Applied Heat-Stress Mitigation Strategies in Vegetable Crops: Toward Integrated Field-Scale Approaches

Abstract

Rising global temperatures and recurrent heat waves increasingly threaten vegetable production, as vegetable crops are more thermosensitive than most field crops. Vegetable crops frequently experience severe reductions in pollen viability, fruit set, marketable yield, and quality under heat waves. Numerous reviews have substantially advanced our understanding of heat stress perception, signal transduction networks, transcriptional regulation, and thermotolerance mechanisms, primarily in model species and major field crops. However, comprehensive review articles of field-applied mitigation strategies specifically tailored to vegetable production remain limited. This review provides a critical analysis of the use of genetic resources (cultivars and grafting), field management approaches (optimized planting dates, crop rotation, canopy management, and intercropping), irrigation, nutrient optimization, biostimulants, microbial inoculants, and physical microclimate modification strategies. The research consolidates current applied and mechanistic evidence on heat-stress mitigation in vegetable crops and identifies targeted, actionable priorities for field adoption. Emphasis is placed on the integration of complementary mitigation strategies at the field scale where combined approaches may generate synergistic effects. Key research gaps include limited studies on combined heat–drought/salinity stress, lack of standardized field protocols for biostimulants, and insufficient farm-scale economic evaluations of mitigation strategies. Advancing interdisciplinary, field-validated, and climate-smart management frameworks will be essential to ensure sustainable vegetable productivity and quality stability in accelerating global warming.

1. Introduction

Climate change is intensifying the frequency and severity of heat waves and extreme temperature events. Concurrently, rapid urbanization and population growth have exerted increasing pressure on agricultural systems, positioning climate change as a major threat to global crop productivity [1]. Recent climate prediction assessments by the World Meteorological Organization indicate a high likelihood that annual global temperatures will temporarily exceed the 1.5 °C threshold at least once during the 2024–2028 period, with an approximately 80% probability of such an event and a substantial chance of the five-year average climbing above this level [2]. Meta-analyses indicate that global yields of major field crops decline by approximately 3.1–7.4% per 1 °C warming [3], and recent empirical evidence shows that these losses persist even after accounting for real-world farmer adaptation [4]. Vegetable crops exhibit narrow thermal tolerance ranges compared to field crops, making them exceptionally vulnerable to elevated temperatures during critical phenological stages such as flowering and fruit set [1,5].

Elevated temperatures adversely affect plant physiology. Photosynthetic rates decline sharply beyond optimal temperatures (24–34 °C for many vegetables) [6], as heat inactivates enzymes and damages chloroplast membranes [7]. Stomata may close prematurely on hot days to conserve water, limiting CO₂ uptake and hence photosynthesis [7,8]. Respiration rates increase with heat, leading plants to consume more sugars at night, which reduces the net assimilate available for growth and yield [9,10]. Prolonged heat causes oxidative stress and cellular damage [11]. High soil temperatures can impair root function and nutrient uptake, especially under dark plastic mulches that exacerbate soil heating [12,13]. In essence, heat stress derails normal metabolic balance. Plants under heat divert energy to stress response (producing heat shock proteins, antioxidants, etc.) rather than growth and yield.

The reproductive stage is particularly vulnerable. Many vegetable crops experience reduced pollen viability and anther dehiscence in hot weather. Pollen germination on stigmas can fail above certain temperature thresholds, leading to fertilization problems [6]. As a result, flowers may abort or fruits may drop shortly after setting under heat stress. Even when fruits are successfully set, high temperatures can impair normal ripening and coloration by suppressing anthocyanin biosynthesis and can also induce sunscald on fruit skins, as observed in tomato [14]. Also, heat can accelerate bolting (premature flowering) in crops like lettuce, causing bitterness and loss of quality [15]. Moreover, cucumber plants exposed to heat stress conditions may produce deformed fruits with drastically reducing marketable yield [16]. Overall, excessive heat stress reduces fruit set and marketable yield and frequently results in smaller, misshapen, and lower-quality produce. Key quality attributes, including taste, color, texture, and nutritional value, are often adversely affected, thereby diminishing both consumer acceptance and economic value. These impacts underscore why mitigating heat stress is critical.

Although heat stress affects all crops, the nature of the response differs between field crops and vegetable crops. In major field crops, warming mainly reduces biomass accumulation and grain yield across vegetative growth and grain-filling stages, with meta-analytic evidence showing substantial yield penalties under elevated temperatures. By contrast, vegetable crops are generally more thermosensitive and more likely to suffer losses in marketable yield and quality because their edible organs are often formed during narrowly timed reproductive or harvest stages [17,18,19]. Consequently, relatively modest heat episodes can disrupt pollen viability, fruit set, bolting behavior, and organ quality in vegetables, making their management requirements distinct from those of staple field crops. These differences justify a vegetable-specific review that focuses on crop quality, reproductive success, and field-scale mitigation strategies tailored to horticultural production systems.

Over the past decade, major progress has been made in understanding plant responses to heat stress through molecular and physiological research. A substantial body of scholarly reviews has comprehensively described stress perception, signaling pathways, and tolerance mechanisms [20,21,22,23,24], particularly in major cereals and field crops [25,26,27,28,29]. In addition, several focused reviews have examined specific mitigation approaches, such as phytohormonal regulation [30] or seaweed-based biostimulants [31,32], contributing valuable insights into individual stress-alleviation strategies. However, despite this substantial progress, integrative reviews that consolidate applied mitigation strategies (e.g., cultivar selection, field management, fertilization, irrigation, biostimulant application, and physical protection) with an explicit emphasis on vegetable cropping systems remain comparatively limited. This gap is particularly notable with the heightened climatic sensitivity and high economic value of vegetable crops. Given the increasing frequency and intensity of heat events under climate change, addressing this gap is both timely and necessary. An integrated evaluation of applied mitigation strategies in vegetable production is essential to bridge the divide between research and practical field implementation.

2. Mitigation Strategies for Heat Stress in Vegetable Crops

Mitigation of heat stress in vegetable species requires an integrated, multi-layered strategy that spans genetic, physiological, agronomic, and technological interventions. Given the narrow thermal optima and reproductive-stage vulnerability of many vegetables, priority measures include the deployment of heat-tolerant cultivars and stress-adapted rootstocks, water and nutrient management, canopy and microclimate modification, and targeted biostimulant applications (Figure 1). Below, we organize mitigation strategies into major categories and detail the innovative techniques in each, emphasizing practical measures that improve yield and quality under high temperatures.

2.1. Cultivar Selection and Propagation Method

2.1.1. Heat-Tolerant Cultivars

Developing and deploying heat-tolerant varieties is a cornerstone of long-term mitigation. Breeders acknowledge that truly heat-proof varieties are still limited, and available cultivars are not sufficient to counteract severe heat stress in many cases [22,33]. In rice and wheat, substantial progress has been made in the identification and deployment of heat-tolerance genes; however, corresponding breeding and genetic pipelines in vegetable crops remain comparatively limited [34]. However, there are some notable successes, for instance, breeders evaluated 15 tomato genotypes under high-temperature conditions to identify superior parental lines, selecting four elite parents. These selected genotypes were subsequently screened for disease-resistance markers and used to generate 13 F₁ hybrid combinations. Performance evaluation under heat stress, employing a composite selection index integrating yield performance, fruit quality attributes, and disease resistance, identified seven heat-tolerant hybrids for advancement to multi-year validation trials [35]. Also, in both field and greenhouse trials in Cambodia, several tomato genotypes maintained reproductive success under heat: CLN1621L showed the most stable fruit set at high temperature, while CLN1621L, CLN2026D, CLN3212C, and KK1 delivered consistently higher fruit yield per plant; the local Neang Tamm had minimal yield reduction under heat, indicating useful tolerance [36]. In snap beans, certain varieties (e.g., ‘PV 857’, ‘Annihilator’) were found to set full pods and retain quality even when high night temperatures caused other varieties to abort seeds [37,38,39]. Another example for beans, the genotype HTA4 (heat-tolerant Andean-type 4) breeding line, under heat stress (31/24 °C Day/night), maintained high pollen viability with successful pollen adhesion, germination, and tube growth, thereby retaining filled pod number and higher seed weight per plant. These results indicate a reproductive-stage (pollen function) mechanism underpinning HTA4’s heat tolerance and support its use both for cultivation in hot regions and as a donor germplasm in breeding [34]. Lettuce breeders have released slow-bolting (more days to first anthesis), heat-tolerant types (e.g., Batavia-type lettuce like cv. Muir’ or romaines like ‘Dov’) that resist premature bolting and bitterness in warm conditions [40,41]. Also, for lettuce, across multi-environment trials (field and protected cultivation; hot seasons), BRS Mediterrânea cultivar consistently showed delayed bolting and superior adaptation under high temperatures versus comparator cultivars. These results support BRS Mediterrânea as a dependable cultivar for hot-region lettuce production [42]. Such cultivars offer farmers a first line of defense: by choosing heat-tolerant varieties, growers can achieve more reliable production during heat waves or in warm regions. A unified framework for evaluating heat tolerance in vegetable crops is needed to overcome inconsistencies among studies arising from differences in screening methods and evaluation criteria. Such a framework should integrate morphological, physiological, biochemical, reproductive, and agronomic traits to provide a comprehensive assessment of plant performance under heat stress. Key indicators include plant vigor, chlorophyll fluorescence, antioxidant activity, membrane stability, pollen viability, fruit set, and yield retention [17,23]. Since reproductive development is particularly sensitive to high temperatures, reproductive traits should be prioritized in screening programs. However, breeding for heat tolerance is challenging and time-consuming, since traits like pollen thermotolerance or membrane stability are complex and often polygenic [43,44]. It also requires extensive multi-environment testing and selection under naturally occurring heat stress conditions because heat tolerance is a complex, quantitative trait controlled by multiple physiological and genetic mechanisms. Modern breeding approaches offer opportunities to accelerate genetic improvement. Marker-assisted selection enables the efficient incorporation of favorable alleles and quantitative trait loci (QTLs) associated with traits such as pollen thermotolerance, membrane stability, and maintenance of reproductive success under high temperatures. Genomic selection further enhances breeding efficiency by using genome-wide marker information to predict breeding values for complex heat-tolerance traits, thereby reducing the number of breeding cycles required for cultivar development. In addition, wild relatives and landraces represent valuable reservoirs of adaptive alleles associated with stress resilience. The integration of these genomic and germplasm-based approaches with conventional phenotypic screening is expected to accelerate the development of heat-tolerant vegetable cultivars adapted to future climate conditions [34,45,46].

2.1.2. Grafting onto Stress-Tolerant Rootstocks

Grafting vegetables onto stress-tolerant rootstocks can confer increased vigor and heat/drought tolerance to the scion. This approach is widely used for soil disease resistance, and now research shows significant heat stress benefits too [47,48,49]. From a physiological perspective, the enhanced stress tolerance of grafted plants is primarily attributable to the inherent characteristics of the rootstock. Stress-tolerant rootstocks often possess more vigorous root architectures, greater root biomass, higher root hydraulic conductivity, and superior nutrient foraging capacity than susceptible genotypes. These traits improve water and nutrient acquisition under hot and dry soil conditions [50,51]. Grafting success requires careful technique and healing conditions (often carried out in specialized nurseries at 24–27 °C and high humidity for healing); ironically, grafts themselves do not like extreme heat during healing. Successful application of grafting also depends on adequate rootstock–scion compatibility, which should be verified before commercial deployment because incompatible combinations may reduce plant performance and graft survival [47,52].

However, once established, grafted plants can be a gamechanger, allowing growers to produce in hotter seasons that ungrafted plants could not tolerate [53].

Table 1. Effects of stress-tolerant rootstocks on the physiological performance, stress mitigation mechanisms, and yield stability of vegetable crops under high-temperature conditions.

Scion/Rootstock (Heat Stress Conditions)	Mechanism (How Tolerance Is Achieved)	Reference
Tomato cv. Celebrity and Arkansas Traveler/Solanum peruvianum (38/30 °C)	Higher antioxidant enzyme activity, chlorophyll content, and sustaining photosynthesis.	[48]
Tomato cv. 023 F1/interspecific hybrid (Maxifort, KFS-16) (42/32 °C)	Improved nutrient uptake and enzyme activity.	[55]
Tomato/goji berry (40 °C for 24 h)	Higher net photosynthesis, stomatal conductance, and Fv/Fm, along with elevated osmolytes (proline, sugars) and antioxidant enzymes. Heat-induced H2O2 and MDA were much lower in the grafted plants.	[56]
Cucumber/bitter melon (summer season)	Reduces photoinhibition and maintains a high photosynthetic via enhanced antioxidant defense.	[57]
Cucumber/sponge gourd (late summer season)	Enhances CO2 assimilation and metabolic enzyme activities, preserving redox balance and reducing heat-induced oxidative damage.	[58]
Cucumber/Cucurbita maxima × C. moschata (e.g., ‘VSS-61 F1’, ‘Ferro’) (38/22 °C)	Provided extensive roots for better water and nutrient uptake, sustaining photosynthesis under combined heat and salinity and yielding the highest marketable output.	[59]
Cucumber/hybrid squash (VSS-61 F1, Ferro, or Super Shintoza) (38/24 °C)	Increased growth and total yield.	[60]
Pepper/A57 rootstock (38/22 °C)	Yielded lower membrane leakage and stable chlorophyll content. Accumulated more ascorbic acid and phenolics, and their anthers had higher proline, resulting in better pollen germination and fruit set.	[61]
Sweet pepper/tolerant pepper rootstocks (A6, A25, A57) (38/24 °C)	Maintained higher growth rate, larger leaf area, and chlorophyll fluorescence, with lower electrolyte leakage. More marketable fruit.	[62]

summarizes reported examples of grafting that enhances heat-stress tolerance in various vegetable crops. Each entry lists the scion species with its heat-tolerant rootstock, and the resulting benefit under high temperature. Currently, the cost of grafted transplants is higher, yet for high-value crops like greenhouse tomatoes, the ROI (Return on Investment) can be justified by yield stability [54]. We should expect to see more heat-resilient rootstock options become commercially available, as research confirms their benefits in different vegetable species.

2.2. Field Management Practices

2.2.1. Planting Dates

One straightforward strategy is to avoid the hottest period for sensitive growth stages. By shifting planting or transplanting dates, growers time flowering and fruiting to occur in cooler parts of the season. Successful “heat-escape” scheduling was reported across vegetable crops. For example, field trials in tomato have demonstrated that earlier spring transplanting (e.g., April) consistently results in higher yields by shifting the flowering and fruit-set stages away from early-summer heat peaks, whereas late plantings (June) experience substantial yield reduction due to exposure to elevated temperatures during anthesis [63]. Similarly, lettuce production underscores the importance of aligning cultivar selection and sowing period with cooler thermal periods. Evidence from cultivar × temperature interaction studies and breeding trials indicates that head formation and bolting should be scheduled to avoid high-temperature episodes to stabilize yield, maintain head uniformity, and preserve market quality [64]. Field studies in pepper have demonstrated significant cultivar × planting-date interactions under high-temperature conditions, whereby shifting planting to locally optimized sowing periods enabled crops to avoid peak thermal stress during flowering, resulting in improved fruit set and higher yields [65]. In cauliflower, field trials in which crops were exposed to elevated temperatures through early-summer sowing demonstrated pronounced reductions in yield and quality, whereas delayed sowing under cooler conditions avoided heat stress during curd initiation and significantly improved crop performance [66]. In okra, multi-date field trials conducted in Egypt and India have demonstrated that elevated temperatures significantly constrain vegetative growth and fruit quality, whereas the selection of earlier or locally optimized sowing dates enhances yield performance compared with late sowings exposed to higher thermal stress [67]. Likewise, short-duration or early maturing cultivars can be chosen to ensure harvest before the hottest weeks. While changing planting dates can help “heat-escape,” it must be balanced with market timing and other constraints. Using local climate forecasts and heat accumulation (growing degree day) data, farmers can plan cropping calendars that minimize the overlap of sensitive stages (e.g., pollination) with expected heat waves. This strategy does not reduce ambient temperatures but mitigates damage by strategic timing.

Effective planting date management requires reliable predictive tools. Growing degree day (GDD) models offer a straightforward framework for estimating the timing of phenological stages relative to accumulated thermal units, enabling growers to calculate whether critical stages such as anthesis will coincide with historically high-temperature periods [68]. Emerging IoT-connected weather station networks and machine-learning-based seasonal temperature forecast models further improve real-time planting calendar adaptation. However, important challenges limit the accuracy of these tools: inter-annual climate variability, the increasing frequency of unpredictable heat wave events, limited availability of local calibration data for many vegetable cultivars, and the computational complexity of downscaling global climate models to field-level resolution all reduce predictive precision. Future development of crop-specific, region-calibrated phenological models integrated with high-resolution seasonal climate forecasts will be essential to realize the full potential of timing-based heat-escape strategies at scale.

2.2.2. Crop Rotation

Growing different crop species in succession on the same field is traditionally used for pest and disease management, but it also confers benefits that improve crop tolerance to heat and drought stress [69]. The contribution of crop rotation to heat-stress mitigation is primarily indirect and operates through improvements in soil health and resource availability. The physiological effects of rotation are not limited to yield stabilization; they reflect a broader reprogramming of the rhizosphere and soil resource environment that can either favor or constrain the subsequent crop depending on species choice, climate, and management [70]. A central mechanism is nutrient complementarity, especially through legume integration. Legumes host symbiotic bacteria in root nodules that convert atmospheric N₂ into ammonia via nitrogenase, after which nitrogen is transformed into plant-available forms such as ammonium. This biochemical pathway reduces dependence on synthetic nitrogen inputs while raising the nitrogen status of the rotation system. Reported fixation values vary with species and environment, ranging from about 10 to 217 kg N/ha in a single season [70,71]. This complementarity extends beyond nitrogen. Crops differ in root depth and nutrient acquisition patterns, so rotations redistribute nutrients through the soil profile rather than exhausting a single layer. Deep-rooted crops such as sunflower and alfalfa can mobilize phosphorus and potassium from deeper horizons and deposit them near the surface after residue decomposition, making these nutrients available to shallow-rooted successors. In parallel, fibrous-rooted crops help stabilize soil particles, while tap-rooted crops such as carrots and radishes break compacted layers, improving porosity, drainage, and root penetration [70]. These root-mediated effects increase water-holding capacity, aeration, and microbial activity, thereby improving nutrient foraging and overall nutrient use efficiency in subsequent crops.

The primary benefits of crop rotation for heat stress come indirectly via improved water and nutrient availability. Diversified rotations with legumes, cover crops, or deep-rooted crops can enhance soil structure and water-holding capacity, thus buffering crops against high temperature extremes [72,73,74]. By ensuring a richer moister soil environment, rotations allow vegetable crops to maintain hydration and cooling transpiration during heat stress. Adequate soil moisture availability sustains key physiological processes, including photosynthesis, transpiration, and nutrient transport, which would otherwise be impaired under heat stress conditions. Nutrient dynamics are also enhanced in rotations (e.g., nitrogen from legumes, deeper mining of minerals by diverse roots), which can improve plant vigor and heat tolerance [69,75,76]. For instance, a study in China found that replacing continuous monocropping with a maize–potato rotation increased the soil water storage in the profile, reduced evaporation, and raised water use efficiency of the system [69,77]. With more water retained in the soil, crops in rotated fields experienced less drought and heat stress, maintaining higher yields even during heatwaves [69]. The mechanisms linking rotation to heat stress mitigation are largely soil-mediated. Rotations build soil organic matter and porosity, which improves infiltration and moisture retention in the root zone [69,78]. Another long-term study in North America showed that diversified crop rotations increased agricultural resilience to adverse growing conditions (like heat and drought) and that yields were more stable and suffered less decline under extreme weather compared to monocultures [69,79]. Overall, robust rotation contributes to a less fragile agro-ecosystem: studies report that even under drought and heatwave conditions, well-rotated fields can maintain yields significantly better than continuously cropped fields.

Despite these well-documented benefits, the adoption of diverse crop rotations faces significant barriers in practice. When designing crop rotations, certain cover crops may influence the establishment and early growth of succeeding crops through residue-derived allelochemicals; therefore, rotation schemes should be selected to maximize agronomic benefits while avoiding adverse effects on crop establishment [70,71]. Legume and non-legume cover crops used in rotation can also exert strong phytotoxic effects on subsequent vegetables. For example, sunnhemp (Crotalaria juncea L.) and rye residues reduced germination of lettuce, and sunnhemp leaf extracts completely inhibited germination of bell pepper, onion, okra, and tomato under controlled conditions [80]. Moreover, crop rotation adoption may be limited in some production systems by economic and management constraints. These factors should be considered when designing rotation-based strategies for climate-resilient vegetable production [72,73].

2.2.3. Planting Density and Canopy Management

The spacing and density at which vegetables are planted can significantly influence the crop microclimate and thus heat stress outcomes. Planting density affects canopy shading, airflow, and humidity [81]. Under higher planting density, a more continuous canopy develops, providing greater soil surface shading that reduces root-zone temperatures, while increasing canopy relative humidity and moderating the crop microclimate. This effect arises because increased leaf area enhances interception of incoming solar radiation, thereby limiting direct soil heating [81,82,83,84]. This approach has been noted to minimize sunscald injury on fruits. In field evaluations, tomato canopies managed with vertical trellising and reduced in-row plant spacing exhibited a significantly lower incidence of sunscald compared with wider spacings and less-structured canopies. The likely mechanism is increased foliar shading of developing fruits, which reduces direct solar irradiance and fruit surface temperature during peak heat periods, thereby mitigating sunlight-induced injury and preserving marketable yield [82]. In melon, late-season vine decline and premature canopy senescence reduce leaf area index (LAI) and fruit self-shading capacity, thereby increasing incident solar radiation on fruit surfaces and elevating fruit surface temperatures. The resulting thermal and radiative stress accelerates peel injury (sunburn) and promotes cracking that lower marketable yield [83]. However, there is a balance to strike: extremely high densities with poor airflow can trap heat and humidity, possibly increasing disease pressure or heat stress if ventilation is inadequate. Research in agronomic crops suggests there is an optimal intermediate density where yield is maximized, and microclimate is most favorable [81,84].

2.2.4. Intercropping

Intercropping mitigates heat injury through multiple interspecific physiological mechanisms that collectively moderate the canopy thermal environment. Structural shading by tall companion crops (e.g., maize) reduces incident photosynthetically active radiation and infrared loading on the vegetable canopy, directly lowering leaf surface and fruit-skin temperatures and alleviating photoinhibitory and sunscald injury [85,86]. Also, root niche complementarity occurs when companion crops with contrasting rooting depths utilize different soil layers, thereby reducing below-ground competition for water and nutrients and enhancing overall resource use efficiency [87,88]. Leguminous companion species contribute rhizosphere nitrogen via biological fixation and root exudation, enhancing soil fertility and supporting vegetable plant vigor. Additionally, volatile organic compound emissions and root-exudate-mediated rhizosphere interactions between companion species can modulate soil microbial communities, promoting plant-growth-promoting rhizobacterial populations that fortify the vegetable crop against abiotic stress [89] The combined effect of microclimate modification (reduced radiation, lower wind speed, higher humidity) and improved rhizosphere biology makes intercropping a biologically well-grounded and practically accessible strategy for heat stress management in vegetable systems. In hot-season, pepper–maize intercropping decreased sunscald incidence and altered the canopy microclimate relative to monoculture, directly linking the companion crop’s structural shade and wind buffering to reduced solar/thermal load on fruit [90]. Consistent evidence from Egypt reported that tall companion rows (maize) alongside tomato create localized shading and calmer air, which protects flowers and fruit during summer heat, stabilizes set, and improves marketable yield [91]. In the semi-arid region, intercropping and related alley-cropping designs measurably cool fields and raise relative humidity at canopy height (1.4 °C lower daily maxima, 8% higher RH, and 34% wind-speed reduction vs. open monocrop), effects that translate into lower evaporative demand and less heat desiccation around sensitive organs [92]. Overall, multiple studies conclude that intercropping moderate extreme air and soil temperatures and improving vegetable performance under climatic stress. This supports intercropping as a practical tool for microclimate management, not just a way to increase land-use efficiency [88,89,93].

2.3. Fertilization, Nutrient Management and Beneficial Elements

Optimal nutrition is key to helping vegetable crops tolerate heat stress. Mineral nutrients (macronutrients and micronutrients) are involved in virtually all physiological processes, and nutrition balance can enhance a plant’s stress defense capacity [94,95]. Nitrogen (N), for example, is crucial for maintaining high photosynthetic rates and building stress-protective molecules. Under heat stress conditions, N-deficient plants suffer greater photo-oxidative damage (ROS injury) than N-sufficient plants [96]. Experiments showed that plants given adequate N were able to utilize more of the absorbed light for photosynthesis and dissipate excess energy safely, whereas low-N plants accumulated excess light energy that led to oxidative stress at high temperature [96,97]. Also, ensuring vegetables have sufficient N (without excess) can improve their leaf canopy density and chlorophyll content, which in turn helps them cope with heat by maximizing shading and carbon fixation [94,96,97]. Potassium (K) is another critical nutrient for heat stress tolerance. K regulates stomatal opening and internal water balance. It is well documented that high K availability helps plants keep stomata functioning during heat stress, facilitating cooling via transpiration. K also plays a role in activating enzymes that detoxify reactive oxygen species generated under heat stress [98]. Supplying sufficient K significantly improved yield and stress outcomes in various crops under temperature stress [99,100]. Calcium (Ca) is important for heat stress because it stabilizes cell membranes and cell walls. Heat stress can cause membranes to become leaky, but Ca binds to membrane phospholipids and proteins, helping maintain integrity. Adequate Ca has been associated with better membrane thermostability and reduced ion leakage in heat-exposed tissues [94,101]. In vegetables, Ca is also linked to lower incidence of blossom-end rot, a disorder exacerbated by heat and water stress in fruits like tomato and pepper; managing Ca nutrition can thus indirectly reduce heat-related fruit damage [101,102]. Other nutrients play supportive roles. Magnesium (Mg) contributes to chlorophyll and enzyme activation in tomato [103,104], while sulfur (S) is a fundamental macronutrient incorporated into the amino acids cysteine and methionine and various coenzymes, thereby underpinning protein biosynthesis, enzymatic activity, and key metabolic processes in onion [105,106]. Micronutrients such as boron (B) support cell-wall formation, sugar transport, and pollen tube growth, while zinc (Zn) is a cofactor for many enzymes in many vegetable crops such as broccoli, sweet potato, and tomato [107,108,109,110,111].

There is also growing evidence that silicon (Si) and selenium (Se), though not essential, can bolster tomato and cucumber against heat stress. In tomato plants, exogenous Si has been shown to mitigate heat stress by raising relative water content and photosynthetic pigments while lowering oxidative markers, translating to better growth and yield under high temperatures [112]. In cucumber exposed to high temperature, foliar Si significantly raised total and marketable yields (approximately 36–40% vs. control) and improved leaf micronutrient status [113]. Exogenous supplementation with Se or Se nanoparticles under high temperature regimes enhances chlorophyll stability, maintains cellular hydration, and promotes biomass accumulation in tomato plants, thereby providing compelling evidence for a direct role in thermal stress mitigation [114]. Additionally, in cucumber, exogenous Se curtailed heat-induced ROS and lipid peroxidation and boosted growth and yield under high temperature [115]. Research studies illustrating nutrients and beneficial element management (dosage and application method) strategies that improve vegetable crop tolerance to heat stress are shown in

Table 2. Applied nutrients and beneficial elements for improving crop growth, physiological performance, and yield under high-temperature stress.

Treatment	Dose	Application	Crop	Effects (Mechanisms)	Reference
Urea/46% N (40/30 °C)	187.5–250 (kg/ha)	Soil fertilization	Tomato	Improved photosynthesis, Water-use efficiency and yield under 40 °C	[116]
Nitrogen Source: NA * (40/30 °C for 4 days)	1.3–2.6 (g/plant)	Soil fertilization	Tomato	Increased biomass, whereas too much N reduced tomato growth and recovery.	[117]
Potassium Nitrate (35 °C for 10 days)	50 mM	Seed priming	Carrot	Improved germination, seedling biomass, and root yield.	[9]
Calcium chloride (42 °C for 36 h)	0.5 mM	Foliar spray	Tomato	Yielded a higher photosynthetic rate, chlorophyll, transpiration, stomatal conductance, antioxidant enzyme activities, and helped stabilize cell integrity.	[118]
Calcium chloride (Summer season)	0.5% (w/v)	Foliar spray	Cowpea	Boosted growth rate, relative water content, chlorophyll, and yield.	[119]
Calcium chloride or Calcium nitrate (40/38 °C for 24 h)	1 mM	Foliar spray	Tomato	Raised PSII photochemical efficiency and CO2 assimilation. Ca-treated plants produced less H2O2 and superoxide.	[120]
Manni-Plex (commercial product) with 7.9% Ca and 4.35%Mg (Summer season)	100–300 mL/100 L water	Foliar spray	Cucumber	Increased plant height, leaf area, flower number, GA3, antioxidant (CAT and POD) activities, and fruit yield, and reduced flower abortion.	[121]
Magnesium sulfate (Summer season)	0.5% (w/v)	Foliar spray	Cabbage	Improved plant head size, head yield, and biomass.	[122]
Magnesium nitrate (Summer season)	28.57 (kg/ha)	Fertigation	Common bean	Improved the chlorophyll content, seed yield, seed protein, and mineral content (K, P, Mg).	[123]
Potassium Silicate (Late summer season)	2 mL L−1 (liquid K-silicate)	Foliar spray	Tomato	Foliar K-silicate (source of K and Si) improved yield in late-summer field trials.	[124]
Nano capsule-potassium (35 °C)	1 μM	Foliar spray	Sweet pepper	Reduced stress indicators (antioxidant enzyme activity and MDA). Decreased electrolyte leakage and helped stabilize membranes.	[125]
Sulfur (S) (99.5% purity) (45 °C)	6 ppm	Foliar spray	Tomato	Maximized shoot/root biomass, photosynthetic rate, transpiration, and greenness (SPAD); increased leaf proline and nutrient (N, P, and K) content.	[126]
Boron (B) + Humic acid (Subtropical climatic conditions)	100 ppm humic + 25 ppm B	Combined foliar sprays	Tomato	Increased plant growth, yield, and fruit quality.	[109]
Zinc oxide nanoparticles (nano-ZnO) (40/25 °C)	45 mg/L	Foliar spray	Mung bean	Increased photosynthetic rate chlorophyll, antioxidant levels, and yield.	[127]
Zinc sulfate (Summer season)	100 ppm	Foliar spray	Common bean	Improve biomass, foliar NPK content, and seed yield.	[128]
Potassium silicate (summer season)	4 mM	Seed priming	Snake cucumber	Membrane stability was enhanced, and oxidative damage markers were reduced.	[129]
Nano selenium (summer season)	200 mg L−1	Foliar spray	Cucumber	Increased plant height, leaf count, chlorophyll/carotenoid content, nutrient uptake (N, P, and K), and osmotic balance. Boosted growth and yield.	[130]

* NA = Not Applicable.

. In summary, fertilization and nutrient management are foundational cultural practices that bolster the inherent stress tolerance of vegetable crops, allowing them to better resist and recover from episodes of heat stress.

2.4. Irrigation and Water Management

Effective water management is fundamental to crop heat resilience, as elevated temperatures markedly increase evaporative demand and plant water requirements. Even transient soil or plant water deficits can suppress transpiration cooling and thereby exacerbate heat-induced physiological injury [131,132]. Supplying moisture in early morning or late evening can help plants start the day turgid and prepared for midday heat. Studies in tomato have shown that the combined effects of heat and water stress are substantially more detrimental than heat stress alone, highlighting the importance of optimized irrigation under high-temperature conditions [133,134]. Thus, irrigation strategies that ensure adequate water or strategically impose stress for tolerance can mitigate heat impacts. Key tactical approaches include optimized precision water management practices designed to enhance water-use efficiency and canopy cooling capacity.

Rather than fixed schedules, irrigation should replace the crop evapotranspiration (ETc) losses in near-real time, especially during heat spells [135,136]. FAO-56 and similar models use weather data (temperature, solar radiation, etc.) to calculate daily ETo (reference ET) and crop coefficients (Kc) for scheduling [137,138,139]. By following ETc-based schedules, growers avoid under-irrigating during hot, high-demand periods and over-irrigating during cooler times [139,140]. In practice, many modern systems use automated weather stations or soil moisture sensors to adjust irrigation volume/frequency to match ETc [138,139]. This not only protects the crop from drought stress during heat but also improves water-use efficiency and heat stress tolerance. Also, integrating soil moisture sensors, weather forecasts, and IoT-based controllers enables “smart” irrigation that responds in real time to plant water needs under heat stress [141,142]. For example, capacitance soil-moisture sensors (SMS automation) reduced irrigation and improved water-use efficiency in irrigation relative to time-based irrigation, with no loss in marketable yield or fruit quality in bell pepper, establishing SMS control as a practical alternative to traditional schedules in pepper field [143]. Sensor-based irrigation systems have demonstrated substantial potential to reduce water use while maintaining crop productivity and quality in lettuce [144]. Smart Irrigation System (SIS) architectures (soil-moisture, climate, and IoT controllers) repeated reduction in seasonal water by 59% without affecting the yield of tomato plants. This therefore illustrates how threshold-based control can decouple water saving from heat-season yield risk when set-points avoid water stress [145,146,147]. In fact, a study using an IoT wireless sensor network on drip-irrigated crops found that the automated system achieved a 12% higher yield and 35% less water use compared to standard ETc-based scheduling [148]. The sensors ensured the crop never went into water stress, even on very hot days, but also avoided water wastage on cooler days. For heat stress, sensor systems can also trigger cooling measures (like misting) when canopy temperatures exceed set points [135,149,150]. Although high-tech, costs are dropping, and even small farms are adopting simple tensiometers or low-cost moisture probes to guide irrigation. Precision irrigation helps maintain plant turgor and evaporative cooling capacity through heat extremes, directly translating to better yield stability.

Evaporative cooling through overhead sprinkler application represents an active canopy temperature management strategy, whereby intermittent wetting of the crop canopy during peak thermal periods enhances latent heat dissipation through evaporation. This process directly lowers leaf and near-canopy air temperatures, thereby alleviating thermal stress and improving physiological performance under high-temperature conditions [151,152]. Overhead sprinkler systems can reduce canopy temperatures by several degrees through evaporative cooling. In a field study on leafy vegetables, intermittent overhead sprinkling applied during hot periods around the crop lowered the air temperature in the plant zone by 3–11 °C and reduced tip burn risk in endive plant, demonstrating direct evaporative cooling at the canopy level [153]. Micro-sprinkler irrigation systems can reduce canopy and leaf temperatures by several degrees through evaporative cooling, thereby alleviating heat stress, improving physiological performance, and increasing tomato yield by 40.4% and cucumber yield by 18.9% [154]. However, evaporative cooling requires sufficient water supply and good water quality (to avoid leaf salt deposits) [151,155]. It is most effective in dry climates where evaporation is rapid; in humid climates, its cooling benefit is smaller, and care must be taken to prevent diseases from the extra leaf wetness.

2.5. Biostimulant Applications

2.5.1. Plant Growth Regulators (PGRs)

A wide range of natural and synthetic plant growth regulators (PGRs) and signaling molecules, including salicylic acid, jasmonates, melatonin, brassinosteroids, cytokinins, and triacontanol, have been extensively investigated for their potential to mitigate heat stress in vegetable crops. These compounds modulate key physiological and molecular processes, such as antioxidant defense, hormonal signaling, membrane stability, and photosynthetic efficiency, thereby enhancing plant thermotolerance and productivity under elevated temperature conditions. Table 3 provides a concise summary of the physiological and biochemical mechanisms through which different PGRs confer heat stress tolerance across a range of vegetable crop species. Salicylic acid (SA) is a signaling molecule (plant hormone) known for activating plant defense pathways [156]. Essentially, SA primes the plant’s intrinsic stress response [157]. Multiple studies show that exogenous SA (at low concentrations of 0.2–2 mM) can induce heat tolerance by ramping up antioxidant enzyme levels (e.g., superoxide dismutase and catalase) and heat-shock protein production [156,157,158]. In practical terms, SA sprays have yielded tangible benefits: e.g., spraying SA on heat-stressed pepper and tomato helped maintain a higher fruit set and reduced oxidative damage compared to unsprayed plants [159]. Given its low cost and proven action, SA is a recommended component in heat stress management, if concentrations are kept in the effective range.

Moreover, jasmonates (JAs), including methyl jasmonate (MeJA), are lipid-derived hormones that play crucial roles in plant defense, senescence, and responses to abiotic stresses [160,161]. Jasmonate signaling confers thermotolerance by inducing antioxidant and osmoprotectant systems and stimulating heat shock protein (HSP) accumulation, while exogenous MeJA application enhances heat resilience through intensified ROS detoxification and transcriptional activation of stress-responsive genes [162]. Another promising molecule is melatonin, an indoleamine derived from tryptophan, which functions as a growth regulator and potent antioxidant in plants [163,164]. Exogenous melatonin application (25–300 μM) via foliar spray has been reported to alleviate heat stress-induced damage in tomato, common bean, Chinese cabbage, and sweet potato [165,166,167]. Moreover, bassinosteroids (BRs), particularly 24-epibrassinolide, are steroidal hormones that regulate cell elongation, division, and stress adaptation [168,169]. Foliar application of BRs (0.20–1.0 μM) has been shown to boost heat resilience in bell pepper and mini-Chinese cabbage by multiple physiological adjustments [170,171]. Adding to this suite of compounds, cytokinins, such as 6-benzylaminopurine (BAP) and 6-benzyladenine (BA), are known to delay senescence and support sink-source dynamics [172,173]. Under heat stress, foliar sprays of BAP at 600–800 ppm and BA at 0.1% significantly improved the heat stress tolerance of tomato and sweet potato [124,174,175]. Lastly, triacontanol, a naturally occurring long-chain alcohol, has emerged as a potent biostimulant with notable effects on photosynthesis and metabolic efficiency [176]. When applied as a foliar spray (5–11 μM), triacontanol improved heat stress tolerance in snap bean, eggplant, and mung bean [177,178,179]. When using hormonal treatments in the field, timing and dosage are critical; typically, a foliar spray a day or two before an anticipated heat wave (or at onset of stress) can be most effective. Overuse or high concentrations, however, may have phytotoxic effects or growth trade-offs. Nonetheless, judicious use of PGRs is an innovative strategy to fortify plants. These compounds are relatively inexpensive and can be applied with standard spray equipment under high-temperature field conditions.

Table 3. Applied use of plant growth regulators (PGRs) for enhancing heat tolerance, physiological resilience, and productivity in vegetable and field crops.

Treatment	Dose	Crop	Mechanisms for Heat Tolerance	Reference
Salicylic acid (foliar spray) (Summer season)	0.20 mM	Bell pepper	Treated peppers accumulated more proline, soluble sugars, phenolics, antioxidant, and soluble proteins. Improved fruit yield.	[180]
Salicylic acid (foliar spray) (42 °C)	1.0 mM	Tomato	Improved photosynthetic efficiency and bolstered the antioxidant capacity, heat-shock proteins and scavenge excess ROS, thereby allowing better growth.	[181]
Salicylic acid (foliar spray) (Summer season)	0.25–0.5 mM	Tomato	Upregulated antioxidants, proline content and improved fruit set and yield.	[159]
Methyl jasmonate (foliar spray) (42 °C/4 h per day)	100 μmol	Tomato	Antioxidant protection by enhancement of vitamin E contents under combined stress (heat and salinity)	[182]
Methyl jasmonate (foliar spray) (40 °C/72 h)	100 μmol	Pepper	Amino-acid (e.g., putrescine, spermine and histamine) contents were increased. The ameliorative impact of methyl jasmonate on heat stress was influenced by cultivar.	[183]
Melatonin (foliar spray) (40 °C)	25 μM	Tomato	Higher activities of SOD, POD, and CAT. This led to more effective scavenging of ROS. Better growth and yield and fruit quality.	[165]
Melatonin (foliar spray) Late Rabi (post-rainy) season	300 μM	Common bean	Canopy temperature was reduced, and the pollen viability was increased. Enhanced seed yield, and micronutrient content of the seeds.	[166]
Melatonin (foliar spray) (42 °C)	100 μmol	Chinese cabbage	Enhanced photosynthetic activity, levels of soluble sugar, vitamin C, proteins, and antioxidants, along with decreased levels of malondialdehyde.	[167]
Melatonin (foliar spray) (37 °C for 7 days)	100 μmol	Sweet potato	Mitigated the decline in chlorophyll levels and elevated antioxidant enzyme activity and osmo-protectants. Increased growth and reduced oxidative damage.	[184]
24-Epibrassinolide (foliar spray) (Summer season)	0.20 μM	Bell pepper	Higher levels of osmolytes (proline, soluble sugars, starch), protective compounds (proteins, phenolics), and enhanced antioxidant enzyme activities (CAT, APX, POX, SOD, and GR). These changes translated to higher fruit yield.	[180]
Brassinosteroids or nano-encapsulated form (foliar spray) (35 °C)	1 μM	Bell pepper	Increased plant biomass, number of fruits, and relative water content and lower flower abscission.	[170]
24-Epibrassinolide (foliar spray) (38/25 °C for 3–5 days)	1 μM	Mini Chinese cabbage	Increased endogenous ABA, better water status, and enhanced protection against heat dehydration. Higher photosynthetic efficiency and antioxidant capacity, resulting in improved growth.	[171]
6- Benzylaminopurine (foliar spray) (Late summer season)	800 ppm	Tomato	Improved vegetative growth, fruit set, and yield.	[124]
6-Benzylaminopurine (foliar spray) (Summer season)	600 ppm	Tomato	Boosted osmoprotectants, antioxidants, proline, and total phenolics. These changes translated into higher yield.	[174]
6-Benzyladenine (foliar spray) (37.5/33 °C)	0.1%	Sweet potato	Improved photosynthetic performance, water status, membrane stability, antioxidant defense, and yield protection.	[175]
Triacontanol (foliar spray) (Summer season)	5 ppm	Snap bean	Enhanced growth, flowering, pod productivity, and pod quality.	[177]
Triacontanol (foliar spray) (Summer season)	10 μM	Eggplant	Increased fruit yield by enhancing water-use efficiency and reducing oxidative damage.	[178]
Triacontanol (foliar spray) (40 °C for 7 days)	11 μM	Mung bean	Improved growth, macronutrients, and amino acids via hormonal modulation (abscisic acid and jasmonic acid).	[179]

Table 3. Applied use of plant growth regulators (PGRs) for enhancing heat tolerance, physiological resilience, and productivity in vegetable and field crops.

Treatment	Dose	Crop	Mechanisms for Heat Tolerance	Reference
Salicylic acid (foliar spray) (Summer season)	0.20 mM	Bell pepper	Treated peppers accumulated more proline, soluble sugars, phenolics, antioxidant, and soluble proteins. Improved fruit yield.	[180]
Salicylic acid (foliar spray) (42 °C)	1.0 mM	Tomato	Improved photosynthetic efficiency and bolstered the antioxidant capacity, heat-shock proteins and scavenge excess ROS, thereby allowing better growth.	[181]
Salicylic acid (foliar spray) (Summer season)	0.25–0.5 mM	Tomato	Upregulated antioxidants, proline content and improved fruit set and yield.	[159]
Methyl jasmonate (foliar spray) (42 °C/4 h per day)	100 μmol	Tomato	Antioxidant protection by enhancement of vitamin E contents under combined stress (heat and salinity)	[182]
Methyl jasmonate (foliar spray) (40 °C/72 h)	100 μmol	Pepper	Amino-acid (e.g., putrescine, spermine and histamine) contents were increased. The ameliorative impact of methyl jasmonate on heat stress was influenced by cultivar.	[183]
Melatonin (foliar spray) (40 °C)	25 μM	Tomato	Higher activities of SOD, POD, and CAT. This led to more effective scavenging of ROS. Better growth and yield and fruit quality.	[165]
Melatonin (foliar spray) Late Rabi (post-rainy) season	300 μM	Common bean	Canopy temperature was reduced, and the pollen viability was increased. Enhanced seed yield, and micronutrient content of the seeds.	[166]
Melatonin (foliar spray) (42 °C)	100 μmol	Chinese cabbage	Enhanced photosynthetic activity, levels of soluble sugar, vitamin C, proteins, and antioxidants, along with decreased levels of malondialdehyde.	[167]
Melatonin (foliar spray) (37 °C for 7 days)	100 μmol	Sweet potato	Mitigated the decline in chlorophyll levels and elevated antioxidant enzyme activity and osmo-protectants. Increased growth and reduced oxidative damage.	[184]
24-Epibrassinolide (foliar spray) (Summer season)	0.20 μM	Bell pepper	Higher levels of osmolytes (proline, soluble sugars, starch), protective compounds (proteins, phenolics), and enhanced antioxidant enzyme activities (CAT, APX, POX, SOD, and GR). These changes translated to higher fruit yield.	[180]
Brassinosteroids or nano-encapsulated form (foliar spray) (35 °C)	1 μM	Bell pepper	Increased plant biomass, number of fruits, and relative water content and lower flower abscission.	[170]
24-Epibrassinolide (foliar spray) (38/25 °C for 3–5 days)	1 μM	Mini Chinese cabbage	Increased endogenous ABA, better water status, and enhanced protection against heat dehydration. Higher photosynthetic efficiency and antioxidant capacity, resulting in improved growth.	[171]
6- Benzylaminopurine (foliar spray) (Late summer season)	800 ppm	Tomato	Improved vegetative growth, fruit set, and yield.	[124]
6-Benzylaminopurine (foliar spray) (Summer season)	600 ppm	Tomato	Boosted osmoprotectants, antioxidants, proline, and total phenolics. These changes translated into higher yield.	[174]
6-Benzyladenine (foliar spray) (37.5/33 °C)	0.1%	Sweet potato	Improved photosynthetic performance, water status, membrane stability, antioxidant defense, and yield protection.	[175]
Triacontanol (foliar spray) (Summer season)	5 ppm	Snap bean	Enhanced growth, flowering, pod productivity, and pod quality.	[177]
Triacontanol (foliar spray) (Summer season)	10 μM	Eggplant	Increased fruit yield by enhancing water-use efficiency and reducing oxidative damage.	[178]
Triacontanol (foliar spray) (40 °C for 7 days)	11 μM	Mung bean	Improved growth, macronutrients, and amino acids via hormonal modulation (abscisic acid and jasmonic acid).	[179]

2.5.2. Amino Acid–Derived Osmoprotectants

Plants under heat often suffer from osmotic imbalances. Therefore, providing certain osmoprotective substances through foliar feeding can alleviate stress. These molecules help maintain enzyme function and water retention in cells. Glycine betaine and proline are typically classified as low–molecular weight compatible solutes (organic osmolytes). In the context of stress physiology, they accumulate in the cytosol to contribute to osmotic adjustment, membrane stabilization, and ROS mitigation without interfering with normal cellular metabolism [185,186,187,188]. Glycine betaine and proline have been tested exogenously on crops like tomato, pepper, and parsley, resulting in improved heat stress tolerance (

Table 4. Applied amino acids for enhancing heat stress tolerance in vegetable crops.

Treatment	Dose	Crop	Mechanism of Heat Tolerance Improvement	Reference
Glycine betaine (foliar spray) (40/32 °C)	15 mM	Chili pepper	Improved chlorophyll, photosynthetic rate, and water-use efficiency.	[189]
Glycine betaine (seed imbibition or add to germination medium) (42 °C for 3–6 h)	1–5 mM	Tomato	Improved germination and seedling survival by enhancing expression of heat-shock genes.	[190]
Glycine betaine (foliar spray) (Summer season)	60 mM	Parsley	Enhanced growth, pigments, antioxidants, minerals, and oil content.	[191]
Proline (foliar spray) (42/32 °C for 30 days)	5–10 mM	Chili pepper	Lowered heat-induced transpiration and stomatal conductance and improved water status.	[192]
Proline (foliar spray) (45/40 °C for 4 h)	1.5 mM	Tomato	Increased growth by improving photosynthetic rate and chlorophyll (SPAD).	[193]
Proline (foliar spray) (45 °C for 30 min–2 h)	50–100 mg/L	Tomato	Increased commercial and total fruit yield, water-use efficiency, and reduced oxidative stress and membrane damage.	[194]

).

2.5.3. Polyamines

Polyamines (e.g., putrescine, spermidine and spermine) play a broad protective role under diverse abiotic stresses, including drought, salinity, chilling, and heavy metal toxicity [195,196]. In general, polyamines contribute to membrane stabilization and macromolecule protection by binding to negatively charged phospholipids, proteins, and nucleic acids, thereby preserving cellular integrity under stress-induced dehydration or ion imbalance [197]. Their accumulation is closely linked to ROS homeostasis, since polyamines can directly scavenge reactive oxygen species and indirectly enhance antioxidant capacity via upregulation of enzymatic (e.g., SOD, CAT, POD, APX) and non-enzymatic antioxidants. Polyamine biosynthesis and catabolism are tightly integrated into stress signaling networks and support osmotic adjustment [196,197,198]. Overall, these studies indicate that exogenous polyamines (putrescine, spermidine, and spermine) applied mainly as foliar sprays (0.3–5 mM) substantially enhance heat tolerance in vegetable crops such as tomato, cauliflower, melon, lettuce, and pea (

Table 5. Applied polyamine treatments for enhancing heat stress tolerance in vegetable crops.

Treatment	Dose	Crop	Mechanism of Heat Tolerance	Reference
Putrescine (foliar spray) (38/28 °C)	1 mM	Tomato	Higher biomass and photosynthetic efficiency and less oxidative damage. Upregulated heat-shock-related genes (HSP70, HSP90, and HsfA1).	[199]
Putrescine (foliar spray) (38 °C for 15 days)	0.3 mM	Cauliflower	Improved photosynthetic and gas exchange and minimized membrane damage.	[200]
Putrescine (foliar spray) (32 and 44 °C for 3 days)	5 mM	Melon	Improved nutritional quality and antioxidant status of melon fruits.	[201]
Spermidine (foliar spray) (35/30 °C)	1 mM	Lettuce	Limited ROS-induced membrane damage and enhanced antioxidant enzymes (glyoxalase system, and AsA–GSH cycle).	[202]
Spermidine (foliar spray) (35/30 °C for 8 days)	1 mM	Lettuce	Changed the polyamine metabolism and protected PSII and CO2 assimilation.	[203]
Spermidine (foliar spray) (38/28 °C)	1 mM	Tomato	Improved biomass by enhancing activities of key enzymes in carbon (e.g., sucrose metabolism) and nitrogen metabolism.	[204]
Spermine (foliar spray) (38 °C for 48 h)	1 mM	Pea	Mitigated leaf pigment degradation and sustained photosynthetic performance, while maintaining antioxidant activities.	[205]

).

2.5.4. Seaweed Extracts and Chitosan

Extracts from kelp (e.g., Ascophyllum nodosum) and other seaweed could be used in vegetables as biostimulants. They contain trace minerals, amino acids, and plant hormones (e.g., cytokinins, auxins and abscisic acid) that can induce stress tolerance [206,207]. Under heat conditions, seaweed extract applications have been shown to increase antioxidant enzyme levels and heat-shock proteins in vegetables (

Table 6. Applied seaweed extracts and chitosan for enhancing heat stress tolerance in vegetable crops.

Treatment	Dose	Crop	Mechanism of Heat Tolerance	Reference
Brown algae (Ascophyllum nodosum) (foliar spray) (31/24 °C for 14 days)	0.106% (w/v)	Tomato	Increased fruit number by 86%. Associated with increased accumulation of soluble sugars and enhanced transcription of genes encoding protective heat-shock proteins.	[212]
Brown algae (Ascophyllum nodosum) (seed priming) (30 °C)	0.3% (w/v)	Spinach	Increased final germination percentage, germination rate, and seedling vigor by reducing oxidative and membrane damage.	[213]
Green seaweed (Ulva lactuca) (seed priming) (30 and 35 °C)	5%	Onion	Increased germination and seedling emergence (height, fresh and dry weight).	[214]
Chitosan (foliar spray) (Summer season)	750 ppm	Cauliflower	Increased vegetative growth, head yield, LRWC, membrane stability index, total chlorophyll, NPK contents, vitamin C, and protein.	[215]
Chitosan (foliar spray) (40/32 °C)	100 ppm	Spinach	Increased fresh and dry weight, chlorophyll, and membrane stability.	[216]
Chitosan (foliar spray) (36/32 °C)	200 ppm	Chinese cabbage	Reduced heat damage, associated with changes in abscisic acid, carbohydrate and glucosinolate metabolite profiles, and improved antioxidant status.	[217]
Chitosan (foliar spray) (Summer season)	175 ppm	Eggplant	Improved growth and yield. Associated with modifying the levels of glycine betaine, proline, soluble carbohydrates, and total phenolics.	[218]

). On the other hand, chitosan is a naturally derived biopolymer obtained from the deacetylation of chitin and is widely recognized as an eco-friendly plant biostimulant and elicitor [208,209]. Exogenous application of chitosan has been shown to enhance plant tolerance to stress through multiple mechanisms, including activation of antioxidant defense systems, improved water retention via partial stomatal regulation, and stabilization of photosynthetic performance. Chitosan-treated plants exhibit elevated activities of key antioxidant enzymes, reduced lipid peroxidation, increased chlorophyll retention, and improved relative water content under high-temperature conditions [208,210,211]. Additionally, chitosan modulates phytohormone signaling and promotes the accumulation of osmoprotectants, thereby enhancing cellular protection and metabolic stability during thermal stress. Studies in vegetable crops such as cauliflower, pepper, spinach, and eggplant confirm that foliar chitosan application improves biomass production, physiological resilience, and yield under heat stress conditions (Table 6).

2.5.5. Humic Substances

Humic substances, a group of natural organic compounds comprising humic acids, fulvic acids, and humin, are widely recognized as plant biostimulants that enhance plant resilience to abiotic stress, including heat stress [219,220,221,222]. Humic substances improved soil structure, stimulated root growth, and enhanced nutrient acquisition, thereby contributing to the maintenance of leaf relative water content and cellular hydration essential for thermotolerance [220,222]. In addition, humic substances exhibit hormone-like activity by modulating auxin- and cytokinin-related signaling pathways, which supports growth continuity and metabolic balance during abiotic stress [220]. As shown in

Table 7. Applied use of humic substances for enhancing heat tolerance in vegetable crops.

Treatment	Dose	Crop	Mechanism of Heat Tolerance	Reference
Humic acid (soil application) (Summer season)	14.4 kg/ha	Tomato	Improved vegetative growth, flowering, yield, and fruit quality.	[223]
Humic acid (growth medium application) (37/30 °C)	500 mg/L	Tomato	Enhanced vegetative growth and chlorophyll fluorescence. Accompanied by increased antioxidant enzyme activities (APX, SOD, GSH, and LPO) and the modulation of heat-responsive genes,	[224]
Humic substances (mixed into peat-based transplant substrate) (35.2/22.9 °C)	1% v/v	Pepper, tomato, watermelon and lettuce	Improved root length, surface area, and root biomass.	[225]
Humic substances (growth medium application) (32 °C)	1% v/v	Spinach	Improved shoot and root weights.	[226]
Fulvic acid (fertigation) (Summer season)	1.83 L/ha	Snap bean	Increased chlorophyll, NPK content, fresh and dry biomass, and pod yield.	[227]
fulvic acid (foliar spray) (Summer season)	2 mL/L	Tomato	Increased chlorophyll, plant growth, yield and fruits quality.	[130]

, multiple experimental studies have demonstrated that these compounds enhance heat tolerance across a range of vegetable crops by improving vegetative growth, photosynthetic efficiency, antioxidant defense systems, and yield-related traits, highlighting their broad potential as effective biostimulants under heat stress conditions.

2.5.6. Microbial Biostimulants

Microbial biostimulants have emerged as effective eco-friendly tools for enhancing heat stress tolerance in vegetable crops through integrated physiological, biochemical, and molecular mechanisms [228,229,230]. As summarized in

Table 8. Application of microbial biostimulants to enhance growth, productivity, and heat stress tolerance in vegetable crops.

Microbial Biostimulants	Crop	Heat Tolerance	Reference
Bacillus cereus (37/30 °C)	Tomato	Improved growth, antioxidant enzyme activities, and nutrients uptake. Modulated stress hormones (ABA and SA) and altered expression of heat-responsive genes.	[224]
Bacillus cereus (Summer season)	Tomato	Improved growth, chlorophyll content, and relative water content. Enhanced antioxidant enzymes.	[231]
Bacillus safensis (42 °C for 5 h)	Tomato	Enhanced plant growth, antioxidant, and chlorophyll content.	[232]
Arbuscular mycorrhizal fungi (Maximum of 45.6 °C)	Tomato Cucumber Pepper	Increased plant vigor, productivity, and fruit quality.	[233]
Arbuscular mycorrhizal fungi (38/30 °C)	Cucumber	Increased biomass and leaf gas exchange. Upregulated heat shock protein genes.	[234]
Acinetobacter sp., Bacillus sp., and Klebsiella sp., (35/30 °C)	Lettuce	Improved biomass, nutrient uptake, PSII, and water use efficiency.	[235]

, plant-growth–promoting bacteria such as Bacillus cereus markedly improved tomato performance under high-temperature conditions by stimulating vegetative growth, enhancing antioxidant enzyme activities, and increasing nutrient uptake. They also modulated stress-related phytohormones, including abscisic acid and salicylic acid, and regulated the expression of heat-responsive genes [224,231]. Similarly, Bacillus safensis application resulted in improved plant growth and reproductive traits, reflected by increased flower and fruit numbers in tomato exposed to heat stress [232]. In addition, arbuscular mycorrhizal fungi (AMF) significantly enhanced plant vigor, productivity, and fruit quality in tomato, cucumber, and pepper, indicating their broad-spectrum efficacy across vegetable species under elevated temperatures [233]. The protective role of AMF was further supported by increased biomass accumulation, improved leaf gas exchange, and upregulation of heat shock protein–encoding genes in cucumber, highlighting their contribution to cellular thermal protection [234]. Moreover, consortia of beneficial bacteria, including Acinetobacter, Bacillus, and Klebsiella species, improved biomass production, nutrient acquisition, photosystem II efficiency, and water-use efficiency in lettuce grown under heat stress [235]. Collectively, these findings demonstrate that microbial biostimulants enhance vegetable heat tolerance by reinforcing antioxidant defenses, optimizing water and nutrient relations, sustaining photosynthetic efficiency, and activating molecular stress-response pathways, underscoring their potential role in climate-resilient vegetable production systems [32,236,237]. Although microbial biostimulants have demonstrated considerable potential for enhancing abiotic stress tolerance in vegetable crops, most studies have focused on drought and salinity stress, with comparatively fewer investigations targeting heat stress [238]. Furthermore, the available evidence is concentrated on a limited number of vegetable species and microbial strains, highlighting the need for broader field-based validation and mechanistic studies under diverse production environments.

2.6. Microclimate Modification and Physical Protection

2.6.1. Mulching

Mulches act as a thermal buffer by insulating the soil surface, thereby moderating diurnal fluctuations in soil temperature, reducing evaporative losses, and enhancing soil moisture conservation [239]. Organic mulches (straw, hay, wood chips, etc.) or reflective plastic mulches can keep the root zone cooler on hot days. By shading the soil surface and reducing direct heating, mulching prevents extreme root-zone temperatures that impair water and nutrient uptake [13,240,241]. For instance, straw mulch has been shown to reduce midday soil temperatures and lower plant canopy stress in vegetables, while also reducing evaporation. This results in less wilting and better maintenance of yield under heat. Mulches also suppress weeds (which compete for water) and improve soil structure over time, indirectly enhancing heat resilience [239,242,243]. It is important to choose mulch type and colors carefully. Dark plastics can increase heat load (black plastic can cause >65 °C at the surface) that intensify heat stress on young plants, whereas reflective mulches (white or white-on-black) are better for cooling [239,240,244]. Several studies have successfully used white plastic or aluminized reflective mulches in high-temperature regions to reduce heat injury and sunscald in crops like peppers, iceberg lettuce, and strawberry [245,246,247]. In summary, mulching is a simple, effective practice to buffer plants against heat spikes by creating a more stable, cooler soil microclimate. Ensuring soils are well-drained and have high organic matter also helps healthy soils retain water and have higher heat capacity, meaning they warm up and cool down more slowly, thus avoiding extreme temperature swings around roots.

2.6.2. Windbreaks

Field layout can influence crop temperature. Windbreaks or shelterbelts (e.g., rows of trees or fences) protect fields from hot, desiccating winds and help create a milder microclimate for crops [248]. Windbreaks reduce wind speed and lower the evaporative demand on plants and soil, thereby conserving soil moisture and reducing crop water stress. Lower wind speeds around crops help reduce excessive transpiration and leaf dehydration during hot weather, so plants maintain better turgor and cooling through controlled transpiration [249,250]. Studies show that in the sheltered zone behind windbreaks, relative humidity tends to be higher and soil temperatures can be lower compared to unprotected fields [248,250,251]. For instance, a field experiment found that areas behind windbreaks had reduced soil temperature and increased air humidity, leading to improved seed germination and taller plants relative to unsheltered plots [252]. Furthermore, many crops show yield increases in the zone extending leeward of a windbreak due to these favorable conditions [251,253,254,255]. In one analysis, grain and vegetable yields in sheltered zones were approximately 3–10 times higher than in open wind-exposed areas, depending on crop and windbreak design [250].

Windbreaks (shelterbelts) can improve the crop microclimate by reducing wind speed and evaporative demand, but their use involves clear trade-offs. They occupy land and may lower yields in the rows nearest to the trees because of shade and below-ground competition for water and nutrients. In dry regions, successful establishment usually requires careful species choice, site preparation, protection from grazing, and supplemental irrigation, followed by routine pruning and periodic renovation as stands age. Performance depends on design: orientation to prevailing winds, adequate height, appropriate spacing, and moderate porosity help create uniform shelter without stagnant, humid zones or excessive shading. Benefits typically increase as the windbreak matures, and overall returns depend on local climate, crop value, and the quality of design and maintenance.

2.6.3. Shading

Deploying shade net over crops can substantially reduce the heat load. By filtering solar radiation, shade nets lower both air and leaf temperatures beneath [256,257]. Shade structures can be as simple as draping shade cloth over hoops for a low-cost tunnel, or more permanent net-houses covering entire fields. The timing of shading is important; nets are usually deployed during the peak heat of the season or at sensitive stages and removed when not needed to maximize light outside of the hot period [256]. Previous studies showed that, compared with open-field conditions, shading can reduce incoming solar radiation and wind speed by approximately 15–39% and 50–87%, respectively, while increasing relative humidity by 2–21% and lowering canopy temperatures by 1–2.5 °C and evapotranspiration by 17.4–50%. These microclimatic adjustments reduce evaporative demand and improve water use efficiency, which can increase stomatal conductance and CO₂ assimilation [258]. Growers can use different shade intensities (e.g., 20%, 30%, 50%) and even different colors; each has specific effects on light spectrum and crop response. A shading intensity of 20–40% is generally considered optimal for many vegetable crops, as it effectively moderates canopy temperature and radiation load without substantially constraining photosynthetic capacity. In contrast, higher shading levels (>40%) may limit light availability, leading to reduced photosynthetic activity and impaired flower initiation and fruit set [259,260].

In addition to thermal stress alleviation, shade nets have been shown to improve crop productivity and postharvest quality traits. Field studies indicate that moderate shade netting (35%) effectively modifies the tomato canopy microclimate by reducing incident radiation and lowering average and maximum canopy temperatures by approximately 1–2 °C compared with open-field conditions. These microclimatic adjustments alleviate heat stress, resulting in improved yield performance and reduced sunburn incidence, particularly under white shade nets [261]. Shade netting has been shown to reduce physiological disorders, enhance fruit quality traits (e.g., soluble solids, titratable acidity), increase antioxidants and phytochemical concentrations, and help maintain postharvest quality by slowing decay processes [262]. In high-tunnel summer lettuce trials, covering structures with 39% white shade cloth reduced daily maximum soil temperatures by approximately 3.4 °C and decreased leaf surface temperatures by 1.5–2.5 °C relative to open-field conditions. These cooler soil and canopy conditions were associated with a lower rate of bolting and reduced bitterness in lettuce compared with unshaded high tunnels, indicating shade-mediated heat stress alleviation and improved sensory quality [263].

Despite their benefits, shade nets may exert negative effects when shading intensity exceeds crop-specific light requirements. Excessive shading can reduce photosynthetically active radiation, leading to lower photosynthetic rates, delayed flowering, and reduced fruit set and yield. Prolonged light limitation may also alter carbon allocation, decrease assimilate availability for reproductive organs, and increase vegetative growth at the expense of yield. In addition, inappropriate shade net selection or density can modify canopy humidity and airflow, potentially favor disease development and reduce overall crop performance [256,257].

3. Toward Integrated Multi-Strategy Frameworks for Heat Stress Management

The strategies reviewed in the preceding sections operate through distinct but often complementary physiological and agronomic mechanisms. Accordingly, achieving robust, field-scale heat resilience in vegetable crops requires deliberate integration of multiple approaches rather than reliance on any single intervention. The value of integration lies in the fact that each strategy targets a different component of heat injury: heat-tolerant cultivars or grafted plants raise the biological threshold of tolerance; precision irrigation preserves plant water status and transpiration-based cooling; nutrient management supports membrane stability and photosynthetic function; and microbial biostimulants enhance root-zone nutrient acquisition, antioxidant priming, and heat-responsive signaling. When microbial biostimulants (e.g., Bacillus spp. or arbuscular mycorrhizal fungi) are layered onto this foundation, their roles in root-zone nutrient acquisition, antioxidant priming, and heat-shock gene upregulation provide an additional physiological buffer that genetics alone cannot supply. In combination, these interventions may act additively by strengthening several defense pathways at once, or synergistically when one practice amplifies the effectiveness of another, such as when improved root function enhances the response to irrigation or when biostimulant-induced physiological priming improves the performance of tolerant genotypes under transient heat episodes. This integrated perspective is essential because heat stress in vegetable systems is rarely expressed through a single physiological effect; rather, it typically involves interacting limitations in water relations, reproductive function, membrane integrity, and carbon assimilation. Emerging multi-factorial field evidence supports the concept that such combinations can produce synergistic, rather than merely additive, protective effects [264]. Similarly, intercropping-induced microclimate moderation, if combined with reflective mulching and optimized planting dates, may reduce peak canopy temperatures by several degrees while simultaneously improving soil moisture retention, a compound benefit that translates into measurable yield stability during heat waves [92,176]. Integration, however, involves trade-offs that must be managed. For example, shading reduces heat load but can raise canopy humidity and disease pressure; high planting densities improve soil shading but may limit ventilation. At the same time, integration must be context-specific, because some combinations may impose trade-offs related to cost or labor. Therefore, the most effective field-scale heat-management packages are likely to be those that combine complementary tools in a crop- and region-specific manner, matching local climate, soil conditions, and production goals. Researchers must therefore evaluate strategy combinations in the context of local climate, crop species, available resources, and market constraints.

4. Economic Feasibility and Adoption Considerations

Economic viability is a decisive determinant of technology adoption at the farm scale. The strategies reviewed in this manuscript differ substantially in their upfront costs, recurrent input requirements, operational complexity, and expected returns. Grafted transplants incur 2–5 times the production cost of seedlings but the stability provided under heat stress can justify this investment for high-value greenhouse vegetables [54]. However, this study provides an early benchmark for grafted tomato economics in the United States, and cannot be directly generalized to current global cost structures. Precision irrigation systems based on drip irrigation combined with soil-moisture sensors or IoT controllers can substantially improve water-use efficiency and crop productivity; however, their installation and operating costs vary widely with system design, automation level, crop value, and local market conditions. Reported benefits include WUE improvements of approximately 61–64.1% and productivity gains of 12–35% [265]. Nevertheless, robust per hectare cost estimates for sensors, controllers, installation, and maintenance remain highly context-dependent, underscoring the need for updated crop- and region-specific techno-economic analyses. Shade netting represents a moderate fixed cost (netting material and support structures), with returns depending on crop value, shade intensity, and local radiation load. Economic assessments indicate that shade net technology can enhance profitability compared with open-field production systems, primarily due to increased marketable yield and improved crop quality [256].

Biostimulants such as PGRs, seaweed extracts, and humic substances are often inexpensive to apply and can be delivered using standard spray equipment under field conditions. However, their economic feasibility and return on investment vary widely depending on the product type, formulation, and doses. Consequently, inconsistent field responses remain a major barrier to predictable adoption, underscoring the need for standardized field-evaluation protocols. Field management practices such as optimized planting dates and intercropping carry minimal additional cost and represent high-benefit, low-risk strategies particularly accessible to smallholder producers. Overall, the economic evidence base for heat-stress mitigation strategies remains fragmented. Comprehensive farm-scale cost-benefit analyses that incorporate risk under variable climatic and market conditions, disaggregated by crop type and region, are urgently needed to support rational decision-making.

5. Conclusions and Future Prospects

Vegetable crops exhibit distinct interspecific variability in thermosensitivity, with elevated temperatures differentially disrupting organ-specific physiological processes across taxa. In leafy vegetables like lettuce, heat stress accelerates premature bolting, which synthesizes bitter compounds, degrades head uniformity, and eliminates marketability. Conversely, solanaceous crops, including tomato and pepper, are primarily constrained by pollen inactivation, which directly impairs fruit set and subsequent yield. Cucurbitaceous species, such as cucumber, respond to thermal stress by developing malformed, unmarketable fruit architectures. Furthermore, leguminous vegetables, notably snap bean and cowpea, demonstrate extreme vulnerability during reproductive phases, characterized by pollen tube growth failure and high rates of pod abortion. Consequently, thermal mitigation frameworks lack universal efficacy; successful deployment requires precise alignment with both crop taxonomy and the specific physiological mechanisms of heat-induced injury.

This review has critically evaluated the evidence base for six major categories of field-applicable heat stress mitigation: genetic resources (heat-tolerant cultivars and stress-adapted rootstocks), field management (optimized planting dates, crop rotation, canopy management, and intercropping), fertilization and beneficial elements, irrigation and water management, biostimulants (PGRs, osmoprotectants, polyamines, seaweed extracts, humic substances, and microbial inoculants), and microclimate modification (mulching, windbreaks, and shade netting). Several key conclusions emerge. First, no single strategy is sufficient; robust heat resilience under field conditions requires the integration of complementary approaches that collectively address genetic, physiological, agronomic, and microclimate dimensions of heat stress. Second, the evidence for synergistic effects among strategies is growing but remains largely inferential; rigorous multi-factorial field trials validating true synergism are urgently needed. Third, crop-specific guidance is essential: anti-bolting cultivar selection and moderate shade netting are the primary tools for lettuce; grafting, precision irrigation, and PGR applications are most impactful for tomato and pepper; antioxidant biostimulants and rootstock selection are well-supported for cucumber; and reproductive-stage irrigation management and polyamine or melatonin applications show the strongest evidence for legumes. Fourth, economic feasibility data are critically lacking for most strategies beyond grafting, and farm-scale cost-benefit analyses across diverse crops and regions are a high-priority research need. Among the most urgent knowledge gaps are: (i) the combined effects of heat with co-occurring drought and salinity stress, which more accurately reflect real farm conditions; (ii) standardized field evaluation protocols for biostimulant efficacy that would allow cross-study comparisons and regulatory harmonization; (iii) evidence for microbial biostimulants under heat stress is currently limited to a few vegetable species and microbial strains; (iv) economic and social feasibility analyses that disaggregate by production system scale and geographic context; and (v) mechanistic understanding of how biostimulant combinations interact at the molecular level. Future research should prioritize multi-site, multi-factor field experiments embedded within realistic farm systems, coupled with digital tools for climate-adaptive crop management. Interdisciplinary collaboration among plant physiologists, agronomists, economists, and data scientists will be essential to translate the substantial body of single-factor evidence into actionable, integrated management packages for sustainable vegetable production under accelerating global warming.