviated salt stress in rice seedlings by promoting root growth, photosynthesis, antioxidant defense, beneficial ion uptake, and hormone balance and decreasing the accumulation of toxic ions [309]. Additionally, Si activated stress signaling pathways involving SA and JA, which enhanced systemic tolerance [345]. Zinc is often absent from salt-affected soils because of competition from alkaline earth metal cations [346,347]. Therefore, external application of Zn improves salt tolerance in rice by improving growth, photosynthesis, antioxidant activity, and ion balance. The KSK-133 genotype showed the best performance to this treatment under salinity stress [348].

Biostimulants derived from the seaweed Ascophyllum nodosum (AN) have shown significant promise in enhancing salt tolerance. AN application enhanced rice biomass, maintained ion equilibrium, increased pigment levels, improved photosynthesis, and strengthened antioxidant defenses [349]. Jithesh et al. [350] have indicated that AN biostimulants significantly influenced the expression of genes responsible for flavonoid biosynthesis, which protects plants from ROS-induced oxidative damage under salinity stress. Additionally, AN treatment altered plant metabolism, leading to increased production of organic acids, sugars, and amino acids, which acted as osmolytes and stress protectants [351,352,353,354]. Generally, various biostimulants such as melatonin, SA, FA, GB, silicon, zinc, and seaweed extracts enhance rice tolerance to salt stress by regulating stress-responsive genes and hormones, activating antioxidant defenses, maintaining ROS and ion balance and promoting osmolyte accumulation. These mechanisms enhance photosynthesis, nutrient uptake, growth, and biomass while minimizing oxidative damage. Overall, such treatments provide effective and environmentally sustainable strategies for managing salt-affected conditions.

In general, a variety of biostimulants, including melatonin, SA, FA, GB, silicon, zinc, and seaweed extracts, bolster rice’s resilience to salt stress by modulating stress-responsive genes and hormones, triggering antioxidant defenses, maintaining the balance of ROS and ions, and encouraging the accumulation of osmolytes. These processes enhance photosynthesis, nutrient absorption, growth, and biomass. Such treatments reduce oxidative damage, facilitate nutrient uptake, and support overall growth in saline environments, thus providing effective and environmentally friendly strategies for managing salt-affected conditions.

6.2. Biostimulants and Soybean Tolerance to Salt Stress

Foliar application of 0.4 or 0.8 mM SA in soybean enhanced its growth, yield, and biochemical characteristics under both normal and saline conditions. These treatments increased phenol, proline, oil, and protein levels and proportion of unsaturated fatty acids in the seed oil [355]. Additionally, external application of SA at 100 and 200 ppm improved tolerance to salinity by elevating chlorophyll, sugar, starch, and proline levels and improving the activity of antioxidant enzymes such as SOD and CAT. Specifically, 100 ppm was effective in enhancing metabolic parameters, whereas 200 ppm was more beneficial for stress-related responses [356]. According to Noreen et al. [357], the SA-induced increase in growth may be linked to a significant increase in the net photosynthetic rate under salt stress, especially at 200 mg L⁻¹ SA. Furthermore, SA application reduced salinity damage by enhancing antioxidant defense and decreasing ROS production [358,359]. The combined use of SA and sodium nitroprusside (an NO donor) further improved stress tolerance by lowering Na⁺ uptake, increasing K⁺ and Ca²⁺ levels, and enhancing antioxidant activity [360]. Similarly, the application of SA and selenium (Se) effectively alleviated salinity stress in soybean by enhancing antioxidant enzyme activities, improving nutrient uptake, and increasing protective metabolites such as proline, phenolics, and non-protein thiols [361]. Additionally, the positive effects of Se supplementation on the activation of various stress resistance mechanisms have been observed in response to salinity [362], ultraviolet (UV) radiation [363], and heavy metal stress [364]. Foliar SA application significantly increased seed protein content, yield, and amino acid composition in soybean under different salinity levels, whereas JA reduced protein yield but increased the concentration of specific amino acids such as methionine and phenylalanine [365]. JA levels typically increase under stressful conditions such as salinity [366], and its application enhanced tolerance to both biotic and abiotic stressors [367]. Foliar application of JA in salt-stressed soybean reduced salt-induced damage and improved photosynthesis and yield [368]. Additionally, SA seed priming significantly increased the proline content in Cicer arietinum under salt stress [369]. Moreover, SA functions as a key signaling molecule in plants and plays a vital role in mediating plant responses to a wide array of biotic and abiotic stressors [370].

Biostimulants containing seaweed extracts improved soybean tallness even under osmotic stress and ion toxicity [371]. Seaweed extract (Ascophyllum nodosum) and FA have shown promising effects in mitigating salt stress. Their application during the V3R1 growth stage reduced electrolyte leakage, improved chlorophyll content, and promoted plant growth under saline irrigation [372]. Ascophyllum nodosum extract is extensively used in commercial applications because of its impact on growth, plant hormone biosynthesis, and its role in the antioxidative defense mechanisms of soybean under water scarcity [373,374]. Yıldıztekin et al. [375] observed an increase in the biomass and efficiency of the antioxidant defense system in pepper (Capsicum annuum L.); however, the effectiveness of biostimulant application in soybean under salt stress remains poorly understood. Foliar application of FA improved soybean resilience to salt and heat stress by enhancing water status, increasing antioxidant activity, and modulating the expression of stress-responsive genes such as Rubisco, cytochrome c oxidase, and Hsp70 [132]. Microbial inoculants such as co-inoculation with Rhizobium sp. SL42 and Hydrogenophaga sp. SL48 enhanced soybean growth, grain yield, and salinity tolerance [376]. Pseudomonas putida TSAU1 improved plant growth, root structure, and nitrogen and phosphorus content of soybean under salt stress in a hydroponic setting [377]. Foliar application of 2.5% potassium sulfate and potassium chloride during the early growth stage exerted a limited effect on reducing salinity stress in soybean, with potassium sulfate eliciting a slightly better effect on antioxidants and pigments than that of potassium chloride [378]. Although potassium (K) is a vital macronutrient essential for crop productivity [379], its foliar application in the early growth stages showed limited success in alleviating salinity stress in soybean and maize [380]. Nevertheless, foliar application of micro- and macronutrients is a commonly used strategy when root nutrient uptake is hindered by salt stress [381]. In contrast, the application of exogenous ethanol significantly enhanced soybean growth under salt stress conditions by improving photosynthesis, antioxidant activity, osmotic adjustment, and nutrient absorption and simultaneously decreasing oxidative damage and Na⁺ accumulation [382]. The enhancement of photosynthesis observed in ethanol-treated soybean plants under salt stress conditions may be attributed to an increased leaf area per trifoliate, which optimizes light capture and consequently enhances photosynthetic activity [383,384,385].

Furthermore, elevated K⁺ levels in the leaves of ethanol-treated plants may facilitate cell enlargement, optimize metabolic processes, and preserve the structural integrity of proteins under salt stress [386,387,388]. Melatonin (MT; N-acetyl-5-methoxytryptamine) functions as a bioregulator that enhances various physiological aspects of plants [389] including vegetative growth and development, photosynthesis, and osmoregulation through ion exchange and adjustment of osmotic and water potential; additionally, it supports the regulation of diverse metabolic pathways involving carbohydrates, lipids, and other compounds [390]. Exogenous application of melatonin (100 μM) mitigated NaCl (60 mM) stress in germinating soybeans by promoting growth, minimizing oxidative damage, enhancing antioxidant enzyme activities, and stimulating isoflavone biosynthesis by upregulating key biosynthetic genes and enzymes [391]. MT-treated plants exhibited stronger salt tolerance, experienced lower ROS and cell damage, and achieved greater plant height, biomass, and organic matter content compared with those of untreated plants [392]. Additionally, the application of 1 mM exogenous melatonin alleviated the effects of salt stress on chlorophyll b and RWC in soybean under high salinity (5.00 dS m⁻¹) [393]. In summary, SA, JA, seaweed extracts, FA, microbial inoculants, potassium fertilizers, ethanol, and melatonin enhance soybean tolerance to salt by improving antioxidant defenses, modulating stress-related genes and hormones, maintaining ion and water balance, and promoting growth and yield in saline conditions. SA and JA enhance antioxidant activity and stress-related metabolites, and SA consistently improves yield and metabolic balance. Biostimulants such as Ascophyllum nodosum extract and FA enhance water status, photosynthesis, and gene expression. Microbial co-inoculation improves nutrient uptake and root architecture, whereas potassium fertilizers show variable effectiveness. Ethanol and melatonin yield promising results by enhancing the antioxidant defense, osmotic balance, and gene regulation. These findings highlight the diverse strategies available for improving salt stress tolerance in soybean.

Table 4. Effects of biostimulant on salt stress tolerance in rice and soybean: application method, growth, yield, and mechanistic insights.

Crop	Stress Level	Bio Stimulants	Application Methods/ Concentrations	Growth /Yield Effect	Mechanisms	Research Gaps	References
Rice	50–100 mM NaCl	Melatonin (MT)	Foliar (MT-25, 50, 100, 200, 300, and 400 μM)	Growth	Enhanced antioxidant enzyme activity, nutrient accumulation, and ion homeostasis and reduced cellular damage	Pot trials only; yield not tested	[316]
Rice	150 mM NaCl	Melatonin (MT)	Foliar (MT-200 μM)	Dry weight	Reduced ROS accumulation, improving membrane stability, and modulating antioxidant enzyme activity	Pot-based study; no yield traits	[317]
Rice	100 mM NaCl	Melatonin (MT)	Root treatment/ (MT-10, 20, 50, or 100 μM)		Reduced K+ efflux and ROS-induced damage and enhanced K+ retention	Controlled hydroponic study; needs field validation	[318]
Rice	(0, 100, 200, 300 and 400 mM NaCl)	Salicylic acid (SA)	Seed treatment (SA-1.0 mmol L−1)	Germination, and growth	Increased antioxidant enzyme, Na+ and Cl− accumulation	Short-term study; needs field validation	[321]
Rice	NaCl 100 mM	Salicylic acid (SA)	Foliar (SA-0.5 mM)		Increased photosynthetic, protein content and gene expression related to antioxidant defense and reduced ROS accumulation and cellular damage	Short-term study; field-level yield and stress tolerance not assessed	[322]
Rice	0, 40, 120 mM NaCl	Salicylic acid (SA)	Foliar (SA-0.5, 1 and 2 mM)	Growth	Improved RWC, ion balance, and antioxidants and reduced ROS and membrane damage	Needs field validation	[324]
Rice	10 dS/m	Fulvic acid	Seed treatment (0.125, 0.25, 0.5, and 1.0 mL L−1)	Growth parameters	Increased phenolic compounds	Mechanisms limited; yield not reported	[326]
Rice	120 mmol/L NaCl	Glycine betaine (GB) and Iron (Fe)	Applied nutrient solution/Foliar (15 mmol L−1 GB and 10% Fe stock solution)		Enhanced RWC and antioxidative enzyme activities and reduced lipid peroxidation	Short-term hydroponic study; needs field validation	[334]
Rice	150 mM NaCl	Paclobutrazol	Foliar (15 mg L−1)	Growth	Improved pigment content and antioxidant activity	Yield traits untested	[335]
Rice	0, 25, 50, and 100 mM NaCl	Silicon	Applied nutrient solution (Si-2 mM)		Increased polyamine (PA) levels, reduced polyamine degradation, and modulated GABA metabolism	Hydroponics only; field application missing	[344]
Rice	60.00 mmol·L−1 NaCl	Nano-silicon	Foliar (2.00 mmol L−1)	Root growth	Enhanced photosynthesis, antioxidant defense, beneficial ion uptake, and hormone balance	Field application missing	[309]
Rice	70 mM NaCl	Zinc	Hydroponic root zone application (An-15 mg kg−1)	Growth	Enhanced photosynthesis, antioxidant activity, and ion balance	Hydroponics only; no yield data	[348]
Rice	200 mM NaCl	Seaweed Ascophyllum nodosum	Foliar (2 mL L−1, 0.2% solution)	Shoot and root biomass	Increased pigment content, photosynthesis, and antioxidant defense	Lon-term effects unknown	[349]
Soybean	NaCl (3 or 6 dS/m)	Salicylic acid (SA)	Foliar (SA-0.4 or 0.8 mM)	Growth, yield, and biochemical traits	Improved phenol, proline, oil, and protein content and increased unsaturated fatty acid proportion in seed oil	Field trials missing	[355]
Soybean	50–100 mM NaCl	Salicylic acid	Foliar (SA-100 and 200 ppm)		Increased chlorophyll, sugars, starch, proline, and antioxidative enzymes	Physiological only; yield not measured	[356]
Soybean	NaCl (0 and 100 mM	Salicylic acid (SA) and Sodium nitroprusside (NO donor)	Pretreatment root uptake (SA, NO-100 μM)		Reduced Na+ uptake, improved K+ and Ca2+ levels, enhanced antioxidant enzyme activities (PPO, PAL)	Lon-term effects on growth and yield are unknown	[360]
Soybean	NaCl (0 and 100 mM)	Selenium (Se) and salicylic acid (SA)	Foliar (Se-0, 25 and 50 mg L−1, SA-0.5 mM)		Enhanced antioxidant enzyme activities, improved nutrient uptake, and increased protective metabolites	Limited to early growth	[362]
Soybean	0, 4, 7, and 10 dS/m NaCl	Salicylic acid (SA) and Jasmonic acid (JA)	Foliar (SA-1 mM and JA-0.5 mM)		Improved seed protein yield and essential amino acid content and reduced protein yield but increased levels of specific amino acids	Lon-term effects on growth, yield and phytochemical content are unknown	[365]
Soybean	5.0 dS/m	Seaweed extract and fulvic acids	Soil (2.5 g pot−1)	Growth parameters	Reduced electrolyte leakage and enhanced chlorophyll content	Pot study; no yield data	[372]
Soybean	150 mM NaCl/35 °C for 2 h for 2 days	Fulvic acid	Foliar (2.0 mg L−1)		Improved water status, antioxidant activity, and regulated stress-responsive gene expression	Field-level validation is needed	[132]
Soybean	100, 250, and 500 mM NaCl	Bradyrhizobium japonicum, Rhizobium sp. And Hydrogenophaga sp., Amphicarpaea bracteat	Seeds soaking (100 µL bacterial suspension)	Growth and yield		Limited understanding of the full microbial diversity	[376]
Soybean	6–12 dS m−1	Potassium chloride and potassium sulfate	Foliar (2.5% solution of potassium sulfate or potassium chloride)		Potassium sulfate improved antioxidant activity and pigment levels more than that of potassium chloride	Limited to greenhouse conditions; no yield data	[378]
Soybean	8–16 dS m−1	Ethanol	Foliar (20 mM)	Growth	Enhanced photosynthesis, antioxidant activity, osmotic adjustment, and nutrient uptake and reduced oxidative damage and Na+ accumulation	Pod conditions only; lacks field variability	[382]
Soybean	60 mM	Melatonin (MT)	Seed soaking (MT-100 μM)	Growth	Enhanced antioxidant defense, reduced oxidative damage, and boosted isoflavone biosynthesis	Short-term study; no yield data	[391]
Soybean	0.50, 3.00, and 5.00 dS m−1	Melatonin (MT)	(MT-0.5, and 1 mM)		Improved chlorophyll b and water status	Short-term study; agronomic feasibility unclear	[393]

Table 4. Effects of biostimulant on salt stress tolerance in rice and soybean: application method, growth, yield, and mechanistic insights.

Crop	Stress Level	Bio Stimulants	Application Methods/ Concentrations	Growth /Yield Effect	Mechanisms	Research Gaps	References
Rice	50–100 mM NaCl	Melatonin (MT)	Foliar (MT-25, 50, 100, 200, 300, and 400 μM)	Growth	Enhanced antioxidant enzyme activity, nutrient accumulation, and ion homeostasis and reduced cellular damage	Pot trials only; yield not tested	[316]
Rice	150 mM NaCl	Melatonin (MT)	Foliar (MT-200 μM)	Dry weight	Reduced ROS accumulation, improving membrane stability, and modulating antioxidant enzyme activity	Pot-based study; no yield traits	[317]
Rice	100 mM NaCl	Melatonin (MT)	Root treatment/ (MT-10, 20, 50, or 100 μM)		Reduced K+ efflux and ROS-induced damage and enhanced K+ retention	Controlled hydroponic study; needs field validation	[318]
Rice	(0, 100, 200, 300 and 400 mM NaCl)	Salicylic acid (SA)	Seed treatment (SA-1.0 mmol L−1)	Germination, and growth	Increased antioxidant enzyme, Na+ and Cl− accumulation	Short-term study; needs field validation	[321]
Rice	NaCl 100 mM	Salicylic acid (SA)	Foliar (SA-0.5 mM)		Increased photosynthetic, protein content and gene expression related to antioxidant defense and reduced ROS accumulation and cellular damage	Short-term study; field-level yield and stress tolerance not assessed	[322]
Rice	0, 40, 120 mM NaCl	Salicylic acid (SA)	Foliar (SA-0.5, 1 and 2 mM)	Growth	Improved RWC, ion balance, and antioxidants and reduced ROS and membrane damage	Needs field validation	[324]
Rice	10 dS/m	Fulvic acid	Seed treatment (0.125, 0.25, 0.5, and 1.0 mL L−1)	Growth parameters	Increased phenolic compounds	Mechanisms limited; yield not reported	[326]
Rice	120 mmol/L NaCl	Glycine betaine (GB) and Iron (Fe)	Applied nutrient solution/Foliar (15 mmol L−1 GB and 10% Fe stock solution)		Enhanced RWC and antioxidative enzyme activities and reduced lipid peroxidation	Short-term hydroponic study; needs field validation	[334]
Rice	150 mM NaCl	Paclobutrazol	Foliar (15 mg L−1)	Growth	Improved pigment content and antioxidant activity	Yield traits untested	[335]
Rice	0, 25, 50, and 100 mM NaCl	Silicon	Applied nutrient solution (Si-2 mM)		Increased polyamine (PA) levels, reduced polyamine degradation, and modulated GABA metabolism	Hydroponics only; field application missing	[344]
Rice	60.00 mmol·L−1 NaCl	Nano-silicon	Foliar (2.00 mmol L−1)	Root growth	Enhanced photosynthesis, antioxidant defense, beneficial ion uptake, and hormone balance	Field application missing	[309]
Rice	70 mM NaCl	Zinc	Hydroponic root zone application (An-15 mg kg−1)	Growth	Enhanced photosynthesis, antioxidant activity, and ion balance	Hydroponics only; no yield data	[348]
Rice	200 mM NaCl	Seaweed Ascophyllum nodosum	Foliar (2 mL L−1, 0.2% solution)	Shoot and root biomass	Increased pigment content, photosynthesis, and antioxidant defense	Lon-term effects unknown	[349]
Soybean	NaCl (3 or 6 dS/m)	Salicylic acid (SA)	Foliar (SA-0.4 or 0.8 mM)	Growth, yield, and biochemical traits	Improved phenol, proline, oil, and protein content and increased unsaturated fatty acid proportion in seed oil	Field trials missing	[355]
Soybean	50–100 mM NaCl	Salicylic acid	Foliar (SA-100 and 200 ppm)		Increased chlorophyll, sugars, starch, proline, and antioxidative enzymes	Physiological only; yield not measured	[356]
Soybean	NaCl (0 and 100 mM	Salicylic acid (SA) and Sodium nitroprusside (NO donor)	Pretreatment root uptake (SA, NO-100 μM)		Reduced Na+ uptake, improved K+ and Ca2+ levels, enhanced antioxidant enzyme activities (PPO, PAL)	Lon-term effects on growth and yield are unknown	[360]
Soybean	NaCl (0 and 100 mM)	Selenium (Se) and salicylic acid (SA)	Foliar (Se-0, 25 and 50 mg L−1, SA-0.5 mM)		Enhanced antioxidant enzyme activities, improved nutrient uptake, and increased protective metabolites	Limited to early growth	[362]
Soybean	0, 4, 7, and 10 dS/m NaCl	Salicylic acid (SA) and Jasmonic acid (JA)	Foliar (SA-1 mM and JA-0.5 mM)		Improved seed protein yield and essential amino acid content and reduced protein yield but increased levels of specific amino acids	Lon-term effects on growth, yield and phytochemical content are unknown	[365]
Soybean	5.0 dS/m	Seaweed extract and fulvic acids	Soil (2.5 g pot−1)	Growth parameters	Reduced electrolyte leakage and enhanced chlorophyll content	Pot study; no yield data	[372]
Soybean	150 mM NaCl/35 °C for 2 h for 2 days	Fulvic acid	Foliar (2.0 mg L−1)		Improved water status, antioxidant activity, and regulated stress-responsive gene expression	Field-level validation is needed	[132]
Soybean	100, 250, and 500 mM NaCl	Bradyrhizobium japonicum, Rhizobium sp. And Hydrogenophaga sp., Amphicarpaea bracteat	Seeds soaking (100 µL bacterial suspension)	Growth and yield		Limited understanding of the full microbial diversity	[376]
Soybean	6–12 dS m−1	Potassium chloride and potassium sulfate	Foliar (2.5% solution of potassium sulfate or potassium chloride)		Potassium sulfate improved antioxidant activity and pigment levels more than that of potassium chloride	Limited to greenhouse conditions; no yield data	[378]
Soybean	8–16 dS m−1	Ethanol	Foliar (20 mM)	Growth	Enhanced photosynthesis, antioxidant activity, osmotic adjustment, and nutrient uptake and reduced oxidative damage and Na+ accumulation	Pod conditions only; lacks field variability	[382]
Soybean	60 mM	Melatonin (MT)	Seed soaking (MT-100 μM)	Growth	Enhanced antioxidant defense, reduced oxidative damage, and boosted isoflavone biosynthesis	Short-term study; no yield data	[391]
Soybean	0.50, 3.00, and 5.00 dS m−1	Melatonin (MT)	(MT-0.5, and 1 mM)		Improved chlorophyll b and water status	Short-term study; agronomic feasibility unclear	[393]

7. Biostimulants Enhance Growth, Yield, and Stress Tolerance in Rice and Soybean Under Waterlogging Stress

Waterlogging is an abiotic stressor that adversely affects global crop yield [394]. It affects >10% of arable lands annually worldwide. However, its effect on global food production and security remain unclear [395]. According to estimated, approximately 12% of global cultivable lands frequently waterlogged, which results in approximate 20% decrease in crop yield [396]. Additionally, waterlogging decreases rice production by >10% [397,398]. In soybean, flooding stress leads to yield reduction of 17–43% during the vegetative phase and ≤50–56% during the reproductive phase [399]. Furthermore, cold-waterlogged fields often show below average yield because of high groundwater levels, inadequate drainage, low soil temperatures, reduced aeration, limited availability of essential nutrients (particularly phosphorus), and buildup of soil-reducing substances [400,401]. Additionally, the severity of waterlogging stress is determined by several factors such as duration and depth of flooding, crop growth stage, and environmental conditions [402,403,404].

Global warming is expected to intensify extreme weather events, which would increase the risk of flooding in both current and previously unaffected regions [405]. Flood disasters pose a threat to food security in developing areas and are expected to become more frequent and severe under the influence of global climate change, thereby increasing the occurrence and impact of waterlogging events [406,407,408]. Waterlogged soils limit oxygen availability, leading to hypoxic conditions that compel plant roots to switch from aerobic to anaerobic metabolism to maintain energy production [408]. However, this metabolic transition is inefficient and often leads to restricted root growth and eventual death. Under these conditions, essential energy-dependent processes such as water and nutrient uptake and transport to the shoot are disrupted, which hinders plant growth and significantly reduces yield [403,409,410]. Additionally, waterlogging affects the shoot system by decreasing leaf nitrogen content, leaf water potential, stomatal conductance, CO₂ assimilation, and photosynthetic capacity, which are often accompanied by accelerated chlorosis and senescence [410]. Moreover, decreased stomatal conductance and chlorophyll levels because of waterlogging results in restricted photosynthesis and accelerated leaf yellowing and aging [410,411,412,413]. In sensitive plants, the downregulation of the photosynthetic machinery leads to excessive ROS accumulation, which causes oxidative damage and disrupts cellular metabolism [403,414]. Furthermore, ROS-induced stress leads to lipid peroxidation, membrane damage, protein breakdown, enzyme deactivation, harmful nucleic acids, and eventual cell death [415,416,417].

Several strategies have been proposed to mitigate these negative effects, and plant biostimulants have shown considerable promise among them. Biostimulants including natural extracts, hormones, microbial inoculants, and nutrient enhancers increase nutrient absorption, promote vegetative growth, strengthen antioxidant defenses, and improve tolerance to abiotic stressors such as waterlogging [374,418]. Hormones such as ABA, ethylene, and 6-benzylaminopurine (6-BA) are crucial for stress management. For example, ABA plays a role in water stress responses, where ethylene aids in adaptation and repair under flood conditions [419,420]. Table 5 and Figure 6 summarize the studies that collectively show that the external application of biostimulants including phytohormones, beneficial microbes, mineral nutrients, and plant extracts significantly improve waterlogging tolerance in major cereal crops by enhancing their growth, yield, and physiological, biochemical, and molecular responses.

7.1. Biostimulants and Rice Tolerance to Waterlogging Stresses

In rice, waterlogging severely hampers growth and yield by affecting root respiration and nutrient absorption. Abdel Megeed et al. [421] have shown that foliar application of the biostimulant Crop Plus (0.5–1.5 mL L⁻¹) and hormones such as cytokinin and ABA (15–25 ppm) at 12-day irrigation intervals significantly enhanced growth and grain quality. Particularly, biostimulant Crop Plus showed the most effectiveness. The use of growth-promoting substances is an effective strategy for improving rice grain quality and increasing the nutritional value of milled rice. Pan et al. [422] found that foliar application of gibberellic acid, paclobutrazol, and 6-benzylaminopurine increased yield, grain quality, and antioxidant enzyme activity in super hybrid rice. Kamboj and Mathpal [423] found that foliar application of gibberellic acid and cytokinin facilitated the translocation of zinc from vegetative parts to rice grains. Nutrient supplementation is crucial for rice recovery under flooded conditions. The application of silicon (Si), phosphorus (P), and nitrogen (N) enhances photosynthesis and plant recovery by increasing chlorophyll and sugar levels and minimizing elongation and senescence [414]. In addition to nutrient management, carbon-based soil amendments (through the use of biochar) have shown potential for improvement of rice performance under waterlogged conditions. Biochar increases soil carbon reserves, nutrient retention, and overall fertility, which frequently results in yield improvements [424,425,426,427,428,429,430,431,432,433,434,435,436]. Liu et al. [427] have reported that applying rice straw biochar at a depth of 5 cm to cold-waterlogged paddies significantly stabilized soil temperature, improved soil properties, and increased grain yield more effectively than the application of bamboo biochar or rice straw alone. Ventura et al. [428] observed that biochar treatment significantly increased the surface soil temperature compared with that of the control, with no differences noted at a depth of 7.5 cm. Cui et al. [429] have shown that applying biochar at 40 t/ha increased soil nitrogen and organic carbon, enhanced carbon sequestration by ≤214%, and reduced CH₄ emissions, global warming potential (GWP), and greenhouse gas intensity (GHGI) without affecting yield. In contrast, straw amendments generally increased emissions and reduced yields, although Han et al. [430] found that applying straw at 3 t ha⁻¹ most effectively increased rice yield. Similarly, Huo et al. [431] have shown that straw amendments optimized water and fertilizer use, enhanced dry matter accumulation, and contributed to higher economic yields. In general, waterlogging negatively affects rice growth and yield by disrupting root function and nutrient uptake. The application of biostimulants (e.g., Crop Plus), phytohormones, nutrient supplements (such as silicon, nitrogen, and phosphorus), and carbon amendments, like biochar, enhances growth, grain quality, antioxidant activity, and soil fertility. Collectively, these approaches improve physiological and metabolic resilience, thereby supporting recovery and productivity under waterlogged conditions. These findings highlight the vital role of integrated nutrient- and carbon-based biostimulants in enhancing rice physiological and metabolic resilience under waterlogging stress.

7.2. Biostimulants and Soybean Tolerance to Waterlogging Stress

Silver nanoparticles (AgNPs) mitigated the adverse effects of flooding in soybean. Furthermore, 15 nm AgNPs at 2 ppm concentration enhanced growth by decreasing stress-related proteins and fermentation activity, which suggests improved metabolic stability and reduced hypoxic stress [432]. Similarly, 20 nm AgNPs enhanced seed germination in wetland plants [433]. However, higher concentrations (100 ppm) were detrimental [434]. Metabolic reprogramming under flooding is essential for soybean adaptation to low-oxygen environments, and ABA treatment further affects stress-related protein responses [433,435]. Aluminum oxide nanoparticles (Al₂O₃ NPs) helped alleviate flooding stress in soybean. At 50 ppm, they promoted root growth and mitochondrial function by regulating membrane permeability and shifting metabolism from fermentation to aerobic pathways, thereby enhancing plant performance under hypoxic conditions [436]. Similarly, Al₂O₃ NPs induced a metabolic shift in soybean from fermentative pathways to normal cellular processes, which improved growth under flooding conditions [432]. NPs influence various biological activities including ROS generation [437], and ROS scavenging antioxidant enzymes are crucial to mitigate the effect of increasing ROS levels in plants exposed to abiotic stress [438]. Buzea et al. [439] have reported that the deposition and accumulation of metal NPs on cellular surfaces and within organelles cause oxidative stress. In tobacco, exposure of cell suspensions to Al₂O₃ NPs triggered programmed cell death and altered plasma membrane permeability [440]. Ethephon (ETP) application enhanced soybean waterlogging tolerance by improving photosynthesis, increasing endogenous gibberellins and amino acids, promoting adventitious root growth, and boosting glutathione-related antioxidant activity [441]. Gibberellins (GA₃) exerted a synergistic effect on adventitious root development in rice [442]. Waterlogging-tolerant soybean genotypes produced more ethylene and less methionine than those of susceptible genotypes, which indicates a role for ethylene regulation in stress response [443].

Melatonin enhanced flood tolerance in soybean by improving gene expression related to cell division, photosynthesis, and carbohydrate metabolism and modulating proteins involved in protein degradation, RNA function, and cell wall metabolism [444]. Additionally, it reduced oxidative damage, restored root lignification, and promoted lateral and adventitious root regeneration [445]. Furthermore, seed coating with MT significantly enhanced plant growth and increased seed yield; however, it did not affect the weight of 100 seeds [444]. Typically, soybean root development is inhibited under flooding stress, as evidenced by shorter root lengths and fewer lateral roots during the initial and recovery phases [446,447]. However, MT promoted the growth of lateral and adventitious roots in etiolated lupin hypocotyls [448]. Although flooding reduced pigmentation in soybean hypocotyls [446], melatonin treatment increased anthocyanin pigment levels in cabbage sprouts [449], which indicates its potential role in stress-triggered pigmentation responses. Phytohormones (particularly SA) and cytokinin-type hormones such as kinetin (KN) effectively mitigate oxidative stress [450]. SA is a crucial phytohormone and signaling molecule that governs various physiological processes including photosynthesis, proline (Pro) metabolism, and upregulation of antioxidant enzymes in plants under stress conditions [231,451]. Under various abiotic stressors, SA positively influences ROS detoxification at the cellular level by upregulating SOD and H₂O₂ scavenging enzymes such as CAT, POD, and APX [420]. Similarly, KN enhances stress tolerance by promoting cell division and nutrient uptake, delaying aging, and boosting antioxidant and glyoxalase systems [450,452,453]. In soybean, the foliar application of SA and KN significantly enhanced waterlogging tolerance by reducing oxidative stress and improving ROS metabolism [454]. SA improved plant growth metrics such as dry weight, stem length, chlorophyll content, antioxidant activity, and grain yield under drought conditions [455]. In addition to abiotic stress, SA and JA are vital regulators of plant defense against biotic stress. However, in dicotyledonous plants, signaling pathways often show antagonistic interactions [456]. In soybean subjected to flooding stress, the external application of JA and SA influenced biophoton emission and stress-responsive proteins, with monodehydroascorbate reductase identified as a key enzyme in the protection against oxidative stress [457].

Overall, biostimulants such as melatonin, phytohormones (e.g., SA, kinetin, JA, and ethylene), and nanoparticles (e.g., AgNPs and Al₂O₃ NPs) enhance stress tolerance by promoting adventitious and lateral root formation, modulating hormonal signals, and strengthening antioxidant defenses. These effects stabilize metabolic processes, reduce damaging ROS and hypoxia-induced damage, and help restore physiological functions, resulting in improved growth, biomass accumulation, and resilience to abiotic stress.

Table 5. Effects of biostimulant on waterlogging stress tolerance in rice and soybean: application method, growth, yield, and mechanistic insights.

Crop	Stress Level	Bio Stimulants	Application Methods/ Concentrations	Growth /Yields Effect	Mechanisms	Research Gaps	References
Rice	Irrigation every 3, 6, 9, 12 days	Biostimulant Crop plus products, cytokinin (CK), and abscisic acid	Foliar (0.5, 1.0, 1.5 mL L−1, CK and AA 15, 20, 25 ppm)	Growth and grain quality		Long-term effects unknow; no mechanistic molecular validation	[421]
Rice	Flooding	Silica (Si), phosphorus (P), and nitrogen (N)	Foliar (Urea-0.49 g), Soil basal (single superphosphate-1.14 g), muriate of potash-0.31 g), and (calcium silicate-3.35 g)		Improved photosynthesis and recovery by enhancing chlorophyll and sugars and reducing elongation and leaf senescence	Focused on physiology; yield response missing	[414]
Rice	Cold waterlogged	Bamboo biochar (BB), rice straw biochar (RB), and rice straw (RS)	Soil basal (BB, RB, and RS, 4.5 t C ha−1)	Grain yield		Limited environmental scope; long-term field data absent, unclear mechanisms	[427]
Rice	Cold waterlogged	Straw and biochar	Straw amendment 6 t ha−1, biochar amendment 2 and 40 t ha−1		Enhanced soil nitrogen and carbon, improved carbon sequestration, and reduced CH4, GWP, and GHG	Short-term field trial; limited variety and biochar evaluation	[429]
Soybean	Flooding	Silver nanoparticles (AgNPs)	Complete submersion of root zone (AgNPs-0.2, 2, and 20 ppm)	Seedling growth	Reduced fermentation-related protein levels in soybean roots, suggesting a shift toward less toxic metabolism	Short-term study; Limited scope	[432]
Soybean	Flooding	Aluminum Oxide Nanoparticles (Al2O3 NPs)	Hydroponic exposure (50 ppm)	Root growth	Modulated glycolysis, antioxidant pathways, and ribosomal protein levels	Hydroponics only; field translation unclear	[436]
Soybean	Water level maintained at 10–15 cm above the soil surface for 10 days	Ethephon (ETP; donor source of ethylene)	Foliar (50 μM, 100 μM, and 200 μM ETP)	Root growth	Improved photosynthesis, increased endogenous gibberellin and amino acid levels	Short-term study; yield impact not measured	[441]
Soybean	Flooding	Melatonin (MT)	Applied via flooding water (MT,10, 50, or 100 μM)		Regulated protein degradation, RNA function, and cell wall lignification	Short-term study; needs yield-level validation	[445]
Soybean	0, 3, 6, and 9 days waterlogging	Kinetin (KN) and Salicylic acid (SA)	Foliar (KN-0.1 mM and SA-0.5 mM SA)		Reduced oxidative damage and enhanced antioxidant defense and glyoxalase enzyme activities	Pot study only; no field-scale trials	[454]
Soybean	Flooding	Jasmonic acid (JA), salicylic acid (SA)	Applied via flooding water (JA-50, 100, 200, and 300 μM), (SA-50, 100, 200 μM)	Growth	Enhanced oxidative stress, with MDHAR crucial for detoxification	Controlled environmental only; yield outcomes absent	[457]

Table 5. Effects of biostimulant on waterlogging stress tolerance in rice and soybean: application method, growth, yield, and mechanistic insights.

Crop	Stress Level	Bio Stimulants	Application Methods/ Concentrations	Growth /Yields Effect	Mechanisms	Research Gaps	References
Rice	Irrigation every 3, 6, 9, 12 days	Biostimulant Crop plus products, cytokinin (CK), and abscisic acid	Foliar (0.5, 1.0, 1.5 mL L−1, CK and AA 15, 20, 25 ppm)	Growth and grain quality		Long-term effects unknow; no mechanistic molecular validation	[421]
Rice	Flooding	Silica (Si), phosphorus (P), and nitrogen (N)	Foliar (Urea-0.49 g), Soil basal (single superphosphate-1.14 g), muriate of potash-0.31 g), and (calcium silicate-3.35 g)		Improved photosynthesis and recovery by enhancing chlorophyll and sugars and reducing elongation and leaf senescence	Focused on physiology; yield response missing	[414]
Rice	Cold waterlogged	Bamboo biochar (BB), rice straw biochar (RB), and rice straw (RS)	Soil basal (BB, RB, and RS, 4.5 t C ha−1)	Grain yield		Limited environmental scope; long-term field data absent, unclear mechanisms	[427]
Rice	Cold waterlogged	Straw and biochar	Straw amendment 6 t ha−1, biochar amendment 2 and 40 t ha−1		Enhanced soil nitrogen and carbon, improved carbon sequestration, and reduced CH4, GWP, and GHG	Short-term field trial; limited variety and biochar evaluation	[429]
Soybean	Flooding	Silver nanoparticles (AgNPs)	Complete submersion of root zone (AgNPs-0.2, 2, and 20 ppm)	Seedling growth	Reduced fermentation-related protein levels in soybean roots, suggesting a shift toward less toxic metabolism	Short-term study; Limited scope	[432]
Soybean	Flooding	Aluminum Oxide Nanoparticles (Al2O3 NPs)	Hydroponic exposure (50 ppm)	Root growth	Modulated glycolysis, antioxidant pathways, and ribosomal protein levels	Hydroponics only; field translation unclear	[436]
Soybean	Water level maintained at 10–15 cm above the soil surface for 10 days	Ethephon (ETP; donor source of ethylene)	Foliar (50 μM, 100 μM, and 200 μM ETP)	Root growth	Improved photosynthesis, increased endogenous gibberellin and amino acid levels	Short-term study; yield impact not measured	[441]
Soybean	Flooding	Melatonin (MT)	Applied via flooding water (MT,10, 50, or 100 μM)		Regulated protein degradation, RNA function, and cell wall lignification	Short-term study; needs yield-level validation	[445]
Soybean	0, 3, 6, and 9 days waterlogging	Kinetin (KN) and Salicylic acid (SA)	Foliar (KN-0.1 mM and SA-0.5 mM SA)		Reduced oxidative damage and enhanced antioxidant defense and glyoxalase enzyme activities	Pot study only; no field-scale trials	[454]
Soybean	Flooding	Jasmonic acid (JA), salicylic acid (SA)	Applied via flooding water (JA-50, 100, 200, and 300 μM), (SA-50, 100, 200 μM)	Growth	Enhanced oxidative stress, with MDHAR crucial for detoxification	Controlled environmental only; yield outcomes absent	[457]

8. Conclusions and Future Perspectives

Biostimulants have become valuable tools for enhancing tolerance in rice, soybean, and other crops to abiotic stressors such as drought, salinity, extreme temperatures, and waterlogging through the improvement of plant growth, yield, and physiological resilience. Both microbial and non-microbial biostimulants contribute to increased antioxidant activity, hormone regulation, improved nutrient uptake, and activation of stress-responsive gene expression. These biostimulants operate through various underlying mechanisms such as the strengthening of antioxidant defense systems, regulation of osmolyte accumulation, modulation of hormonal pathways, and enhancement of nutrient absorption. Despite their proven effectiveness in controlled environments, the practical application of biostimulants in agriculture faces several challenges. Field results are often inconsistent due to the simultaneous occurrence of multiple abiotic stressors, which are rarely replicated in short-term greenhouse experiments. Responses can vary among cultivars, emphasizing the importance of genotype-specific evaluations. Additionally, the molecular mechanisms of biostimulants, particularly those derived from complex organic extracts, remain poorly understood, limiting the ability to predict their effectiveness under diverse field conditions. Environmental variability, soil diversity, and the interaction of multiple stresses further contribute to the disparity between laboratory and field outcomes. For large-scale crops such as rice and soybean, uniform application over extensive areas presents additional challenges, including timing, coverage, and equipment requirements, which may reduce field effectiveness compared to smaller vegetable plots. Future studies using advanced molecular tools, including transcriptomics, proteomics, and metabolomics, could help elucidate biostimulant mechanisms, improving predictions of their performance across genotypes, developmental stages, and environmental conditions. Another limitation is the absence of standardized regulatory frameworks and uniform product classifications across countries, creating uncertainty in product quality, safety, and efficacy. Addressing these constraints requires standardized, multi-site field trials that replicate real-world stress conditions, integrate multiple abiotic stresses, and account for genotype-specific responses. Clear guidelines for dosage, timing, and formulation are essential for generating reproducible and comparable results. Promoting adoption among farmers will require addressing practical challenges such as application logistics, cost, and availability, alongside extension programs, field demonstrations, and transparent labeling. As the global biostimulant market continues to expand, developing clear definitions, robust quality control measures, and transparent labeling is crucial for gaining the trust and support of farmers for the widespread adoption of biostimulants. Current agricultural practices increasingly warrant sustainable strategies to reduce chemical inputs and adapt to climate variability, and biostimulants offer promising eco-friendly alternatives by enhancing crop resilience and resource-use efficiency and supporting circular economy goals by utilizing natural or waste-derived materials. In summary, biostimulants show significant potential to improve the sustainability of rice and soybean production under abiotic stress conditions. Bridging the gap between experimental success and practical field implementation is essential to harness this potential to its maximum extent and will require interdisciplinary collaboration among scientists, agronomists, industry stakeholders, and policymakers. Thus, through coordinated efforts, biostimulants may become the cornerstone of climate-resilient and productive agricultural systems.