How Novel Biostimulants Enhance Resilience and Quality in Hydroponic Crop Production—A Review

Abstract

Hydroponic cultivation is expanding rapidly as a resource-efficient alternative to soil-based farming, but challenges related to nutrient management, abiotic or biotic stresses, and organic production still limit the system’s performance and efficiency. Biostimulants are increasingly being explored as a promising strategy to support productivity and sustainability in soilless systems. This review summarizes the current evidence on the use of plant biostimulants to support crop performance in hydroponic systems. Microbial biostimulants, such as plant growth promoting rhizobacteria, Arbuscular Mycorrhizal Fungi, and Trichoderma spp., have been reported to promote root growth by synthesizing phytohormones, enhance nutrient uptake, and reduce the impacts of salt and heat stress, with reported improvements in biomass and nutrient use efficiency. Seaweed extracts and protein hydrolysates modulate plant hormonal balance, improve antioxidant defense, and have been associated with improvements in yield and quality. Humic and fulvic acids increase micronutrient bioavailability through chelation and stimulate root activity through auxin-like effects. In organic hydroponics, biostimulants may help address the nutrient gap by accelerating organic matter mineralization. Existing key challenges include the lack of hydroponic-specific dosage guidelines and high commercialization costs. Future efforts should further evaluate system-specific strategies, including emerging tools such as artificial intelligence-optimized strategies and the use of clustered regularly interspaced short palindromic repeats-edited microbes to support the long-term sustainability of controlled environment agriculture.

1. Introduction

The expansion of the global population, compounded by escalating climate risks, presents mounting challenges to world food systems [1,2,3]. Conventional soil-based farming faces persistent limitations, including recurring soilborne pathogens, weed infestation [4,5], and environmental degradation caused by the excessive use of agrochemicals and water resources [6]. Consequently, there exists an urgent need for alternative cultivation techniques that are both resource-efficient and environmentally sustainable.

Hydroponics, the production of plants using a soilless system in which nutrients are supplied to the plant via an aqueous solution directly to the roots, offers a viable alternative to conventional agriculture [7,8]. By eliminating soil from the production system, hydroponics enables precise control over water, nutrient delivery, and oxygen supply, achieving up to 90% reduction in water use compared with field cultivation [7,9]. Furthermore, hydroponics offers advantages such as reduced soil erosion and nutrient runoff, weed control, optimized space utilization, and controlled growing conditions, making it an efficient and sustainable option in urban or resource-constrained areas [10,11]. However, hydroponic systems face distinct challenges, including complex nutrient management, susceptibility to disease outbreaks, and high energy consumptions [8]. Accordingly, there is a clear need for sustainable approaches that can both address these challenges and enhance crop resilience and quality.

Biostimulants are substances or microorganisms applied to plants to enhance nutrition efficiency, abiotic stress tolerance, and crop quality traits, regardless of their nutrient content [12]. Based on this definition, biostimulants can be broadly divided into microbial and non-microbial groups, as shown in Figure 1. Microbial biostimulants mainly include plant growth-promoting rhizobacteria (PGPR), mycorrhizal fungi, and Trichoderma spp. In contrast, non-microbial biostimulants are classified according to their origin and chemical composition and include seaweed and plant extracts, protein hydrolysates and amino acids, humic and fulvic acids, and novel biostimulants such as chitosan and nanoparticles [12,13,14]. Their effectiveness varies widely depending on crop species, developmental stage, and environmental conditions. Although biostimulant research has traditionally focused on conventional field production systems, studies on their application in soilless and hydroponic systems have gained increasing attention in recent years [15]. This recent increase in research activity is also reflected in the publication trend of studies on biostimulants in hydroponic and soilless systems (Figure 2). As shown in Figure 2, the number of publications remained very limited before 2017, but increased rapidly after 2020, highlighting the growing scientific and practical interest in this topic. In addition, a keyword co-occurrence shown in Figure 3 highlights the major research themes and emerging focus areas in this field.

The lack of information is particularly significant because the synergy between hydroponics and biostimulants offers a clear path toward sustainability. However, a critical research gap persists regarding the unique ecology of the soilless root zone. Specifically, the hydroponic environment is fundamentally distinct from soil, characterized by a low-microbial or sterile matrix and direct root exposure to the nutrient solution. In soil-based systems, many biostimulants, particularly microbial inoculants and humic substances, rely on complex interactions with the soil matrix and diverse microbial communities to modulate their efficacy [16]. Consequently, recommendations derived from soil-based production may not reliably translate to hydroponic systems. These differences are likely to influence not only the mechanisms of action of biostimulants, but also their appropriate application strategies, including product selection, dosage, delivery method, and timing under hydroponic conditions. Therefore, a system-specific understanding of biostimulant performance in hydroponics is needed to clarify their mechanisms, improve crop-specific recommendations, and optimize their practical use in commercial production.

To address this critical knowledge gap, this review is guided by the hypothesis that the effects of biostimulants in hydroponic systems are shaped by their interactions with hydroponic-specific factors, such as nutrient solution recipes, soilless substrate types, the unique rhizosphere microbial community, and other controlled environment parameters. From this perspective, this review first outlines the major categories of biostimulants and their proposed mechanisms of action under hydroponic conditions. It then synthesizes current evidence on how these materials influence key production outcomes, including plant growth, yield, quality, and tolerance to abiotic and biotic stresses across different high-value hydroponic crops. In addition, this review examines their relevance in organic hydroponics, postharvest quality management, and broader economic and environmental sustainability. Finally, the major limitations, research gaps, and future directions for improving the effective use of biostimulants in hydroponic production are discussed. By connecting these factors in a clear and logical way, this review aims to provide a more useful reference for researchers and growers and to support the development of sustainable and resilient hydroponic production systems.

2. Classification and Mechanisms of Biostimulants in Hydroponics

2.1. Microbial Biostimulants

Microbial biostimulants used in hydroponics mainly include PGPR, mycorrhizal fungi, and Trichoderma spp. These groups are discussed together because they all promote plant performance through beneficial root-associated interactions, although they differ in their biological nature and dominant modes of action. This grouping is intended as a functional classification and does not imply that these three groups currently have an equivalent evidence base in hydroponic systems [17,18].

2.1.1. PGPR

PGPR are free-living bacteria that colonize plant roots and promote plant growth through multiple mechanisms, including improved nutrient availability, phytohormone production, enhanced root development, and greater stress tolerance. Species such as Azospirillum brasilense, Pseudomonas fluorescens, and Bacillus subtilis contribute to these effects by producing phytohormones, including auxin, which stimulate root elongation and branching, increase root surface area, and enhance nutrient uptake [19]. Although most PGPR functional evidence originates from soil systems, similar effects have been observed in hydroponic systems. For example, recent studies show that Azospirillum baldaniorum and A. brasilense stimulate root length, root hair formation, and stem growth in hydroponic tomato seedlings, thereby increasing seedling survival and overall vigor [20]. These effects are mainly attributed to the microbial synthesis of auxins and other phytohormones that stimulate cell elongation and lateral root initiation, improving nutrient and water uptake efficiency.

Beyond growth promotion, PGPR also contributes to stress mitigation in hydroponic systems. Commercial biostimulant products containing Bacillus amyloliquefaciens, B. subtilis, and Bacillus licheniformis demonstrated strong mitigation of salt stress in floating-slab hydroponics, significantly increasing leaf area, plant height, and biomass of lettuce (Lactuca sativa) while improving water and nitrogen (N) use efficiency, which can be induced by the hormones produced by Bacillus spp. [21]. These improvements may arise from synergistic mechanisms, including enhanced ion homeostasis (maintaining K⁺/Na⁺ balance), activation of antioxidant enzymes, and the chelation activity of fulvic acid that facilitates micronutrient availability. For example, Yasmin, et al. [22] found that soybean plants inoculated with the halotolerant strain Pseudomonas pseudoalcaligenes maintained K⁺/Na⁺ homeostasis under salinity stress.

In addition to regulating growth and stress responses. PGPR can improve micronutrient acquisition in hydroponic systems, particularly that of iron (Fe). Fe availability is often a limiting factor in soilless production, and even relatively stable Fe-EDDHA can gradually degrade under prolonged light exposure, reducing its long-term effectiveness [23,24]. PGPR can enhance Fe uptake through siderophore secretion, which facilitates Fe binding and transport [25]. For instance, in tomato, siderophores produced by Chryseobacterium C138 provided an efficient Fe source for Fe-deficient plants, matching or surpassing the efficacy of Hoagland nutrient solution [26]. These findings indicate that PGPR enhances the adaptability of hydroponic vegetables by synthesizing phytohormones, activating enzymes, improving nutrient assimilation, mitigating salt stress, and increasing Fe availability.

The combined application of different types of biostimulants deserves separate discussion because their effects are often not simply additive but depend on whether complementary interactions can be achieved among different components. In greenhouse soilless onion seedling production, the combined use of microbial biostimulants and Kelpak seaweed extract resulted in overall improvements in seedling morphology and biomass compared with the control, with particularly strong effects on root traits. In white onion, the combination of Tribus Continuum and Kelpak increased root length by 17% relative to the control, suggesting that the “microbial + seaweed extract” combination may improve seedling quality through the joint promotion of root development [27]. Similarly, in soilless cucumber cultivation, the combined application of Glomus intraradices + Azospirillum brasilense with seaweed extract improved fruit diameter, yield, and titratable acidity; however, most vegetative growth and fruit quality traits did not exceed those obtained with the best individual treatment [28]. These findings suggest that the benefits of combined application are strongly trait-dependent, and not all parameters are enhanced simultaneously. In addition, in floating hydroponic lettuce, combinations of fulvic acid, amino acids, and earthworm castings extract also showed clear interaction effects. For example, fulvic acid 40 with vermicompost 2 mL increased total yield by approximately 18.2% and root weight by 33.9%, whereas fulvic acid 40 ppm + amino acid 100 ppm increased total yield by approximately 17.4% [29]. These results indicate that, even without microbial inputs, combinations of non-microbial biostimulants may still improve crop performance, although the strongest effects may occur in different traits depending on the specific combination applied.

2.1.2. Mycorrhizal Fungi

Mycorrhizal fungi are symbiotic fungi associated with plant roots. Specifically, arbuscular mycorrhizal fungi (AMF), belonging to the phylum Glomeromycota, form an endomycorrhizal symbiosis characterized by the formation of arbuscules within root cells, which serve as the main site of nutrient exchange between the two partners [30]. In hydroponic systems, AMF promote plant performance through root colonization, improved nutrient and water uptake, and stress-related physiological regulation. Host root exudates can activate AMF metabolism and stimulate hyphal branching, thereby increasing the probability of host root encounter and facilitating subsequent colonization. Among these exuded signals, strigolactones are recognized as key components that strongly stimulate fungal metabolism and hyphal fine branching [31]. Functionally, AMF enhance water uptake and improve nutrient absorption efficiency, facilitating the exchange of essential nutrients, such as phosphorus (P) and N, for host carbon compounds [32]. This symbiotic pathway is highly regulated by the host plant’s nutritional requirements, particularly the availability of phosphate. Physiologically, AMF improve parameters like photosynthesis and stomatal conductance [33]. Furthermore, under abiotic stress (e.g., salinity, drought, heavy metals), AMF confer tolerance through ionic regulation by increasing critical tissue ratios, such as K⁺:Na⁺, Ca²⁺:Na⁺, and Mg²⁺:Na⁺. This effect is complemented by the accumulation of osmolytes, including glucose, fructose, and proline, which aid in osmotic adjustment [34]. These integrated mechanisms allow AMF to maintain their beneficial influence, enhancing plant resilience and overall performance in soilless environments. Previous research also found that Rhizophagus irregularis effectively colonized in roots of Lotus japonicus grown in hydroponic systems, achieving colonization levels comparable to sand culture [35]. Similarly, Glomus mosseae achieves high root colonization rates (36–73%) in hydroponic systems for wheat, sorghum, and flax, demonstrating significant root biomass advantages under low P conditions [36]. These findings indicate that AMF maintain their beneficial effects in soilless cultivation environments, helping crops improve nutrient acquisition capacity and stress tolerance under controlled environments.

2.1.3. Trichoderma spp.

Trichoderma spp. belongs to a class of plant growth-promoting fungi, and some strains exhibit a predominant biostimulant action by promoting root development, improving nutrient uptake, and enhancing plant adaptation to stress [18]. Unlike PGPR or AMF, Trichoderma spp. functions primarily through fungal root colonization, stimulation of root hair development, auxin-related signaling, secretion of secondary metabolites, and regulation of nutrient transport and nitrate metabolism. These features make Trichoderma spp. a distinct fungal biostimulant subgroup within microbial biostimulants, rather than a direct mechanistic equivalent of PGPR or AMF in hydroponic systems. The fungi stimulate primary meristematic activity and increase both the number and volume of root hairs, thereby enhancing water and nutrient uptake. This enhanced root architecture is mediated by auxin-related signaling and the production of phytohormones such as indole-3-acetic acid (IAA) [37,38]. Trichoderma spp. also improves nutrient dynamics by increasing the use efficiency of P, sulfur (S), zinc (Zn), and Fe [38,39]. Secondary metabolites act as carriers for micronutrients such as Fe and Zn, facilitating their absorption and translocation within the plant. Additionally, Trichoderma can regulate expression of nitrate transporter genes (e.g., NRT2.1 and NRT2.2) and enhance nitrate reductase activity, thereby reducing nitrate accumulation [40]. These physiological responses, combined with enhanced photosynthetic activity, result in vigorous plant growth. For example, Trichoderma asperellum strains TaMFP1 and TaMFP2 significantly increased plant height, leaf area, root length, and biomass in DWC spinach while improving P and calcium (Ca) uptake and ultimately boosting yield by 34.5% [41]. In a floating-root hydroponic lettuce system, inoculation with T. asperellum increased fresh biomass by 76.4% and dry matter by 82.6% without affecting visual quality [42]. These results highlight that Trichoderma not only promotes root development and nutrient uptake in soilless systems but also enhances overall yield and crop marketability.

2.2. Non-Microbial Biostimulants

Non-microbial biostimulants used in hydroponics mainly include seaweed and plant extracts, protein hydrolysates and amino acids, humic substances, and other emerging materials.

2.2.1. Seaweed and Plant Extracts

Seaweed and plant extracts are non-microbial biostimulants derived from marine algae or terrestrial plants and are widely used because they contain diverse bioactive compounds that can enhance plant growth, nutrient uptake, and tolerance to abiotic stress [43]. They contain plant hormones such as auxins, cytokinins, and brassinosteroids that modulate hormonal balance and promote root and shoot development [44,45,46,47]. In addition, compounds in these extracts including glycine betaine, phlorotannins, and alginate oligosaccharides, act as natural elicitors. For example, glycine betaine functions as a compatible osmolyte that assists in the repair of Photosystem II and strengthens the antioxidant system during abiotic stress [48,49,50]. Phlorotannins, including eckol, encourage root formation, improve the accumulation of photosynthetic pigments, and maintain the stability of endogenous auxin [51]. Alginate oligosaccharides further contribute to root development by increasing endogenous auxin levels and activating defense-related enzymes such as phenylalanine ammonia lyase and peroxidase [52]. Through these combined mechanisms, seaweed extracts enhance plant stress tolerance, nutrient use efficiency, and overall plant performance.

As shown in

Table 1. Summary of seaweed extract types, application methods, mechanisms, and reported benefits in hydroponic crop production.

Crops	Source	Application Methods	Key Benefits	Mechanism	Reference
Lettuce	Kappaphycus alvarezii (red seaweed)	Added to nutrient solution	Maintained/increased biomass with 50% fertilizer replacement.	Direct nutrient supply plus hormone-like biostimulant activity that promotes growth processes.	[56]
	Ascophyllum nodosum  (brown seaweed)	Foliar spray	Mitigated K deficiency and improved fresh-cut lettuce quality.	Strengthened antioxidant defence and reactive oxygen species homeostasis and improved photosynthetic function under K limitation.	[54]
	Ascophyllum nodosum (brown seaweed)	Added to nutrient solution	Enhanced growth and beneficial microbiomes.	By shifting microbial community structure toward taxa linked to higher productivity and nutrient cycling.	[55]
	Eucheuma cottonii (red seaweed)	Partial replacement of nutrient solution	Reduced fertilizer use while maintaining yield.	Driven by changes in nutrient solution strength and ion supply as replacement rate increases.	[57]
	Ecklonia maxima (brown seaweed)	Added to nutrient solution	Increased biomass/leaf area and extended shelf life.	Improved resource use efficiency and delayed senescence processes that slow postharvest deterioration.	[53]
Peppermint (Mentha × piperita); Purple basil (O. basilicum var. purpurascens)	Ulva intestinalis (green seaweed)	Liquid extract (10% v/v) added to nutrient solution	Enhanced peppermint growth and chlorophyll.	Through species-specific effects on pigment metabolism, photosystem performance, and plant water relations.	[59]
Spinach (Spinacia oleracea L.)	Ascophyllum nodosum (brown seaweed)	Soluble powder; incorporated into growth medium (0.1–0.5 g L−1)	Increased shoot biomass/chlorophyll and enhanced antioxidant traits.	Through activation of antioxidant and phenylpropanoid metabolism and regulation of redox and osmotic adjustment pathways.	[58]

, recent studies have demonstrated that seaweed extracts function as effective biostimulant inputs in hydroponic systems, increasing crop yields, nutrient-use efficiency, and postharvest quality. The most robust evidence supports their use in the model crop, lettuce [53,54,55]. For instance, adding Ecklonia maxima extract to DWC nutrient solutions significantly increased biomass and leaf area while maintaining high postharvest quality [53]. Similarly, Sargassum and Ascophyllum nodosum extracts enhanced hydroponic lettuce growth while making root/leaf bacterial communities more diversified associated with increased productivity [55]. Regarding fertilizer substitution, Kappaphycus alvarezii combined with bovine urine replaced approximately 50% of mineral nutrient solutions without yield reduction [56]. Likewise, Eucheuma cottonii substituted 25–50% of mineral nutrients in a static Kratky system showed no significant yield decline [57]. Regarding stress mitigation and postharvest quality, foliar application of A. nodosum in a deep flow technique system restored growth and improved fresh-cut lettuce quality under K deficiency [54]. In spinach hydroponic production, A. nodosum treatments significantly decreased leaf N content by 40% and increased P by 52% [58]. Additionally, seaweed extracts also display species-specific responses. For example, Ulva intestinalis promotes mint growth and increases chlorophyll in nutrient film technique (NFT) systems but reduced growth in purple basil, indicating that seaweed extract application requires crop- or variety-specific optimization [59]. In certain cases, hydroponic systems exhibited stronger synergistic responses to seaweed extracts than soil-based cultivation. In soil systems, algal polysaccharides, biopolymers derived from marine algae, bind to clay and organic matter, delaying physiological responses, whereas hydroponic application facilitates more rapid uptake and activity [60].

2.2.2. Protein Hydrolysates & Amino Acid-Based Biostimulants

Protein hydrolysates and amino acid-based biostimulants are an important category of non-microbial biostimulants and mainly contain peptides and free amino acids derived from the hydrolysis of plant- or animal-based proteins [61]. Protein hydrolysates and amino acid-based biostimulants function as organic N sources that stimulate key enzymatic activities involved in N assimilation, thereby improving nutrient-use efficiency in both soil and hydroponic systems [62,63]. They also contribute to abiotic stress alleviation, including drought and salinity tolerance [61]. In hydroponic systems, these biostimulants such as yeast-derived amino acids and peptides, can be directly absorbed by plant roots or foliage [63]. These compounds rapidly activate plant defense pathways, promoting the accumulation of osmolytes and antioxidant enzymes that help plants maintain photosynthesis and growth under temperature or salinity fluctuations [61,63,64]. For example, low concentrations of L-methionine added to the nutrient solution stimulate growth in hydroponic lettuce, and amino acid mixtures applied to tomato foliage under salinity stress increase soluble sugar accumulation, consistent with improved osmotic adjustment [65,66].

As summarized in Table 2, hydroponic experiments further demonstrate the multifaceted effects of protein hydrolysates and amino acid-based biostimulants, highlighting their roles in optimizing nutrient metabolism and physiological resilience across different crops and stress conditions. Specifically, plant-derived protein hydrolysates applied with nutrient solutions in DWC systems can maintain or even increase yields and nutritional quality while reducing fertilizer concentration. For example, a legume-derived enzymatic protein hydrolysate (Trainer^®) increased anthocyanin content by over 450% in carrot microgreens and enhanced yield and antioxidant capacity in dill microgreens, while in hydroponic basil, it improved biomass and total phenolics by around 15% [67,68]. Additionally, animal-derived protein hydrolysates were also effective in hydroponic systems, as applying approximately 1 mL·L⁻¹ milk protein hydrolysate to NFT grown lettuce increased both shoot and root biomass and enhanced the accumulation of essential mineral nutrients including P, K, Ca, magnesium (Mg), and Fe [69].

In the case of free amino acids, hydroponic crops display clear dose-dependent and molecule-specific responses. In lettuce, low doses of L-methionine promoted growth and increased leaf contents of N, P, and K, while higher doses of glycine or tryptophan showed inhibited plant growth under the tested conditions [70]. Moreover, under salt stress (50 mM NaCl), application of commercial amino acid formulations (Aminoset^®) in DWC systems increased yields by approximately 34% compared to salt-stressed controls while improving stomatal conductance, chlorophyll content, and antioxidant defenses, demonstrating their potential for stress mitigation in soilless environments [25]. Furthermore, foliar application of legume-derived protein hydrolysates under Fe-deficient conditions in hydroponic tomatoes and cucumbers also increased chlorophyll content, maintained biomass and ion balance, thereby alleviating micronutrient deficiency [71].

Table 2. Summary of recent studies on Protein Hydrolysates and amino acid-based biostimulants in hydroponic cultivation.

Crops	Biostimulant	Key Outcomes	References
Carrot & dill microgreens	Enzymatic hydrolysate of leguminous biomass	Increased biomass and antioxidant content.	[67]
Basil	Vegetal protein hydrolysate	Improved yield and phenolics under reduced nutrients.	[68]
Tomato, cucumber	Protein hydrolysate (enzymatic hydrolysate of legume seeds)	Alleviated iron deficiency stress.	[71]
Lettuce	Milk protein hydrolysate	Increased biomass and mineral uptake; reduced nitrate.	[69]
	Free amino acids (L-Met, L-Gly, L-Trp)	L-Met promoted growth; others inhibited dose-dependently.	[70]
	Amino acid-based biostimulant	Improved yield and physiological performance under salinity.	[25]

Table 2. Summary of recent studies on Protein Hydrolysates and amino acid-based biostimulants in hydroponic cultivation.

Crops	Biostimulant	Key Outcomes	References
Carrot & dill microgreens	Enzymatic hydrolysate of leguminous biomass	Increased biomass and antioxidant content.	[67]
Basil	Vegetal protein hydrolysate	Improved yield and phenolics under reduced nutrients.	[68]
Tomato, cucumber	Protein hydrolysate (enzymatic hydrolysate of legume seeds)	Alleviated iron deficiency stress.	[71]
Lettuce	Milk protein hydrolysate	Increased biomass and mineral uptake; reduced nitrate.	[69]
	Free amino acids (L-Met, L-Gly, L-Trp)	L-Met promoted growth; others inhibited dose-dependently.	[70]
	Amino acid-based biostimulant	Improved yield and physiological performance under salinity.	[25]

2.2.3. Humic and Fulvic Acids

Humic acids (HAs) and fulvic acid (FAs) are non-microbial biostimulants derived from humic substances formed during the decomposition of organic matter. They can promote root growth, improve nutrient uptake, and increase plant tolerance to environmental stress [72]. They enhance micronutrient bioavailability through chelation, forming stable metal-organic complexes that prevent nutrient immobilization in both soil and soilless environments [73,74]. Beyond their chelating capacity, HAs and FAs act as biostimulants that modulate multiple physiological and molecular processes. They exhibit auxin-like activity, stimulating root elongation and lateral root formation through the activation of plasma membrane H⁺-ATPase, which increases ion uptake and nutrient translocation within the plant [72,75].

In hydroponic systems, humic substances further chelate micronutrients such as Fe, Zn, and Mn, contribute to pH buffering around the root zone, and enhance chlorophyll biosynthesis, photosynthetic efficiency, and antioxidant enzyme activity, collectively improving nutrient-use efficiency and overall plant vigor [76,77,78]. Due to their low molecular weight and high water solubility, FAs effectively chelate and stabilize trace elements, thereby promoting their transport into inert substrates within hydroponic systems and reducing precipitation risks [79]. This direct pathway via foliar uptake or root absorption bypasses soil adsorption losses when HAs or FAs are applied [80]. For example, in strawberry NFT hydroponics, adding FA at 1.5–3 mg L⁻¹ significantly increased both fruit firmness and soluble sugar levels [81], highlighting the advantages of precise nutrient dosing possible in hydroponic systems compared with soil-based cultivation. Similarly, in hydroponically grown tomato (Solanum lycopersicum), supplementing the nutrient solution with HA at 25–50 mg L⁻¹ increased the photosynthetic rate to 4 times, increased chlorophyll content by approximately 77%, and improved fruit sugar concentration by 47%, resulting in a 64% higher yield compared with the control [82]. These findings further confirm that HAs in hydroponic systems stimulate photosynthesis and nutrient assimilation through enhanced chlorophyll biosynthesis and ion transport efficiency.

2.2.4. Novel Biostimulants

Beyond the major biostimulant categories, hydroponic applications also include nano-enabled formulations, natural biopolymers such as chitosan, and selected organic extracts formulated for nutrient-solution delivery. In soil systems, nanostructured biostimulants often interact with soil aggregates and organic matters in multiple ways, exhibiting controlled release behaviour and synergizing with soil microbial communities. By contrast, in hydroponic systems, nanocarriers remain stably suspended and contact roots directly, enabling more precise and efficient delivery of active compounds [83].

Chitosan, a natural biopolymer, not only promotes aggregate stability, microbial activity, and plant defence responses in soil, but when formulated as 30–50 nm nanoparticles in hydroponic solution, its controllable surface charge and structure allow it to stay suspended for extended periods and slowly release signalling molecules and nutrients, greatly enhancing tomato plants’ oxidative stress tolerance and growth performance [83,84].

The principal mechanisms of action of biostimulants in hydroponic systems are summarized in Figure 4. Representative studies reporting crop-specific responses to different biostimulants are compiled in

Table 3. Key studies demonstrate the effects of different biostimulants on hydroponic crops.

Crops	Biostimulant	Biostimulant Effects	References
Lettuce	PGPR (Bacillus subtilis, Bacillus amyloliquefaciens, Azospirillum brasilense)	Increased growth and biomass; improved salt-stress tolerance.	[21]
	Seaweed extract (Kappaphycus alvarezii)	Improved yield and leaf quality; enhanced chlorophyll and antioxidant activity.	[85]
	Vegetal protein hydrolysate	Increased biomass; enhanced phenolics and antioxidant capacity.	[25,86]
	Moringa oleifera leaf extract	Promoted leaf growth; increased phenolics and antioxidant capacity.	[87]
Spinach	PGPR and AMF	Improved nutrient uptake; reduced nitrate; enhanced chlorophyll under salinity.	[88]
	Microalgae (Chlorella vulgaris)	Increased chlorophyll and antioxidant activity; reduced nitrate.	[25]
Bok choy	Bacillus amyloliquefaciens and Trichoderma harzianum	Bacillus improved growth and N uptake; Trichoderma enhanced P/Ca uptake with variable growth response.	[89]
Kale	Rhizosphere microbial consortium	Improved plant growth and yield; maintained N-use efficiency.	[90]
	Seaweed extract (Kappaphycus alvarezii)	Increased chlorophyll and antioxidant activity.	[85]
Mint	Left-handed amino acids	Reduced leaf nitrate; enhanced pigments and essential oil yield.	[91]
Basil	Cyanobacteria hydrolysate (Nostoc sp.)	Promoted vegetative growth and secondary metabolite accumulation.	[92]
Tomato	Seaweed extract, amino acids, polysaccharides, glycosides, and mineral elements	Enhanced flowering/fruit set; increased yield and fruit quality traits.	[93]
	PGPR, HA/FA, chitin derivatives, seaweed extracts, amino acids	Improved yield and/or antioxidant-related quality, depending on treatment.	[94]
	Seaweed extract, AMF	Advanced flowering; improved fruit set; enhanced firmness and flavor.	[95]
	Amino acids, organic carbon compounds, vitamins, and bioactive substances	Increased yield and photosynthetic/pigment status; improved antioxidant-related quality under heat stress.	[96]
	Amino acids, organic carbon compounds, vitamins, and bioactive substances	Increased yield; improved mineral content; reduced blossom-end rot.	[97]
Cucumber	Brown algae extracts	Slight yield reduction but improved fruit quality and antioxidant-related traits.	[98]
	Amino acids, seaweed	Enhanced vegetative growth and root development.	[99]
	Putrescine	Improved seedling vigor and vegetative growth.	[100]
Melon	Nicotinamide	Improved photosynthesis and fruit quality/yield under salinity stress.	[101]
Strawberry	Alfalfa protein hydrolysate, B-group vitamins, chitosan, and silicon	Improved soluble solids and postharvest quality.	[102]
	AMF	Enhanced fruit firmness and antioxidant-related compounds.	[103]
Raspberry	Glycine betaine, kelp extract	Improved photosynthesis and anthocyanin accumulation under heat stress.	[104]

, and these studies are discussed in more detail in the following section.

3. Effects of Biostimulants on High-Value Hydroponic Crops

3.1. Leafy Greens

Biostimulants have reported to improve the growth and leaf quality of hydroponic leafy greens from various families, including Asteraceae, Amaranthaceae, and Brassicaceae, although their effects vary with crop genotype, hydroponic system design, nutrient solution regime, treatment level, and application method. For instance, in a perlite-based drip system during March–May, foliar application of a Fabaceae protein hydrolysate (AgricostanD at 2.5 mL L⁻¹) increased lettuce fresh weight and leaf area by 37% and 22%, respectively, and enhancing antioxidant capacity [87]. Under different hydroponic conditions, in a floating culture system with 50% mineral nutrition (EC 1.3–2.0 dS m⁻¹), root-zone supplementation with a PGPR mixture (Rhizofil^TM at 1.0 mL L⁻¹) every 10 days improved spinach leaf quality, particularly by reducing nitrate content by approximately 67% [88]. Taken together, these responses in leafy greens are generally consistent with the mechanisms outlined in Section 2, particularly enhanced nutrient acquisition, improved chlorophyll retention, and stronger antioxidant regulation, which together help maintain vegetative growth and leaf quality under hydroponic conditions.

Under saline stress, the application of a legume-derived protein hydrolysate (Trainer^®) maintained approximately 25% higher fresh biomass, 30% greater chlorophyll content, and around double stomatal conductance, and at the same time reduced lipid peroxidation by about 35% relative to the untreated control, indicating improved photosynthetic efficiency and oxidative stress tolerance in lettuce [25,86]. Moreover, in hydroponic bok choy (Brassica rapa var. chinensis), inoculation with Bacillus amyloliquefaciens increased shoot fresh weight from 72.2 g to 82.7 g, about 14.5%, and it also increased plant height by approximately 12%, leaf area index by 15%, and N uptake efficiency by 18% in NFT systems, whereas Trichoderma spp. treatments reduced these parameters by about 15% compared with the control [89]. In addition, in hydroponically grown kale, supplementing 50% inorganic fertilizer with a rhizosphere microbial consortium increased plant height by around 10%, leaf number by 6%, and yield by 3%, and at the same time maintained high N uptake efficiency [90]. Likewise, applying 1% Kappaphycus alvarezii seaweed extract in NFT hydroponics enhanced fresh biomass by 25%, chlorophyll by 10%, and antioxidant activity by 30%, indicating improved growth performance and nutritional quality of kale [85].

3.2. Herbs

Essential oils and aroma are secondary metabolites that can be enhanced by biostimulants in hydroponic herbs. In mint (Mentha spp.), 0.5% protein hydrolysate (Amino16^®) reduced leaf nitrate concentration by 77%, and increased chlorophyll content by 17.6%, carotenoid content by 21.1%, and essential oil production by 26.5%, with no changes in key oil components [91].

In basil (Ocimum basilicum) grown in a deep-flow hydroponic system under half-strength Hoagland nutrient solution, weekly foliar application of carbohydrate-rich cyanobacterial hydrolysates at 1 g·L⁻¹, particularly Nostoc sp., increased leaf number by about 24% and significantly enhanced root development, resulting in a 21% increase in root-to-shoot ratio compared with the control, thereby supporting nutrient uptake and vegetative growth [92]. These responses in hydroponic herbs likely reflect improved nutrient acquisition and pigment metabolism, together with greater carbon allocation to secondary metabolite production, which may explain the concurrent increases in vegetative growth and aroma-related traits.

3.3. Fruiting Vegetable Crops

Biostimulants of various types have been applied to increase fruit yield and quality, as well as to reduce abiotic stress in a number of fruiting vegetable species, most notably within the Solanaceae and Cucurbitaceae families, including tomato, sweet pepper, cucumber, and melon. Under hydroponic cultivation conditions, several studies indicate that biostimulants can significantly promote tomato flowering and fruit set, increase yields, and improve fruit quality. Abdelkader, Gaplaev, Terekbaev and Puchkov [93] found that, in a greenhouse hydroponic study using the tomato hybrid ‘Merlice’ grown on mineral wool, applying the commercial plant-derived biostimulant Radifarm^® at 2.5, 5.0, and 7.5 mg·L⁻¹ through seed soaking and two foliar sprays increased the number of fruits per plant and individual fruit weight, boosting total yield by 20–70% while enhancing fruit dry matter, soluble solids, and vitamin C content. In soilless trials on tomato grown in coconut coir substrate, treatments including PGPR, HA/FA, chitin derivatives, seaweed extracts, and amino acids outperformed controls [94]. Earthworm castings extract yielded the highest increase (approximately 43%), while PGPR treatment boosted total phenolic content by 88% [94]. Furthermore, Dash, Guo and Leskovar [95] addressed heat stress under organic nutrient solutions, and found that products containing seaweed extracts and AMF (e.g., MycoApply^®) advanced tomato flowering by 5 days, increased fruit set by 19%, and reduced flower drop by 10%. After 12 consecutive weeks of foliar application, treated tomatoes yielded 30% higher than controls, with improved fruit firmness, soluble solids, and flavor [95]. Similarly, in hydroponically grown tomato and lettuce, root-applied protein hydrolysates increased marketable yield by 32% in tomato and 21% in lettuce, and improved water use efficiency by about 20%, demonstrating their effectiveness in promoting sustainable production and reducing water consumption [105].

In hydroponically grown sweet peppers, biostimulants also demonstrated significant effects. Parađiković, Vinković, Vinković Vrček, Žuntar, Bojić and Medić-Šarić [96] evaluated four commercial biostimulants, Radifarm^®, Megafol^®, Viva^®, and Benefit^®, containing amino acids, polysaccharides, humic substances, and vitamins, respectively. Under a 30% reduction in fertilizer use, these treatments increased marketable yield by approximately 22% and enhanced leaf pigments by 20–32%, total phenolics by 36–63%, vitamin C content by 3–18%, and antioxidant activity by 7–25% under heat-stress conditions [96]. A subsequent study applying Radifarm^® and Megafol^® under the same 30% reduction in chemical fertilizer inputs reported comparable outcomes: yields increased by about 20% across two growing seasons, the proportion of marketable fruits improved, fruit nutrient content increased, and the incidence of blossom-end rot was significantly reduced [97].

In hydroponic cultivation of cucumbers and other cucurbit crops, biostimulants exert multifaceted effects on yield and quality. Zamljen, Šircelj, Veberič, Hudina and Slatnar [98] observed that application of brown algae extracts (Phylgreen^®, Fitostim^®) slightly reduced yield (approximately 13%) but significantly improved fruit quality, including deeper skin coloration, higher dry weight in early harvested fruits, and increased chlorophyll a and b as well as α-tocopherol content. Similarly, García-Cano, Ferrández-Gómez, Jordá, Pablo, Cerdán and Sánchez-Sánchez [99] demonstrated in growth chamber trials that treatment with Lombrico^® ARREL, containing amino acids and seaweed, significantly promoted cucumber plant height, leaf number, and biomass. Fresh root weight and dry root weight increased by up to 97% and 53%, respectively, indicating its potential for later fruit set [99]. In substrate-based hydroponics using coco peat, putrescine also proved to be an effective biostimulant, increasing cucumber seedling vigor indices by 54% and improving stem thickness, leaf area, and shoot fresh weight [100]. Furthermore, for hydroponic melons under stress conditions, the foliar application of nicotinamide (vitamin B3) (300 mg·L⁻¹) and Azospirillum sp. (2 mL·L⁻¹) significantly enhanced the net CO₂ assimilation rate by 20%, the fresh fruit weight by 12%, and increased chlorophyll content as well as total soluble solids, showing that these two biostimulants are very promising in salt stress management and the improvement in hydroponic melon production [101]. In fruiting vegetable crops, the reported gains in fruit set, yield, and quality are generally associated with improved reproductive stability under stress, stronger antioxidant and photosynthetic maintenance, and more efficient assimilate partitioning toward developing fruits.

3.4. Berries

Recent studies on the soilless cultivation of berry crops reveal that the choice of cultivation system, the application of biostimulants, and precise nutrient management are three core factors determining final yield and quality. Research clearly indicates that in hydroponic strawberry production, substrate-based systems utilizing media like coco coir and perlite significantly outperform pure water-culture methods such as NFT and aeroponics in terms of yield and plant biomass. For instance, in an indoor substrate-based study using ‘Cabrillo’ strawberry, foliar potassium silicate applied at both 50 and 100 mg·L⁻¹ under three temperature regimes (20, 24, and 28 °C) reduced time to first flower by approximately 29% and increased fruit set by 47% specifically at 28 °C [106]. Similarly, in ‘Malwina’ strawberries, the application of biostimulants such as TriBoost and Hicure increased fruit weight by up to 37% even under a 50% irrigation regime, highlighting their capacity to sustain productivity under water-limited conditions [107].

In addition to cultivation systems, biostimulants play a key role in stress mitigation. Under heat stress, formulations containing glycine betaine or kelp extract increased raspberry photosynthetic rate by up to 70% and enhanced anthocyanin accumulation by more than 200%, demonstrating substantial improvements in physiological resilience [104].

Biostimulants also contribute significantly to fruit quality enhancement in hydroponic strawberries. Under nutrient-limited conditions, foliar application of mixed amino acids, Zn–amino acid complexes, and B vitamins increased total soluble solids by 23%, 13%, and 10%, respectively, relative to the untreated control [102]. Chitosan application improved fruit firmness by about 18%, correlating positively with extended shelf life. These improvements are attributed to biostimulant-mediated regulation of fruit metabolism, including chitosan-induced reinforcement of cell wall structure and amino acid-stimulated expression of sugar-transport proteins [102].

Moreover, inoculation with AMF not only mitigated selenium toxicity in hydroponic strawberries but also improved nutritional quality, increasing total phenolic content by 48.4%. These findings underscore the importance of precise nutrient management in soilless berry systems. While low selenium concentrations can serve as effective biofortification agents, excessive levels (e.g., 10 mg·L⁻¹) proved detrimental, causing up to a 42.4% reduction in fruit firmness and inducing oxidative stress [103]. In berries, these effects appear to be mediated mainly through improved photosynthetic performance, better osmotic or oxidative stress regulation, and changes in fruit metabolism related to sugar accumulation, cell wall integrity, and antioxidant synthesis.

4. Advantages of Biostimulants in Hydroponics

4.1. Abiotic and Biotic Stress Mitigation

The major abiotic and biotic stresses relevant to hydroponic production are summarized in Figure 5. Among these, salinity stress is one of the major abiotic stresses that affects crop production. It can also occur frequently in hydroponic cultivation due to improper management of the nutrient solution, particularly for long-term production crops like tomatoes, peppers, and berries, and in recirculating systems [108]. Salt stress results in increased osmotic pressure surrounding plant roots, leading to ion toxicity and impaired uptake of water and nutrients and thus restricted plant growth [109]. Previous studies demonstrated that applying brown seaweed (Ascophyllum nodosum) extract as a biostimulant enhanced antioxidant enzyme activities and improved cell membrane stability in tomato (Solanum lycopersicum), which effectively alleviated salt stress damage [110]. These findings suggest that integrating biostimulants into hydroponic nutrient management can buffer crops against salinity fluctuations, improve plant resilience, and contribute to more stable productivity in soilless systems.

Although drought stress happens rarely in hydroponic culture systems, high temperature and low air current speed can adversely affect plants’ water homeostasis, reduce transpiration, and trigger osmotic stress. Extreme temperatures, including both heat and cold, severely impair photosynthesis, damage cellular membranes, and increase the production of reactive oxygen species. Benefiting from hydroponic systems that are typically set up in controlled environments, temperature fluctuations are generally more manageable than in open-field systems; however, challenges can still arise. For example, Dash, Guo and Leskovar [95] reported that foliar application of the arbuscular mycorrhizal biostimulant MycoApply^® for twelve consecutive weeks alleviated heat stress in organically hydroponically grown tomato (Solanum lycopersicum ‘Valdeon RZ’). This treatment increased net photosynthetic rate by 20%, stomatal conductance by 40%, and fruit-set rate by 19%. It also reduced electrolyte leakage by 31% and increased yield by approximately 30% [95]. The application of the amino acid-based biostimulant proline also strengthens plant heat tolerance by enhancing antioxidant responses and accelerating cellular repair processes [110,111,112,113,114].

In addition to temperature-related stresses, micronutrient imbalance, especially Fe deficiency, represents another significant abiotic constraint in hydroponic production. Traditional Fe chelates such as Fe-EDTA and Fe-DTPA rapidly lose stability under high pH and degrade under greenhouse lighting, forming insoluble Fe precipitates that ultimately cause Fe deficiency chlorosis in plants [115,116,117]. These issues highlight Fe deficiency as a critical bottleneck in hydroponic nutrient management. These findings indicate that PGPR can function as Fe-mobilizing biostimulants in hydroponics, complementing chemical chelating agents to alleviate Fe deficiency stress.

Beyond reducing abiotic stresses, biostimulants also play an important role in enhancing resistance to biotic stresses. Among these, PGPR protect plants through two main strategies. First, Pseudomonas and Bacillus strains (such as P. putida, P. fluorescens, and Bacillus subtilis) directly suppress pathogens in hydroponic production of cucumbers and lettuce by synthesizing antibiotics, chitinase, and Fe carriers [118,119]. For instance, strains from these genera constitute key components of highly effective composite biocontrol agents against Fusarium wilt in cucumbers and Pythium root rot in lettuce [120,121]. Second, strains including Pseudomonas putida and P. fluorescens have been demonstrated to induce systemic immune response in hydroponic systems. This process enhances the accumulation of lignin and phenolic compounds (alongside phytoalexins), thereby further strengthening plant defences [118,122,123]. Additionally, the application concentration and inoculation method of PGPR critically influence efficacy. Although no universally accepted optimal concentration exists, extensive research confirms that successful application of specific PGPR strains or composite inoculants consistently improves root function, nutrient uptake, and yield in hydroponic crops such as tomatoes, cucumbers, and lettuce [119,124].

Besides PGPR, other compounds also enhance plant defences when combined with microbial biostimulants. Notably, the fungus Aspergillus niger and the plant hormone methyl jasmonate (MeJA) are known to act as elicitors, enhancing total phenolic contents by more than 30% and boosting chlorophyll carotenoid accumulation in Silybum marianum (milk thistle) when cultivated hydroponically [125].

Biostimulants may also contribute to integrated pest management by reducing pest pressure and altering host suitability. In hydroponic tomato seedlings, root application of rosemary essential oil, Bacillus subtilis, Trichoderma harzianum, and microalgae significantly reduced aphid nymph abundance at 120 h compared with the untreated control, with reductions ranging from approximately 81% to 100% depending on the product [126]. In addition, studies in cucurbit crops showed that organic biostimulants reduced both the attractiveness and host suitability of potyvirus-infected plants to aphid vectors [127]. Overall, these findings suggest that biostimulants can serve as useful complementary tools in integrated pest management programs, although they should be integrated with other pest management strategies rather than used alone.

4.2. Biostimulants Are Needed for Hydroponic Organic Production

In the United States, the National Organic Program permits the certification of hydroponic production systems as organic, provided they comply with established organic management standards [128]. In contrast, countries such as the European Union, Canada, and Australia require that organic farming must rely on soil-based systems, therefore hydroponics is not certified as organic [129,130]. This policy divergence has established the United States as a primary hub for both research and commercial deployment of organic hydroponics, particularly within controlled environment agriculture and vertical farming [131,132]. Compared with conventional hydroponics, organic hydroponic systems not only aim for productivity and resource efficiency but also emphasize sustainability and the biological conversion of nutrients.

Unlike conventional hydroponic systems, organic hydroponic systems do not rely on readily soluble inorganic salts. Instead, they use nutrient sources derived from animal manures, fish byproducts, compost extracts, algae, vermicompost teas, vinasses, and food waste [133,134,135,136,137,138,139,140,141]. The nutrients in these materials exist largely in complex organic forms that roots cannot directly absorb and must be mineralized by microorganisms before uptake. However, hydroponic systems lack a soil-like microbial community, which slows nutrient mineralization. This often leads to N deficiency, ammonium and nitrite accumulation, and instability in pH and EC. Together, these problems can reduce plant growth and impair system performance [142]. Furthermore, decomposition of organic materials can deplete dissolved oxygen, and insufficient aeration may inhibit root respiration and aggravate nutrient imbalance. For example, in a deep flow hydroponic system where corn steep liquor was added as an organic fertilizer, doubling the N rate significantly reduced bok choy biomass, potentially due to dissolved oxygen depletion and a mismatch between nitrification rate and plant N uptake [143].

Microbial biostimulants (such as PGPR and AMF) can significantly accelerate the conversion of nutrients from organic to inorganic forms, thereby bridging the gap between organic nutrient sources and plant nutrient requirements [144,145,146]. Following this trend, the application of biostimulants has become a critical component of organic hydroponic systems in the United States. Microbial biostimulants, particularly PGPR such as Azospirillum brasilense and AMF like Rhizophagus intraradices, promote the mineralization of organic N, P, and K, enhance nutrient uptake by modulating phytohormone synthesis and signalling, and improve rhizosphere stability [147,148,149,150,151,152]. Recent studies have also shown that some PGPR can stimulate plant metabolism by releasing volatile organic compounds, providing new insight into microbial regulatory mechanisms in hydroponic systems [153]. However, in highly controlled environments, the effectiveness of microbial inoculants can be limited because certain strains may compete with plants for key resources such as nitrate and oxygen, thereby reducing or counteracting their growth-promoting potential [129].

Several studies have demonstrated that combining microbial inoculants with organic nutrient sources can significantly enhance organic hydroponic performance. For instance, in an indoor vertical hydroponic system, organic fertilizers formulated from corn steep liquor and fish byproducts, when combined with Azospirillum and Rhizophagus inoculation. This combination improved N uptake efficiency and increased micronutrient concentrations such as Fe and Zn. However, total biomass remained lower than that under inorganic solutions [142]. Similarly, Antón-Herrero, et al. [154] reported that an organic-mineral hybrid biostimulant promoted root growth and N assimilation in the short term. Under saline hydroponic conditions, İkiz, Dasgan, Balik, Kusvuran and Gruda [25] noted that the application of biostimulants such as amino acids, FAs, chitosan, PGPR, AMF, and vermicompost effectively mitigated salt stress and improved lettuce yield and quality.

In summary, microbial biostimulants play an indispensable role in organic hydroponic systems. They not only promote nutrient mineralization and release but also regulate the rhizosphere ecology, improve nutrient absorption, and enhance plant adaptability to environmental stress. Their effectiveness depends on microbial strain, application method, and environmental conditions such as dissolved oxygen levels, pH stability, and crop growth stage. Optimizing microbial biostimulant application is therefore essential for improving productivity, nutrient efficiency, and long-term sustainability in organic hydroponic production.

4.3. Post-Harvest Quality Improvements

Hydroponic production methods offer a promising approach to maintaining post-harvest quality, primarily because they eliminate soil as a source of microbial contamination and reduce the risk of soil-borne pathogens. When combined with the use of treated or disinfected water for nutrient solution preparation, the system can further limit the initial microbial load, thereby reducing spoilage risk and slowing quality deterioration [5,155,156]. Furthermore, hydroponically grown produce can be combined with other postharvest technologies, such as cold storage or modified atmosphere packaging. These approaches can prolong shelf life while causing minimal changes in nutritional and sensory attributes [157].

In addition to the inherent advantages of hydroponics, biostimulants can further improve postharvest quality. For example, in hydroponic tomato cultivation, preharvest treatments with seaweed extracts and amino acid-based biostimulants significantly increased soluble solids, antioxidant activity, ascorbic acid, and carotenoid concentrations, thereby improving both flavor and nutritional value [93]. In citrus, foliar applications of Si-Ca-based formulations delayed maturation, maintained peel firmness, elevated antioxidant content, and reduced decay during cold storage [158]. Leafy vegetables also show positive responses with combined preharvest applications of Ascophyllum nodosum extracts and HA reduced fresh weight loss in lettuce and spinach during storage while enhancing antioxidant capacity [159]. Similarly, Moringa oleifera leaf extracts improved storability and increased resistance to fungal decay in hydroponic lettuce, performing comparably to commercial protein hydrolysates [87]. In addition, supplementation with Ecklonia maxima extracts delayed senescence and preserved marketable quality of hydroponic lettuce for up to 21 days of cold storage [53]. Emerging compounds such as melatonin have also been proposed as novel biostimulants due to their roles in scavenging reactive oxygen species, enhancing antioxidant enzyme activity, and delaying senescence during storage [160].

4.4. Economic & Environmental Benefits

Combining biostimulants with hydroponics can increase crop yields and quality and therefore generate greater economic returns for growers. Experiments show that plant-derived protein hydrolysates increased marketable fresh yield by up to 82.7% in green ‘Ballerina’ lettuce and 55.4% in red ‘Canasta’ lettuce compared with untreated controls [161]. These can decrease water usage and associated costs, resulting in an integrated economic and environmental benefit. In addition, using biostimulants in hydroponic systems can lead to more efficient nutrient uptake, which reduces the need for chemical fertilizers and benefits the environment.

More efficient nutrient utilization also supports a lower-carbon production model, with studies showing that biostimulants can reduce CO₂-equivalent emissions by 7–12% in zucchini and 7–24% in spinach production [162]. Furthermore, integrating microalgae and cyanobacteria as biostimulants promotes a circular economy by utilizing nutrient-rich hydroponic wastewater as a substrate for cultivating algal biomass [163,164]. This process serves as a form of bioremediation, as some microalgae strains can achieve 100% removal of N and P from this wastewater [164]. The resulting biomass can then be processed into new biostimulant formulations, effectively converting agricultural effluents into value-added products and further reducing the system’s environmental footprint.

Integrating nutrient management techniques with biostimulant application can significantly improve crop yield and resource utilization, thereby strengthening the sustainability of hydroponic production systems. For example, Rajabi Hamedani, Rouphael, Colla, Colantoni and Cardarelli [162] conducted a life cycle assessment on greenhouse-grown vegetables and found that applying biostimulants significantly reduced their carbon footprint. In zucchini, using a mycorrhizal fungus biostimulant lowered the global warming potential by 7% to 12%, while in spinach, foliar application of a vegetal-derived protein hydrolysate reduced the carbon footprint by 7% to 24%. These environmental benefits were primarily attributed to increased crop productivity associated with biostimulant application. By further enhancing crop yield, typically in the range of 5% to 10%, biostimulants derived from agricultural and industrial byproducts can also help reduce production costs, further turning agricultural systems into greener and more economically viable practices [165].

5. Challenges and Future Perspectives

5.1. Knowledge Gaps

One of the major challenges for using biostimulants in hydroponics is the lack of dosage recommendations specifically designed for soilless systems. Most manufacturers provide guidelines based on conventional field cultivation, usually expressed as volume or weight per unit land area, which do not account for the recirculating dynamics and dilution patterns of nutrient solutions in hydroponic systems. As a result, growers are left without clear instructions on appropriate dosage, application frequency, or target concentration, creating uncertainty in practice [166,167]. In addition, the technical design of hydroponic systems can further limit the effectiveness of biostimulants. For instance, microbial inoculants may be inactivated by UV sterilization or high-pressure pumping. In addition, filtration systems may remove or degrade macromolecules and particulate components. These processes can reduce their persistence and biological activity in the nutrient solution [168,169].

Effectiveness also varies considerably among crops, making broad recommendations difficult. Leafy vegetables tend to respond more quickly with improvements in chlorophyll accumulation and photosynthetic efficiency, whereas fruiting crops rely more on carbon–N balance and antioxidant regulation to sustain yield and quality [170,171]. Even within the same crop, varietal differences in root architecture, exudation, and gene expression profiles can lead to highly inconsistent outcomes [12]. Therefore, future research should focus on developing system-specific application strategies that account for the interactions among biostimulant formulation, hydroponic system design, and the physiological and genetic traits of different cultivars.

5.2. Commercialization Barriers

Adoption of biostimulants in hydroponic systems is often limited by their comparatively high cost relative to conventional fertilizers [166,171]. This challenge is compounded by the lack of a standardized regulatory framework, which results in inconsistent data and limited third-party validation of manufacturers’ claims [12,167]. Consequently, growers face uncertainty about whether biostimulant application will reliably translate into yield or quality improvements, making cost-benefit assessments difficult and slowing adoption.

To address these issues, future research should focus on applied techno-economic analyses that improve energy efficiency, enhance nutrient recycling, and develop low-cost, high-performance biostimulant formulations tailored to hydroponic environments. Establishing clear regulatory standards, certification frameworks, and transparent labelling for both hydroponic practices and biostimulant products would strengthen market credibility, increase grower confidence, and support the long-term commercial viability of the hydroponic industry.

In addition to financial barriers, regulatory hurdles also limit commercialization. These include the lack of a clear designation for hydroponic produce, the absence of clear standards and certification systems, and fragmented oversight by different agencies. Approval requirements related to water use, discharge, and facility construction further complicate adoption [172]. Furthermore, adoption among farmers is hampered by a lack of technical knowledge and the perception that the investment may be too risky for these agrarian families. Compounding the problem, there are also gaps in localized training programs to reach small farmers in such areas. Ultimately, even with growing consumer interest, price sensitivity remains a key barrier to widespread commercialization in some markets [173]. To overcome these social and institutional challenges, coordinated actions are needed at both policy and community levels. Governments and research institutions should provide technical guidance, financial support, and risk-sharing programs to assist small growers in transitioning to hydroponic systems. Strengthening extension services, demonstration events, and farmer-to-farmer knowledge exchange can help translate research outcomes into practical production protocols. Moreover, public–private partnerships and consumer education initiatives can enhance market access, foster trust in hydroponic and biostimulant-based produce, and support the sustainable expansion of the industry.

5.3. Emerging Trends

Through advances in technology, hydroponic systems have started integrating artificial intelligence (AI) and synthetic biology, allowing for more precise, data-driven regulation of environmental factors and crop inputs. Among these technologies, AI-enabled biostimulant optimization has emerged as a major research focus. By combining multi-source data, including plant phenotypes, metabolomics, and environmental monitoring, AI can generate predictive models for biostimulant use. These models can optimize both formulation selection and spraying schedules [174,175]. For instance, Internet of Things (IoT) sensors and computer vision-based smart systems can be developed to monitor plant growth phases and nutrient solution composition in real time, while machine learning models can be trained to predict how a plant will react to a specific biostimulant, enormously enhancing the nutrient use efficiency and yields of hydroponic crops [174].

Meanwhile, clustered regularly interspaced short palindromic repeats (CRISPR)-edited bio-stimulatory microbes are also gaining attention in hydroponic research. Since the natural microbiome of hydroponic systems is extremely limited, researchers are also interested in genome editing approaches that could enable beneficial microbes to be optimized further to promote plant growth and enhance nutrient transfer [176]. For example, CRISPR/Cas9 can be utilized to introduce N fixation gene clusters into non-symbiotic species, which can provide a consistent supply of N to crops under hydroponic practice, or to improve the capacity of Bacillus strains to synthesize plant hormones (e.g., auxins, cytokinins) to facilitate root growth and stress resilience [176]. Furthermore, engineered strains can be designed to tolerate concentrated nutrient solutions, enhancing their versatility under hydroponic conditions [163]. Despite still facing regulatory and biosafety concerns ahead of large-scale commercialization, the intersection of AI and CRISPR-based microbial biostimulants is positioned to form a core pathway for next-generation precisely controlled environment agriculture.

5.4. Policy and Sustainability Implications

Hydroponics supports urban agriculture and the circular bioeconomy in several ways. It enables space-efficient crop production in rooftops, warehouses, and vertical farms. It also reduces land demand and shortens supply chains, which can strengthen local food self-sufficiency [177]. It can also reduce pesticide inputs and save up to 90% of water compared with field cultivation [7,9]. However, regulatory frameworks for indoor and vertical farming are often limited, creating barriers related to zoning, building codes, and food safety guidance [177]. The application of biostimulants, particularly microbial ones, poses a specific challenge for hydroponic food safety. Ensuring the safe use of these products within hydroponic systems will require coordinated management and monitoring practices that align with existing food safety standards.

From a circular bioeconomy perspective, hydroponics fosters production as a closed cycle, with the inclusion of further resource recovery leading towards zero waste [178]. For example, recent studies have demonstrated the possibility of using microalgae production with high-nutrient-content hydroponic effluent, which can eliminate N and P pollutants while producing raw materials for biostimulants [163]. In addition, hydroponic crop residues can be processed into compost or biochar to enhance the microbial substrate and serve as a carbon sequestration source [178]. In the future, policy coordination between sectors, linking waste management strategies with agriculture initiatives on one hand and economic incentives for resource reuse on the other, will be instrumental in integrating hydroponics into urban agriculture and the circular bioeconomy, thereby promoting sustainability goals.

6. Conclusions

Overall, the use of biostimulants in hydroponics has been shown to enhance crop yield and quality, improve water and nutrient use efficiency, and strengthen plant resilience. Beneficial microorganisms that promote root establishment and growth, seaweed extracts that regulate plant physiological processes, protein hydrolysates, humic substances, and other biostimulant classes have all demonstrated significant impacts on productivity, quality attributes, and resource savings across a variety of crops. In addition, these applications in hydroponics, together with advanced hydroponic environmental control technologies such as targeted nutrient control, climate control, and automated operations, can have potential to achieve efficient and low-waste cultivation systems.

Although these promising findings, the application of biostimulants in hydroponic systems still faces several significant challenges. Due to the lack of specific dosage recommendations for hydroponic systems, it is difficult to directly apply guidelines from conventional production systems. Furthermore, different crops and varieties often respond differently to biostimulants; combined with variations in product composition, application methods, and study designs, this makes direct comparisons across studies difficult. Therefore, there is an urgent need for more systematic research on hydroponic systems and the establishment of clearer reporting standards to advance future development and commercial applications in this field.

In the future, the design of biostimulants for sustainable hydroponics will be more targeted and varied. It is also likely that, with the help of AI-optimized and CRISPR-edited microorganisms, technology can “customize” application methods according to the real-time conditions of the crop to maximize response effects and minimize waste. Circular bioeconomy approaches, including the conversion of nutrient-rich hydroponic effluent into new biostimulant products, present a further opportunity to reduce environmental impacts and close resource loops. To make this vision a reality, multi-crop studies and longer-term research can help address relevant knowledge gaps on biostimulants and their responses in different crop types and environments. The industry needs to develop efficient, easy-to-use, and cost-effective products, and policymaking plays a crucial role in providing transparent legislation and financial incentives, along with integrating hydroponics into urban agriculture policies. With close collaboration between science, business, and policy, biostimulants in hydroponics can be a key component in building a more resilient and sustainable food system.