Enhancing Horticultural Sustainability in the Face of Climate Change: Harnessing Biostimulants for Environmental Stress Alleviation in Crops

Abstract

Climate change significantly impacts agriculture by increasing the frequency and intensity of environmental stresses, which can severely reduce agricultural yields. Adopting sustainable practices is crucial to mitigating these risks and enhancing crop resilience. Applying natural compounds and microorganisms as biostimulants has gained popularity as an eco-friendly approach to alleviating abiotic stress in agricultural plants. This study reviews the current research on applying biostimulants in horticulturally significant crops to boost their resistance to abiotic stressors such as salinity, drought, and high temperatures. It explores the mechanisms through which these stimulants offer protection, focusing on the roles of key bioactive substances in regulating physiological and molecular processes for stress adaptation. The study addresses biostimulant formulation, regulation, and application challenges. Future research directions are suggested to harness biostimulants’ potential fully, aiming to develop climate-resilient horticultural systems that follow sustainability principles. This comprehensive review underscores the use of biostimulants as a sustainable strategy to increase crop yields in the face of climate change, reducing reliance on synthetic agrochemicals.

1. Introduction

Climate change has become an essential element impacting crop yield and production, posing significant risks to the long-term stability of the agricultural system and the assurance of global food supply. Its effects on the agricultural sector are profound, bringing challenges such as extreme weather events, altered rainfall patterns, and rising temperatures, all threatening food availability and stability [1]. Droughts, heat waves, floods, temperature fluctuations, and soil salinity are major contributors to reduced crop productivity. Plants face numerous stressors, including biotic factors like bacteria, fungi, nematodes, and weeds, along with non-living challenges like extreme temperatures, drought, soil salinity, and waterlogging [2]. These stress conditions disrupt essential physiological and biochemical processes, diminishing crop performance and yield potential [3]. Countries such as India, where most agricultural land relies on rainfed systems, are vulnerable to these climate-induced challenges [4,5]. According to estimations, at the peak of the twenty-first century, worldwide temperatures rise by 1.5–2 degrees Celsius [6], further exacerbating these issues. Horticultural crops requiring specific environmental conditions are susceptible to these changes. For instance, rising temperatures can adversely affect plant development and stages of blooming and fruiting, eventually diminishing yields and impacting food security on a global scale [7]. A United Nations report projects that 9.7 billion people are predicted to inhabit the planet by 2050, significantly increasing the demand for food [8]. To meet this growing need, food production must rise by 60–70%, minimizing yield losses caused by environmental stresses [9]. One promising approach to address fruit quality and blossom count per plant and the challenges of food availability for both now and future generations is adopting integrated and sustainable crop production systems, especially regarding climate change. Researchers focus on developing adaptation and mitigation strategies to mitigate environmental degradation effects on horticultural crops. These efforts involve utilizing advanced technological interventions to restore ecological balance and ensure sustainable agricultural practices.

Biostimulants (BSts) are emerging as a promising approach to enhance crop performance in modern agricultural practices. Biostimulants are substances or combinations of naturally occurring organic compounds that promote plant growth, particularly under challenging environmental conditions [10]. As per the Fertilizer Amendment Order of 2021 (Inorganic, Organic, or Mixed), biostimulants are defined as materials or microbes or their combinations designed primarily to improve nutrient absorption, enhance growth, increase yield and quality, and help plants withstand stress. Importantly, these do not include plant growth enhancers or pesticides controlled by the 1968 Insecticide Act. Biostimulants come in various forms, including botanical extracts (such as seaweed extract, protein hydrolysates, vitamins, amino acids, anti-transpirants, non-microbiological products, humic acid, fulvic acid, and their products. Unlike fertilizers or manures, they are applied in small quantities, which sets them apart from these inputs [11]. Reducing the use of artificial fertilizers, pesticides, and heavy metals is essential for keeping farms healthy and sustainable. Excessive use of these chemicals can harm the environment, soil, and beneficial organisms. By enhancing the use of biostimulants and other natural alternatives, we can maintain the stability of agroecosystems while ensuring healthy plant growth and high productivity.

Decreasing dependence on artificial fertilizers, pesticides, heavy metals, and biostimulants is essential to preserving agroecosystems’ natural equilibrium. Additionally, through various processes, a range of bio-based compounds have demonstrated potential for increasing the growth of plants. These substances included gamma-aminobutyric acid (GABA) [12], silicon [13,14], silicon oxide [15,16], silica [17], selenium [18], and melatonin [19,20]. These substances contribute to different plant production and health types, offering significant potential in sustainable agricultural practices. Biostimulants help plants cope with stress and increase resistance to biotic and abiotic stressors, promoting sustainable agricultural production across diverse agroecological regions worldwide [21]. Research has shown that they can increase yields by 17–19% in crops such as legumes, cereals, fruits, and vegetables [22]. Beyond boosting production, biostimulants have shown the potential to reduce greenhouse gas emissions by minimizing the need for synthetic fertilizers. For instance, ref. [23] concluded that incorporating seaweed extract from Kappaphycus alvarezii into sugarcane farming significantly reduced emissions while decreasing reliance on chemical fertilizers. This highlights the importance of adopting eco-friendly agricultural practices that reduce the harmful effects of agrochemicals on the environment [24]. Plant-based biostimulants, in particular, offer a sustainable solution, as they can enhance crop yields while supporting environmentally responsible farming methods [25].

2. Methodology

This study focused on reviewing the existing literature, specifically, those studies addressing biostimulant applications and their role in enhancing horticultural sustainability amidst climate change. The review also explored utilizing biostimulants to reduce crop environmental stress. Keywords such as “Horticultural Sustainability”, “Climate Change Impacts”, “Biostimulants”, “Environmental Stress Alleviation”, “Crop Resilience”, and “Sustainable Agriculture” were employed to gather relevant studies. The study has used 160 different papers from various journals. Initially, the titles of these studies were assessed for relevance, followed by collecting detailed references, including author names, publication year, titles, and abstracts, for further evaluation. Multiple reliable databases, such as Google Scholar, Web of Science, and EBSCO host, were utilized for thorough coverage due to their widespread recognition and frequent use in academic research. Given the advancements in archival and retrieval technologies, the review methodology was tailored to stay current with these changes. Finally, abstracts from 175 studies were systematically reviewed to evaluate their suitability for the particular area of study.

3. Concepts and Types of Biostimulants

Biostimulants are organic substances applied to plants to improve the quality of crops, improve resistance to abiotic stresses, and boost nutrient efficiency, regardless of the plants’ nutrient content. They can be used in many ways, such as directly as a soil treatment, irrigation, or foliar application, improving crop development and quality [26]. Two primary categories may be used to broadly classify biostimulants according to their origin: microbial and non-microbial. Trichoderma species, rhizobacteria that promote plant development, and arbuscular mycorrhizae fungus are examples of microbial biostimulants. Conversely, materials like silicon, humic and fulvic acids, protein hydrolysates, phosphites, chitosan, and seaweed extract are examples of non-microbial biostimulants [27]. These biostimulant formulations are used directly in soil applications or as seed priming agents to promote synergistic plant growth, independently or in combination with micronutrients (Figure 1 and Figure 2).

3.1. Non-Microbial Biostimulants

3.1.1. Humic Substances (HS)

The aerobic microbial breakdown of organic compounds produces humic substances (HSs). These substances are typically divided into three categories depending on their soluble capacity at various pH levels: humin, fulvic acid (FA), and humic acid (HA). HSs are naturally found in organic soil material created by the breakdown of plant, animal, and microbial residues, along with the metabolic activities of soil microorganisms [28]. Studies have indicated that applying humic chemicals to plants can encourage the growth of their roots and overall plant development. HSs also help increase soil organic carbon, which can influence various processes, from gene expression related to plant adaptation to critical physio-biochemical events. Because of these benefits, HSs might make substantial contributions to developing sustainable agriculture methods [29].

3.1.2. Seaweed Extract (SWE)

Seaweed species, which include various types of multicellular macroalgae that are red, brown, and green, are a significant source of nutrients for fertilizers and organic matter. Seaweed extracts are frequently utilized as plant promoters or soil-conditioning agents in the agricultural sector. In a spray-on-the-leaves application, these extracts can boost plant growth, enhance their ability to cope with abiotic stresses, improve the activity of photosynthesis processes, and increase resistance to bacterial, viral, and fungal infections. As a result, they can significantly increase the quantity produced and productivity of various crops. Commercially available seaweed extracts (SWEs) comprise 30–40% polysaccharides. Red seaweed, including Kappaphycus alvarezii, Garciliaria edulis, and Acanthophora spicifera, as well as brown seaweed like Ascophyllum nodosum, Fucus spp., and Macrocystis pyrifera, are frequently applied in agricultural practices [30]. Seaweed extracts can change chemical and physical soil traits, enhancing nutrient content, promoting microbial growth, and improving water retention. These effects help alleviate nutrient deficiencies and biotic or abiotic stresses in plants, often through complex signal transduction pathways. SWEs are increasingly recognized for their role in sustainable agriculture, as they improve soil health, boost beneficial microorganisms, and enhance plant nutrient levels, all of which support plant metabolism and help crops better withstand environmental challenges [31].

3.1.3. Botanical Derivatives

Botanical derivatives are naturally occurring secondary metabolites, or phytochemicals, derived from plants and commonly used across various industries, including food, cosmetics, pharmaceuticals, and agriculture. While much botanical research has focused on their pesticide properties, there is growing interest in their potential as biostimulants. Unlike seaweed, the biostimulant effects of botanicals are less well understood, but they offer promise for promoting plant growth and resilience. One of the key advantages of using botanicals as biostimulants is that they are typically thought to be less detrimental to humans and the atmosphere than traditional chemical pesticides [32].

3.1.4. Protein Hydrolysates and Nitrogen-Containing Compounds

Protein hydrolysates (PHs) are combinations of peptides and amino acids created through animal or plants’ enzymatic or chemical breakdown of protein. These substances help stimulate plant defense mechanisms, promote growth, and improve various physiological processes, such as leaf efficiency of water consumption, gas exchange, and antioxidant activity [33]. PHs can be used through multiple methods, including prepping seeds, soil drenching, and foliar treatment [34,35]. They all support better root formation and increased proliferation of plants. However, one must remember that heavy application of PHs can lead to phytotoxicity and hinder plant growth [36]. Additionally, PHs possess hormone-like properties that help regulate carbon and nitrogen metabolism while boosting plants’ antioxidant defenses, particularly under stress conditions.

3.1.5. GABA

A four-carbon non-protein amino acid called gamma-aminobutyric acid (GABA) is essential for plant communication and defense systems [37]. It assists plants to save moisture under drought conditions [38]. Ref. [39] studied the impact of GABA on sunflowers and discovered that higher concentrations increased sugar and chlorophyll synthesis. This was achieved by increasing the activity of antioxidant-related resistance enzymes significantly below drought and temperature-related stress levels.

3.1.6. Selenium

Selenium (Se) is a beneficial trace element, and when applied in low concentrations, it can enhance crop health and improve stress tolerance. For example, supplementing with organic selenium has significantly increased radish (Raphanus sativus) resistance to arsenic stress and toxicity by boosting superoxide dismutase activity [40].

3.1.7. Chitosan

Chitosan is a natural chitin derivative in insect, crustacean, fungal, and nematode eggshells [41]. It activates plant protective systems, including promoting the collection of phytoalexins, pathogenesis-related proteins, reactive oxygen species, and other plant defense chemicals. This increases plant resilience to stress and disease. Chitosan is utilized in agriculture as a biostimulant in several crops, including fruits, vegetables, cereals, and plantation crops, where it promotes growth and development while protecting against disease [42].

3.2. Microbial Biostimulants

Microbial biostimulants (MBSts) are essential in developing and maintaining different crops. Certain microorganisms in crops’ root zones, notably plant growth-promoting rhizobacteria (PGPR), serve as organic plant growth stimulants [43,44]. These soil microbes are crucial for transporting essential macro- and micronutrients into the rhizosphere, facilitating critical processes such as siderophore synthesis, nitrogen fixation, phosphate solubilization, and disease suppression. Among the key groups of MBSts are Trichoderma species and various arbuscular mycorrhizal fungi (AMF). Other soil microorganisms, such as Bacillus, Pseudomonas, and Paraburkholderia phytofirmans, have been identified as effective biostimulants that help plants cope with different abiotic stresses [45]. Endophytic MBSts, which reside inside plants, further increase a plant’s resistance to abiotic and biotic stresses by producing secondary metabolites that help neutralize reactive oxygen species, the culprits behind stress damage.

4. Sources of Biostimulants

Biostimulants come in a range of chemical forms and compositions. Beneficial bacteria, seaweed extracts, valuable fungus, and humic acid are all common constituents. Many biostimulant materials are derived from industrial and agricultural activity waste and byproducts. Ref. [46] identify potential sources of agricultural residue, industrial byproducts, urban solid trash, and aquaculture and livestock production waste. Plant and animal-based biostimulants are compounds produced from animal and plant tissues, frequently isolated from waste or byproducts. These biostimulants include dissolved proteins from plant and animal sources, such as secondary metabolites of plants, chitin, and chitosan [47]. These compounds are valued for their specific properties and functions that support plant growth and resilience. Protein hydrolysates contain peptides, oligopeptides, and amino acids created through chemical, thermal, or enzymatic hydrolysis. These hydrolysates are primarily sourced from both plant and animal byproducts. Plant-based protein sources often include byproducts from maize wet milling and other vegetable processing. However, significant byproducts produced by processing fish, poultry, and cattle, including fish scraps, animal skins, furs, and blood meal, are the source of animal-derived protein [48]. For instance, protein hydrolysates have been recovered from vegetable waste, including beetroot, cabbage, broccoli, and cauliflower [49].

Additionally, byproducts from leather production, such as animal skins, can also be processed into biostimulants [50], demonstrating how chicken feather waste could be converted into hydrolyzed products that are high in proteins, peptides, and amino acids to promote growth and development of plants. Secondary metabolites are substances plants form during their growth that have notable biological activity. These metabolites are valued for their wide range of effects and are used in various applications, including biostimulants. According to their chemical constituents, secondary metabolites are often classified as terpenoids, flavonoids, phenolic acids, alkaloids, and steroids [51]. Furthermore, to their many uses, these substances are crucial for plant development because they act as biological messengers and offer protection against environmental stresses [52]. A naturally occurring biopolymer, chitin is present mainly in the exoskeletons of animals such as insect species, molluscs, crustaceans, and arthropods. Deacetylation is a process that changes the structure of chitin and yields chitosan, a derivative of chitin. Chitin and chitosan are necessary for increasing agricultural yields and preserving the general quality of crops [53].

By altering their metabolic pathways, chitosan has been shown to boost photosynthesis in plants and increase their resistance to environmental stressors [54]. In the naturally occurring composting method, microbes decompose organic matter into more minor organic compounds, transforming it into nutrient-rich compost. This compost can serve as a valuable source of biostimulants, as it contains various beneficial compounds and microbes that can promote plant growth and overall health. For example, humic acid has been extracted from municipal solid waste, compost waste, and sewage sludge [55]. Additionally, vermicompost, a type of compost produced by worms, contains betaine. This compound is vital for preserving plant lipids and cell walls and fixing enzymatic and protein molecules [56,57]. Seaweed, mainly brown species like Fucus, Laminaria, and Ascophyllum, are widely recognized in agriculture for boosting plant growth. These seaweed-based biostimulants are usually available in liquid form or as soluble powders. The effectiveness of these products largely depends on the raw materials and the extraction methods used, such as acid or alkali treatment [32].

Additionally, to be rich in phenolic substances, seaweed extracts frequently include phytohormones that support plant growth and development. They also increase nutrient absorption, improve soil conditions, and help to chelate metals. Furthermore, seaweed supports plant water retention by producing hydrogels, which help maintain moisture in soil. Humic compounds are naturally occurring organic materials in soil, formed through the degradation of microorganisms, plants, animal materials, and soil microbial activity. Compared to fulvic acids, humic acids possess more carbon, a more significant molecular weight, and more polymerization, giving them a darker colour. Most humic farming materials come from non-renewable sources such as leonardite, peat, and soft coal. However, humic compounds derived from compost and vermicompost offer a more sustainable, renewable option. These natural compounds are frequently more successful in encouraging plant development, as they enhance soil properties, improve nutrient uptake by roots, and promote the development of lateral roots. The benefits of humic compounds are primarily due to their polyanionic nature, which increases soil Cation Exchange Capacity (CEC) and improves overall soil health and plant growth [47,58,59]. The Different sources of biostimulants have been summarized in

Table 1. Source of biostimulants and their activity.

Source of Biostimulant	Example	Activity
Hydrolysis product	Enzymatic (alfalfa, hay, pulses, fruits, and vegetables) and chemicals (feathers, bone meal, collagen from skin, animal tissues, and fish waste)	Enhanced productivity
		Improved leaves’ nutrient content and uptake	[1,21,30,32,33,43]
		Enhanced protein levels
		Defense against both abiotic and biotic stressors.
		Enhanced soil fertility by promoting the growth of soil microbes.
Aerobic digestion product	Animals and plants and lignin biomass	Induced the effect of auxin.
		Enhanced nutrient availability.
Biopreparations from marine algae	Ascophyllum nodosum, Sargassm wightii, Ecklonia maxima, Gelidium pectinutum	Enhanced antioxidant capacity and ability to scavenge free radicals.
		Chelation effect.
		Enhanced mitigation from abiotic and biotic stress.
		Prolonged shelf life
		Enhanced plant resilience to heat.
		Defended against the stress of drought.
Consortia of useful fungus	Rhizophagus intraradices, Rhymbocarpus aggregatus, Glomus spp., Trichoderma spp., Heteroconium chaetospira	Enhanced plant growth and yield, both individually and in symbiosis.

.

5. Biostimulants as Growth, Yield, and Quality Enhancers of Fruit Crops

Biostimulants, including seaweed extracts, humic acids, PGPR, protein hydrolysates, and oak wood extracts, have shown considerable promise in enhancing fruit quality, increasing yield, boosting antioxidant activity, and promoting overall plant health in various fruit crops. These substances help improve plant resilience, nutrient uptake, and stress tolerance, ultimately leading to better crop performance. This review provides an overview of how these biostimulants are applied in fruit production, highlighting their positive impacts on growth and fruit characteristics and their potential for improving agricultural sustainability [60,61,62,63,64,65,66] (Figure 2 and

Table 2. Application of biostimulants derived from various waste sources to promote plant growth.

Biostimulants	Sources	Effect of Biostimulants	Maximum Increases in Germination/Yield/Biomass (%)	Dosages/Concentration	Treatment Conditions	References
Compost tea	Composting compounds from organic waste	Enhance sugar beet growth and improve its quality.	4% improvement in yield.	500 ppm	Foliar application was used in the field testing.	[60]
Deproteinized leaf juice	Byproducts of leaf protein isolation from alfalfa	Enhance photosynthesis in sweet basil and promote the growth of roots and leaves.	50.3% growth increase in shoot length.	50, 100, 250 ppm	Foliar spray, pot trails.	[61]
Protein hydrolysate	Chicken Feather Waste	Enhance soil fertility and yield in mung bean.	40% improvement in germination.	10, 20, 30 mL	Potting trials; irrigation.	[62]
Lignin salts	Pulp mill byproducts	Reduce the buildup of reactive oxygen species and increase rice’s photosynthesis.	32.36% growth in shoot height.	100, 200, 300, 400 ppm	Added to the rhizosphere.	[63]
Humus-like substance	Tea waste	Stimulate the growth of tea trees.	117% improvement in plant-dried weight.	20, 40, 80, 160 ppm	Field trials; soil applications.	[64]
Seaweed extract	Seaweed waste	Stimulate cress seed germination and growth.	25% increase in germination percentage.	100, 1000, 2500, 5000, 10,000 ppm	Potting trials; irrigation.	[65]
Chitosan	Exoskeleton of insects, shrimps and crabs	Promote Salicornia bigelovii root development.	35% increase in biomass.	5000, 10,000 ppm	Field trials; root dipping.	[66]

and Table 3).

5.1. Strawberry

The use of biostimulants in growing strawberryxc (Fragaria × ananassa Duch.) has been widely researched, yielding promising results. For instance, foliar treatment of humic acid and salicylic acid has enhanced leaves’ sugar content, acidity, vitamin C levels, and nutrient uptake (like the minerals calcium, magnesium, potassium, and phosphorous). While these treatments increased fruit yield, they were associated with decreased overall antioxidant capability without appreciably altering fruit brightness [67]. Applying seaweed extract (from Ascophyllum nodosum) and silicon dioxide four times via foliar spraying has significantly boosted fruit quality, particularly by increasing anthocyanin levels, total yield, and fruit count [68]. Weekly foliar sprays combining seaweed extracts (Ascophyllum nodosum), alfalfa, protein hydrolysates, and chitosan have been shown to enhance fruit texture, color, and nutritional quality, particularly in terms of phenolic compounds [69]. These treatments have also led to notable enhancement of vitamin C in fruit [70].

5.2. Pome and Stone Fruit

Using seven foliar applications of extract from seaweed enhanced the spread and severity of the red coloring in apples (Malus domestica Borkh.) [71]. Likewise, weekly foliar treatment of Ascophyllum nodosum seaweed extract, alfalfa protein hydrolysates, and vitamins (B1, B2, and B6) after blooming enhanced red color and increased anthocyanin content in apple skin [72]. Although seven foliar applications of A. nodosum did not significantly impact yield, there was a noticeable reduction in sunburn and frost damage [73]. Dipping apple roots in suspension PGPR strains, such as Bacillus M3, Bacillus OSU-142, and Microbacterium FS01, increased shoot growth, productivity, and fruit weight [74]. In apricots (Prunus armeniaca L.), applying foliar sprays of humic and fulvic acids boosted the antioxidant activity of fruits at harvesting [75]. For bananas (Musa × paradisiaca L.), applying rhizobacteria promoted plant growth and development (Pseudomonas fluorescens) in a chitin-enriched medium, resulting in heavier fruits and overall yield, especially in rain-fed conditions [76].

5.3. Berries and Vine Crop

In blackberries (Rubus fruticosus L.), applications of PGPR (Pseudomonas fluorescens) to both roots and leaves have boosted sugar levels and increased flavonoid and phenol content [77]. In grapevines, applying humic acid as a foliage application has led to improvements in the weight of berry fruit clusters, an overall increase in the quantity of fruit, and higher TSS [78,79]. When protein hydrolysates from plants were applied to the soil, they boosted total polyphenol and anthocyanin content, resulting in a richer red color in berries and stimulating the production of petunidin [80]. Additionally, foliar sprays of Ascophyllum nodosum seaweed extract increased anthocyanin levels within berries’ shells by harvest [81].

French oak wood extracts have been shown to modify the aroma profile of wines by enhancing wood-derived aromatic notes and increasing glycosidic non-volatile aroma precursors [82]. Other studies have reported that these extracts contributed to increased caramel, butterscotch, and vanilla flavours, along with higher polyphenol content and improved wine color [83,84]. Similarly, extracts from American oak wood have been found to alter the taste of grapes (Vitis vinifera L.). Additionally, foliar application of phenylalanine has enhanced most amino acid and stilbene levels, further boosting phenolic compound concentrations in wine [85].

5.4. Cherry, Papaya and Olive

Foliar applications of rhizobacteria (PGPR) like Bacillus subtilis OSU-142 and Pseudomonas putida BA-8 have enhanced the nutritional health of sweet cherry (Prunus avium L.) trees [86]. Additionally, applying seaweed extracts two to three times through foliar spraying helped reduce the occurrence of cracks in fruit [87]. Soil treatments with potassium silicate also contributed to improved firmness of fruit [88]. Applying humic acid to peach trees through foliar or soil treatments has enhanced fruit size and yield, soluble solids, skin anthocyanin levels, and NPK [89]. In plums, three foliar applications of seaweed extract (Ascophyllum nodosum) have boosted seed count, fruit weight, and overall yield without impacting fruit maturity or vegetative growth [90].

In papayas (Carica papaya L.), inoculation with arbuscular mycorrhizal fungi (Glomus mosseae and Entrophospora colombiana) enhanced the fruit’s storability and increased yield. However, there were no significant changes in the quality of fruit [91]. In olives (Olea europaea L.), foliar treatment of seaweed extract enhanced both the oil content and the rate of fruit maturation [92].

Table 3. Effect of biostimulants on fruit crops.

Fruit Crop	Biostimulant	Dosage	Method of Application	Effect of Biostimulants on Fruit Crop	Reference
Kiwifruit Actinidia chinensis var. deliciosa (A.Chev.)	Humic acid	4 mL per litre	Foliar and drenching	Increased the yield, TSS, and vitamin C content.	[93]
Strawberry (Fragaria ananassa Duch.)	Humic acid	100 kg per ha	Soil application	Increased the carotenoids, chlorophyll a, and leaf area.	[94]
Olive (Olea europaea L.)	Arginine +humic acid	5 mL per litre	Foliar	Increased the total chlorophyll and fruit protein content.	[95]
Grapevine (Vitis vinifera L.)	Seaweed	1 g/L	Foliar application	Increased the yield, berries number, and anthocyanin content.	[96]
Sour orange (Citrus × aurantium L.)	Seaweed extracts	4 g/L	Fertigation	Increased fruit quality and blossom count per plant.	[97]
Mango (Mangifera indica L.)	Seaweed extract	4 mL/L	Foliar application	Increased the nitrogen, potassium, iron and zinc content in the leaf.	[98]
Grape (Vitis vinifera L.)	Protein hydrolysate	2 g/L	Foliar application	Increased the amino acid and C: N ratio.	[99]
Washington Navel Orange Citrus sinensis (L.) Osbeck	Chitosan	2 g/L	Foliar application	Increased the leaf inflorescence and fruit set.	[100]
Pomegranate (Punica granatum L.)	Chitosan	Chitosan at 0.5%	Foliar application	Improved the fruit quality and reduced the fruit cracking.	[101]
Cherry (Prunus avium L.)	Chitosan coatings	At 2%	Dipping	Reduced the pectin content and increase the firmness of the fruit.	[102]
Grapevine (Vitis vinifera L.)	Protein hydrolysates	1.6 g per litre	Foliar application	Increased the yield per vine and fruit colour and TSS.	[83,103]

Table 3. Effect of biostimulants on fruit crops.

Fruit Crop	Biostimulant	Dosage	Method of Application	Effect of Biostimulants on Fruit Crop	Reference
Kiwifruit Actinidia chinensis var. deliciosa (A.Chev.)	Humic acid	4 mL per litre	Foliar and drenching	Increased the yield, TSS, and vitamin C content.	[93]
Strawberry (Fragaria ananassa Duch.)	Humic acid	100 kg per ha	Soil application	Increased the carotenoids, chlorophyll a, and leaf area.	[94]
Olive (Olea europaea L.)	Arginine +humic acid	5 mL per litre	Foliar	Increased the total chlorophyll and fruit protein content.	[95]
Grapevine (Vitis vinifera L.)	Seaweed	1 g/L	Foliar application	Increased the yield, berries number, and anthocyanin content.	[96]
Sour orange (Citrus × aurantium L.)	Seaweed extracts	4 g/L	Fertigation	Increased fruit quality and blossom count per plant.	[97]
Mango (Mangifera indica L.)	Seaweed extract	4 mL/L	Foliar application	Increased the nitrogen, potassium, iron and zinc content in the leaf.	[98]
Grape (Vitis vinifera L.)	Protein hydrolysate	2 g/L	Foliar application	Increased the amino acid and C: N ratio.	[99]
Washington Navel Orange Citrus sinensis (L.) Osbeck	Chitosan	2 g/L	Foliar application	Increased the leaf inflorescence and fruit set.	[100]
Pomegranate (Punica granatum L.)	Chitosan	Chitosan at 0.5%	Foliar application	Improved the fruit quality and reduced the fruit cracking.	[101]
Cherry (Prunus avium L.)	Chitosan coatings	At 2%	Dipping	Reduced the pectin content and increase the firmness of the fruit.	[102]
Grapevine (Vitis vinifera L.)	Protein hydrolysates	1.6 g per litre	Foliar application	Increased the yield per vine and fruit colour and TSS.	[83,103]

6. Biostimulants as Stress Alleviators in Fruit Crops

Fruit crops can suffer significantly from environmental problems such as excessive heat, drought, salt, poor soil quality, and nutrient shortages, occasionally resulting in production losses of up to 50% [104]. By strengthening internal defenses and promoting vital physiological and biochemical functions, biostimulants provide a safe, natural means of assisting plants in withstanding these pressures. For instance, they reduce oxidative stress by increasing antioxidants, which counteract dangerous reactive oxygen species (ROS) that build up in challenging environments and destroy cells [105]. Key enzymes like catalase and superoxide dismutase are activated by biostimulants like protein hydrolysates and seaweed extracts, encouraging the production of other protective substances like glutathione and ascorbate. By increasing the synthesis of osmoprotectants such as proline and enhancing soil moisture retention through humic acids, these compounds also help manage water, which makes them particularly helpful in salty or droughty environments [106].

Furthermore, plant hormones essential for growth and stress tolerance are primarily regulated by biostimulants (

Table 4. Effect of biostimulants on stress alleviation in plants.

Stress	Name of Crop	Biostimulant	Mode of Application	Effect of Biostimulants on Plant	Reference
Cold stress	Avocado (Persea americana Mill.)	Fulvic acid	Dipping	Decreased the chilling injury and increase the shelf life.	[109]
	Grapevine (Vitis vinifera L.)	PGPB	Root immersion	Increased metabolic activity.	[110]
	Strawberry (Fragaria × ananassa Duch.)	Amino acid	Foliar spray	Delayed the spring frost injury.	[111]
	Grapefruit (Citrus × paradisi Macfad.)	Methyl Jasmonates	Dipping	Reduced the chilling injury.	[112]
Heat stress	Apple (Malus domestica Borkh.)	Humic acid	Foliar	Reduced the sunburn	[113]
	Pomegranate (Punica granatum L.)	Kaolin	Foliar spray	Reduced the sunburn and fruit cracking.	[114]
Drought stress	Grapevines (Vitis vinifera L.)	AMF (Glomus mosseae)	Soil application	Increased leaf water potential and stomatal conductance.	[115]
	Citrus (Citrus spp.)	Seaweed extract	Soil drenching	Increased the stem water potential and growth.	[116]
	Grape (Vitis vinifera L.)	Chitosan	Dipping	Increased the rooting in cutting and no. of new canes	[117]
	Loquat Eriobotrya japonica (Thunb.) Lindl.	AMF (Funneliformis mosseae)	Soil application	Increased the osmatic adjustment in the root.	[118]
Salinity stress	Strawberry (Fragaria × ananassa Duch.)	PGPB	Root dipping	Decreased the sodium and chloride content in roots and leaf.	[119]
	Apple (Malus domestica Borkh.)	AMF (Glomus versiforme)	Drenching	Reduced the ROS effect and increased the root length.	[120]
	Grapevines (Vitis vinifera L.)	Potassium silicate	Drenching	Increased the plant height and yield.	[121]
	Strawberry (Fragaria × ananassa Duch.)	Potassium silicate	Drenching	Decreased the proline ion content and increased the fruit yield.	[122]
Nutritional	Olive (Olea europaea L.)	Humic acid	Drenching	Increased the shoot growth.	[123]
	Pear (Pyrus communis L.)	Amino acid	Foliar spray	Increased the Fe, Cu, Zn and Mn content in the leaf.	[124]

). They support stress-related hormones, including abscisic acid (ABA), which aids plants in preserving water during dry times. The plant’s capacity to respond to environmental stressors is further reinforced by seaweed extracts, which are high in cytokinins and protein hydrolysates that activate defense-related chemicals like salicylic acid [107]. Biostimulants benefit the plant and the soil by encouraging beneficial microbial activity, which enhances root growth and nutrient availability. Beneficial bacteria and mycorrhizal fungi are microorganisms that improve plant nutrient absorption, and humic materials improve soil structure, retain more water, and foster ideal growth conditions. Even in challenging situations, fruit harvests flourish thanks to these combined impacts [108].

7. Mechanisms of Biostimulants for Stress Tolerance: Molecular and Physiological

Numerous biotic stressors, such as insect and disease assaults, and abiotic stressors, like extreme temperatures, drought, and salt, are continuously present in crops (Figure 3). These challenges often require plants to adjust and reorganize their defense mechanisms and modify their metabolism to cope with these conditions [125]. Over time, plants have evolved a complex and flexible defense mechanism that responds to these environmental challenges, much like animals do [126]. Recent studies suggest that biostimulants can act as plant priming agents, helping to enhance and sensitize plant defenses, thereby improving their resistance to different abiotic stresses [127].

Plants encounter challenges from harmful microbes and tough abiotic elements that might adversely affect their growth and development, including lack of moisture, excessive salinity, excessive heat, and a shortage of vital nutrients—productivity [128]. In response to these stressors, plants initiate well-regulated defense responses. When faced with these challenges, plants activate various crucial processes, such as signaling pathways and defense gene activation. They also synthesize protective metabolites and undergo both physiological and structural changes. These adaptive mechanisms enable plants to survive and flourish in challenging environments [129].

7.1. Molecular Mechanisms

7.1.1. Gene Expression Modulation

Biostimulants are essential for increasing gene expression that helps plants respond to stress. For example, microbial biostimulants from Bacillus species have been found to turn on genes linked to resilience to drought. This leads to increased production of osmolytes that help plants preserve osmotic balance and adapt to stressful conditions like drought [130]. For example, in grapevines (Vitis vinifera L.), Bacillus subtilis increases the expression of proline biosynthesis genes, improving drought tolerance [131]. Similarly, in strawberries (Fragaria × ananassa Duch.), Bacillus amyloliquefaciens activates DREB (Dehydration-Responsive Element Binding) and LEA (Late Embryogenesis Abundant) genes, which boost water retention and cellular stability under water-deficit conditions. Additionally, in citrus (Citrus spp.), Bacillus strains stimulate the expression of genes involved in synthesizing heat shock proteins (HSPs) that protect cells from dehydration [132].

7.1.2. Antioxidant Defense Activation

Plants frequently release reactive oxygen species during a drought that may harm cells [133]. Thankfully, antioxidant enzymes such as Catalase (CAT), Peroxidase (POD), and Polyphenol oxidase (PPO) are essential for neutralizing ROS, shielding plants from stress, and enhancing their capacity to withstand such circumstances [134]. Additionally, these antioxidants promote quicker recovery from dehydration and water deprivation, allowing plants to recover more quickly than those without these defenses [135]. In mangos (Mangifera indica L.), biostimulants derived from Bacillus species enhance antioxidant enzyme activity, reducing oxidative damage and improving drought resilience [136]. Likewise, in pomegranates (Punica granatum L.), surge expression of antioxidant genes after biostimulant application improves cell membrane stability under drought stress [137].

7.1.3. Phytohormone Regulation

Numerous plant hormones, including auxin, gibberellins (GA), jasmonic acid (JA), salicylic acid (SA), and abscisic acid (ABA), are naturally produced by PGPRs and are essential for assisting plants in responding to stress. By altering the element’s nitrogen metabolic rate, boosting antioxidant synthesis, and encouraging the accumulation of glycine betaine, salicylic acid (SA), for instance, helps plants cope with difficult circumstances and reduces drought stress [138]. Bacillus species increase ABA levels in papaya (Carica papaya L.), enhancing stomatal regulation and water use efficiency [139]. Similarly, in apples (Malus domestica Borkh.), biostimulants promote the production of jasmonic acid (JA), strengthening plant defenses against environmental stressors. In blueberries (Vaccinium spp.), auxin and gibberellin production stimulated by microbial inoculants improves root development, supporting water uptake during dry periods [140].

7.1.4. Metabolic Reconfiguration

Targeted metabolomics studies reveal that biostimulant-treated plants exhibit altered profiles of metabolites such as amino acids, flavonoids, and phenolic compounds. These changes are associated with improved physiological responses under stress conditions, suggesting that biostimulants induce a metabolic shift that enhances plant resilience [141]. For example, in cherries (Prunus avium L.), microbial biostimulants boost the accumulation of phenolic compounds, which protect against oxidative damage during drought stress [142]. In kiwifruits (Actinidia deliciosa A.Chev.), increased flavonoid production following biostimulant application improves stress tolerance and fruit quality [143]. Additionally, in avocados (Persea Americana Mill.), the upregulation of amino acid metabolism helps maintain cellular homeostasis under water-limited conditions [144].

7.2. Physiological Mechanisms

7.2.1. Enhanced Nutrient Uptake

Biostimulants improve root architecture and function, leading to better nutrient absorption. This is particularly beneficial during periods of abiotic stress when nutrient availability may be limited [145]. Enhanced root growth facilitates greater access to water and nutrients for plant survival [146]. In pineapples (Ananas comosus L.), microbial inoculants enhance root biomass, improving phosphorus uptake under drought stress. In figs (Ficus carica), biostimulants increase root surface area, facilitating better potassium absorption to sustain physiological functions. Similarly, in guava (Psidium guajava L.), applying Bacillus biostimulants improves nitrogen uptake, supporting growth even under water-deficit conditions [147].

7.2.2. Osmotic Adjustment

Growth arrest is frequently brought on by drought stress, especially in shoots, which helps plants save energy and lower metabolic demands when water is scarce. This modification encourages the synthesis of essential metabolites, including osmoprotectants. These soluble, low molecular weight substances, including polyols, amino acids, betaines, and non-reducing sugars like proline, inositol, and glycine betaine, are essential for preserving osmotic equilibrium. Under stressful circumstances, they protect the plant from dehydration by stabilizing macromolecules and cellular membranes [148]. Additionally, to inhibit shoot development, stopping root development preserves the function of the root meristem, facilitating a speedier recovery and further root extension when the stress is removed [149]. Furthermore, the elongation of the primary root is prioritized by decreasing lateral root development, an adaptation approach that aids in the primary root’s ability to reach moisture stored in deeper soil layers [150]. In peaches (Prunus persica L.), biostimulant applications have been linked to increased proline accumulation, aiding in osmotic balance [151]. In watermelons (Citrullus lanatus Thunb.), glycine betaine synthesis is enhanced, reducing water loss and improving drought tolerance. Additionally, in passion fruit (Passiflora edulis Sims.), increased inositol levels contribute to osmotic regulation, ensuring sustained growth under limited water availability [152].

7.2.3. Stress Acclimation

Abiotic stressors like drought frequently cause physiological changes in plants, including slowed growth rates, decreased stomatal conductance, decreased transpiration and photosynthesis rates, and reduced leaf water content [153]. In response to extreme weather, plants trigger cellular and molecular processes similar to their defense systems against biotic stress, resulting in various physiological changes to help them survive. Significant alterations include swelling of the root zone to enhance mineral absorption, stomatal closure, and leaf rolling to minimize moisture through evaporation. Decreased leaf area is another effect of drought stress that can assist with rerouting resources from damaged or older leaves to promote the stem or new development [154]. In olives (Olea europaea L.), microbial biostimulants help regulate stomatal closure, preventing excessive water loss [155]. In pears (Pyrus communis L.), leaf rolling induced by biostimulant treatment reduces transpiration rates, enhancing drought tolerance. Similarly, in lychee (Litchi chinensis Sonn.), reduced leaf expansion helps optimize water use efficiency, ensuring sustained growth under drought conditions [156].

8. Estimation of Biostimulant Effects on Fruit Crops with the Help of Systematic Techniques

The impact of biostimulants on fruit crops requires a systematic approach, combining experimental methods and detailed analysis. Biostimulants, which include natural products like seaweed extracts and beneficial microbes, boost crop yield and increase quality, especially under stressful situations (Table 5). Here, we explore essential factors that influence their effects on fruit crops.

8.1. Enhancing Growth and Yield

Biostimulants like humic acid and seaweed extracts have demonstrated significant potential to boost plant development and crop production. For example, combining organic fertilizers with humic acid in strawberry cultivation led to a remarkable yield increase, reaching 262.42 g per plant [157]. Similarly, in apple farming, treatments with sodium carboxymethyl cellulose and biogenic stimulants enhanced yields by 10.69% to 27.62% compared to plants without treatment [158].

8.2. Improving Fruit Quality

Biostimulants also play a vital role in improving fruit quality, such as increasing TSS and boosting nutritional value. In apples, using moringa leaf extract and seaweed extract significantly enhanced the characteristics, both molecular and physiological, of the fruit [159]. Similarly, applying organic liquid nutrients in strawberry cultivation yielded an impressive yield of 5418 kg per hectare, highlighting improvements in fruit quality and market appeal [160].

8.3. Mitigating Abiotic Stress

Biostimulants are instrumental in helping plants manage environmental stressors like salt and drought. They work by stimulating enzymes in plants and strengthening antioxidant defenses, which are essential for building resilience against such challenges [161,162,163,164,165,166,167,168,169,170,171,172]. Although biostimulants provide significant advantages, their performance can differ depending on the plant type and environmental factors. Further research is essential to refine application strategies tailored to specific fruit crops and growing conditions to maximize their potential.

Table 5. Effects of different biostimulants on growth and yield of various fruit crops.

Name of Crop	Biostimulants	Mode of Application	Effects of Biostimulants	References
Strawberry (Fragaria × ananassa Duch.)	Chitosan	Foliar	Yield increases 20%	[162]
Grape (Vitis vinifera L.)	Chitosan	Dipping	Increases the no. of canes and internodes	[163]
Citrus (Citrus spp.)	Sea weed extract	Soil drenching and soil application	Enhances the plant growth	[118,164]
Loquat Eriobotrya japonica (Thunb.) Lindl.	AMF	Soil application	Enhances the dry mass of the leaf	[165]
	(Funneliformis mosseae)
Mango (Mangifera indica L.)	Potassium silicate	Drenching	Increases the vegetative and reproductive growth	[166]
Tangerine orange (Citrus reticulata) Blanco	Humic acid	Soil application	Increases the plant height, stem diameter	[167]
Strawberry (Fragaria × ananassa Duch.)	PGPB B (Bacillus subtili)	Root dipping	Increases the final yield	[168]
Apple (Malus domestica Borkh.)	Humic acid	Drenching	Increases the root length	[169]
Pear (Pyrus communis L.)	Amino acid	Foliar application	Increases the shoot growth and yield	[170]
Almond Prunus dulcis (Mill.) D.A.Webb	Seaweed extract	Foliar spray	Increases the shoot length and shoot biomass	[171]
Apricot (Prunus armeniaca L.)	Humic acid	Foliar spray	Increases the vegetative growth and yield	[172]

Table 5. Effects of different biostimulants on growth and yield of various fruit crops.

Name of Crop	Biostimulants	Mode of Application	Effects of Biostimulants	References
Strawberry (Fragaria × ananassa Duch.)	Chitosan	Foliar	Yield increases 20%	[162]
Grape (Vitis vinifera L.)	Chitosan	Dipping	Increases the no. of canes and internodes	[163]
Citrus (Citrus spp.)	Sea weed extract	Soil drenching and soil application	Enhances the plant growth	[118,164]
Loquat Eriobotrya japonica (Thunb.) Lindl.	AMF	Soil application	Enhances the dry mass of the leaf	[165]
	(Funneliformis mosseae)
Mango (Mangifera indica L.)	Potassium silicate	Drenching	Increases the vegetative and reproductive growth	[166]
Tangerine orange (Citrus reticulata) Blanco	Humic acid	Soil application	Increases the plant height, stem diameter	[167]
Strawberry (Fragaria × ananassa Duch.)	PGPB B (Bacillus subtili)	Root dipping	Increases the final yield	[168]
Apple (Malus domestica Borkh.)	Humic acid	Drenching	Increases the root length	[169]
Pear (Pyrus communis L.)	Amino acid	Foliar application	Increases the shoot growth and yield	[170]
Almond Prunus dulcis (Mill.) D.A.Webb	Seaweed extract	Foliar spray	Increases the shoot length and shoot biomass	[171]
Apricot (Prunus armeniaca L.)	Humic acid	Foliar spray	Increases the vegetative growth and yield	[172]

9. Biostimulants: Current Challenges, Future Prospects for Sustainable Agriculture

Biostimulants face several challenges that need to be addressed for more effective use. These include difficulties related to production, sourcing raw materials, understanding toxicity levels, determining optimal dosages, and clarifying how they work. While many biostimulants have shown promising results in experiments, the mechanisms behind their effectiveness remain unclear for many of them [173]. The production procedure is further related to obtaining raw materials, which are influenced by environmental factors. Additionally, the potential toxicity of biostimulants to plants has not been widely explored. To overcome these hurdles, advancements in techniques such as agronomics, metabolomics, and phenomics are necessary. These methods will help clarify how biostimulants work and guide the optimization of their dosages [174].

Biostimulants offer dual benefits by promoting plant growth and strengthening plants’ defenses against environmental stresses. Extracts from seaweed, humic substances, and other natural compounds have proven effective biostimulants for fruit crops, improving their nutritive value. These biostimulants have demonstrated no adverse impact on the development or growth of plants, making them a promising option for enhancing crop productivity. Additionally, the synthesis of green nanoparticles from seaweed extract has been investigated for its potential benefits [175]. When used alongside inorganic fertilizers, biostimulants can help reduce cultivation costs, and they are a crucial component of sustainable fruit production [98]. Concern over the overuse of dangerous chemicals like fertilizers and pesticide products in fruit production is rising in nations like India. The goal is to shift toward eco-friendly agricultural practices, reflecting the increasing emphasis on sustainability in horticultural crop production.

Biostimulants are innovative tools that can significantly enhance traditional farming practices. Their application in fruit orchards has shown promising results, such as improved plant growth, better fruit production, and increased environmental stability. Despite these benefits, widespread adoption of biostimulants for sustainable fruit farming still faces hurdles, particularly in navigating the legal processes required for large-scale use. However, the potential advantages of biostimulants could transform the fertilizer industry and help combat food insecurity in developing countries, positioning it as a crucial component in sustainable agriculture’s future. Moreover, ongoing developments in biostimulant technology, such as new product formulations, production advancements, and marketing and distribution improvements, could play a vital role in incorporating sustainable practices into modern farming. These efforts are critical to boosting plant resilience and adaptability to environmental challenges, making biostimulants an essential part of securing and optimizing global agricultural production, especially in the face of climate change.