Heat stress in chilli (Capsicum annuum L.): Reproductive vulnerability, fruit quality and mitigation strategies

Introduction

Chilli (Capsicum annuum L.) is grown both as a vegetable and as a spice and its commercial value depends on more than total biomass. Growers require consistent flowering, fruit set, marketable pod yield and, for pungent types, a desirable level of capsaicinoids. A plant may remain visibly alive during a hot spell while experiencing substantial injury in anthers, pollen, stigmatic tissues or young ovaries; the economic damage becomes obvious only later as flower drop, poor fruit set, malformed fruits or reduced yield. Recent reviews have established high temperature as a major constraint to chilli productivity and have summarized vegetative, physiological and molecular heat responses (Khattak et [1,2,3,4] . Presently, the more specific challenge is to understand how those responses connect to the reproductive sequence that ultimately determines harvestable yield. Temperature sensitivity in chilli depends on genotype, developmental stage, exposure duration and the balance between day-and night-time temperatures. For that reason, a single numerical value should not be treated as a universal heat-stress threshold. Controlled experiments have used a wide range of heat regimes. In the studies considered here, treatment temperatures ranged from 36 °C to 42 [5,6,7,8] . Foundational work also showed that particular floral stages are especially vulnerable and that pollen performance can be a useful indicator of reproductive thermotolerance (Erickson et al., 2002; Gajanayake et al., 2011; Reddy et al., 2007) [9,10,11] . More recent studies reinforce this view while also showing that heat-stable fruit set cannot be explained by pollen alone (Manoharan et al., 2026; Yamazaki et al., 2025) [12,13] . The urgency of this issue has increased because heat events are becoming more frequent, longer and more likely to overlap with flowering in open-field and protected systems. Process-based simulations indicate that warming can substantially reduce chilli yield while climate-focused reviews of horticultural crops emphasize that temperature effects interact with radiation, water availability and other stresses rather than acting in isolation (Park et [14,15,16,17] . This interaction matters in practical production. A high-temperature episode accompanied by strong radiation and soil-water limitation imposes a different physiological load from the same air temperature under adequate root-zone water and moderated radiation.

A further reason to focus on reproductive heat stress is that the quality consequences have received less attention than yield. Chilli fruits are valued for colour, vitamin C, phenolics and pungency and the biochemical pathways responsible for these traits remain active during fruit development. Multi-omics evidence now shows that heat can alter capsaicin and ascorbic acid accumulation in developing pepper fruit (Liu et al., 2022) [6] . However, this evidence base is still much smaller than the literature on vegetative heat responses. A review that links flowering, reproductive organ function, fruit set, yield and capsaicinoid accumulation can therefore identify where the evidence is strong where it is contradictory and where further experimentation is needed.

Heat sensitive stages

Reproduction compresses several highly coordinated processes into a short period. Before anthesis, microsporogenesis and microgametogenesis must produce viable pollen; the anther must mature and dehisce; pollen must be released and reach a receptive stigma; pollen grains must hydrate, germinate and extend a tube through the style; ovules must remain functional; and fertilization must be followed by sufficient hormonal and assimilate support to retain the young fruit. Heat can disturb any of these steps. Because failure at one stage cannot always be compensated later, reproductive heat damage often causes a disproportionate yield penalty compared with a similar episode during vegetative growth (Driedonks et al., 2016; Kaur et al., 2016; Reddy et al., 2007) [18,19,11] . Recent anatomical work strengthens this interpretation. In heat-tolerant and heat-susceptible C. annuum material, high temperature caused contrasting changes in anther organization, pollen abundance and viability, vascular integrity and ovary tissues. Tolerant plants retained a more intact reproductive structure whereas susceptible material showed greater tissue degeneration (Manoharan et al., 2026) [12] . These observations move the discussion beyond the simple statement that pollen viability declines and suggest that maintaining reproductive tissue organization and carbohydrate status is also part of heat tolerance. Genotype changes the extent of reproductive damage considerably. Reproductive-stage screening has repeatedly separated tolerant and susceptible material on the basis of pollen performance, flower retention and fruit set (Kaur et al., 2016; Singathiya et al., 2026; Srivastava et al., 2020) [19,20,21] . Whole-plant comparisons also show useful variation in photosynthesis, cooling, recovery and oxidative injury under heat (Kim et al., 2023; Poudyal et al., 2023; Srivastava et al., 2022) [22,7,23] . Earlier screening and breeding studies likewise reported exploitable variation within chilli germplasm (Chowdhury et al., 2020; Kumar et al., 2020) [24,25] . [22,14,23] . Stress-related hormonal signalling also changes, although the direction and magnitude depend on genotype, treatment and interacting stresses (Otalora et al., 2020; Otalora et al., 2022) [27,28] . Direct ovary profiling in pepper shows that heat can disrupt auxins, cytokinins, gibberellins, abscisic acid, jasmonic acid, salicylic acid and the ethylene precursor ACC during early fruit set; a thermotolerant rootstock also reduced losses in fruit set and seed number (Padilla et al., 2026) [29] . Field and controlled-environment studies support the practical importance of flower retention. Heat-tolerant chilli genotypes selected over successive summer generations have shown superior fruiting ability under hot conditions (Srivastava et al., 2020) [21] . More recent physiological comparisons similarly reveal differences among genotypes in recovery, photosynthesis, proline accumulation and oxidative injury (Kim et [22,7,8,23] . These whole-plant traits do not substitute for reproductive measurements but they help explain why one genotype can continue to support flowers while another experiences rapid abscission.

Flower initiation, anthesis and flower retention

Pollen development, viability and germination

Pollen is one of the most consistently heat-sensitive components of chilli. High temperature can disrupt tapetal function, carbohydrate supply to developing microspores, pollen wall formation, dehydration status and membrane integrity. The resulting pollen may appear morphologically present but have low viability or weak germination. In vitro studies across Capsicum species established strong genetic variation in temperature responses of pollen germination and pollen-tube growth (Reddy et al., 2007) [11] . Recent reproductive-stage screening further showed that pollen viability, pollen germination, pollen-tube growth, fruit set and healthy seed set are strongly interrelated under heat stress (Singathiya et al., 2026) [20] . Screening of ornamental pepper and hot-pepper germplasm likewise demonstrated that pollenbased traits can discriminate more tolerant from susceptible material (Gajanayake et al., 2011; Kaur et al., 2016) [10,19] . The value of pollen traits is confirmed by recent grafting work. Under heat, a susceptible sweet-pepper scion grafted onto a tolerant rootstock maintained better leaf physiology, higher anther proline, improved pollen germination and higher fruit set than less tolerant plant combinations (Gisbert-Mullor et al., 2023) [5] .

Pollen-tube growth, stigma, ovary and fertilization

After pollination, heat can alter the probability that a pollen grain completes fertilization. Pollen-tube growth is temperature sensitive and both very high temperatures and heat-damaged pollen can shorten the effective fertilization window (Erickson et al., 2002; Reddy et al., 2007) [9,11] . At the same time, the pistil is not passive. Stigma receptivity, style physiology, ovule longevity and ovary vascular function can all be affected. The 2026 histological study in C. annuum pro-vides direct evidence that tolerant and susceptible genotypes differ in the structural integrity of both male and female organs under temperature stress (Manoharan et al., 2026) [12] .

Fruit yield and quality

At fruit set stage, multiple heat responses become agronomically visible. Flower can be produced normally yet fail to set because of pollen injury, poor pollen-pistil interaction, ovule damage or inadequate assimilate and hormonal support after fertilization. Aruna et al., 2023 [30] ; Kaur et al., 2016 [19] ; Srivastava et al., 2020 [21] ; Yamazaki et al., 2025 [13] reported that fruit set remain a primary selection trait in reproductive heat-tolerance breeding rather than being inferred from vegetative measurements alone. Yield losses also accumulate through effects on fruit number, individual fruit mass and the duration of productive flowering. Process-based modelling across locations in Korea predicts progressively larger pepper-yield penalties as temperature increases, emphasizing that chronic warming affects more than a single flowering event (Park et al., 2025) [14] . Beside the yield parameters, fruit colour, dry matter, soluble solids, vitamin C, phenolics and pungency influence market value. Temperature can alter these traits directly through fruit metabolism and indirectly through changes in source-sink relations and fruit-development rate. Older temperature-regime studies already demonstrated that pepper fruit development and quality are temperature responsive (Oh et al., 2019) [31] . Temperature also interacts with cultural practices that modify light and plant water status. Moderate shading has improved plant water relations and marketable yield in bell pepper under warm conditions (Kabir et [32,33,34] and a hot-pepper study found that intermediate shade levels improved several physiological, yield and quality attributes compared with excessive shade (Ahmed et al., 2023) [35] . Colored shade nets can also affect fruit colour and chemical composition (Diaz-Perez et al., 2020) [36] . These responses illustrate an important management trade-off: reducing canopy heat and radiation can protect reproduction and fruit from sun injury but excessive shade may constrain photosynthesis and carbon supply. The appropriate shade intensity therefore depends on climate, cultivar, season and production objective. Rootstock studies provide another useful example of a possible yieldquality and benefit-cost ratio. Under heat, thermotolerant rootstocks improved marketable yield and, in recent work, maintained major biochemical quality attributes while improving marketable performance (Gisbert-Mullor et al., 2021; Gisbert-Mullor et al., 2023; Gisbert-Mullor et al., 2026) [37,5,38] . This suggests that heat mitigation need not inevitably trade quality for yield. However, pungent chilli types should be tested directly because sweet-pepper fruit-quality responses cannot automatically be extrapolated to capsaicinoid-rich cultivars. The available quality evidence is summarized in Table 2. The study was not a direct heat-stress experiment.

Mitigation strategies

For recurrent heat, genetic improvement offers the most durable longterm option. Chilli contains substantial usable variation for heat response. Reproductive-stage studies have separated tolerant and susceptible genotypes on the basis of pollen function and fruiting ability (Kaur et [19,20,21] while physiological comparisons reveal differences in recovery, cooling and carbon balance (Kim et [22,7,23] . QTL work extends this variation to mapped genomic regions (Aruna et al., 2023) [30] . Earlier screening and breeding reports, together with experience from heat-tolerance breeding in related crops, also support the value of broad genetic diversity (Ayenan et al., 2019; Chowdhury et al., 2020; Kumar et al., 2020) [39,24,25] . Genome editing could eventually speed up both validation and breeding. In chilli, CRISPR/Cas systems have already been demonstrated through PDS editing and newer virus-mediated or heritable editing platforms (Bulle et al., 2024; Kang et al., 2025; Zhao et al., 2024) [40,41,42] . Broader crop-editing literature outlines how negative regulators or stress-responsive transcription factors might be targeted for temperature resilience (Chakraborty et al., 2024; Ikram et al., 2024; Zafar et al., 2020) [43,44,45] . A recent chilli-focused review makes a similar case for transcription-factor optimization (Bulle et al., 2025) [46] . Even so, direct editing for reproductive heat tolerance in commercial chilli is still a research opportunity rather than a routine breeding tool. Transformation efficiency, genotype dependence, offtarget assessment and regulatory acceptance remain practical constraints. Grafting is one of the management options supported by several pepper-specific heat studies. Appropriate rootstocks can improve growth, membrane stability, chlorophyll fluorescence and marketable yield during prolonged heat (Gisbert-Mullor et al., 2021) [38] . A heat-tolerant rootstock can also improve anther proline, pollen germination and fruit set (Gisbert-Mullor et al., 2023) [5] , directly connecting rootstock choice with reproductive performance. The most recent hybridrootstock study further demonstrates improved marketable yield and lower blossom-end rot under hot greenhouse conditions (Gisbert-Mullor et al., 2026) [37] . For protected cultivation where high rootzone and air temperatures recur predictably, thermotolerant rootstocks therefore offer a practical bridge while new heat-tolerant cultivars are being bred. Heat load in the field is shaped by solar radiation and plant water status as well as air temperature. Shade nets can lower radiation and modify canopy and root-zone conditions. In bell pepper, moderate shading has improved plant water status, photosynthetic performance and marketable yield under warm environments while very strong shade can restrict carbon gain (Kabir et al., 2020; Kabir et al., 2022; Kabir et al., 2023) [32,33,34] . Hot-pepper experiments similarly indicate that intermediate shade levels can improve physiology, yield and selected quality traits (Ahmed et al., 2023) [35] . Coloured nets may also alter fruit colour and composition (Diaz-Perez et al., 2020) [36] . These studies support site-specific shading rather than a single universal shade percentage. Adequate irrigation is equally important because transpirational cooling depends on water supply. Bell-pepper work shows that irrigation level affects plant growth, physiology, yield and quality (Kabir et al., 2021) [47] . Under heat, maintaining root-zone water can help sustain stomatal conductance and leaf cooling but excessive irrigation may create nutrient or root-health problems. Practical climate-smart management therefore involves matching irrigation frequency and volume to evaporative demand, soil or substrate water-holding capacity and shading level. Where water is limited, breeding for reproductive tolerance and water-use efficiency must be pursued together rather than treating heat and drought as separate problems (Dos Santos et al., 2022; Poudyal et al., 2023; Raja et al., 2020) [48,7,49] . Foliar application of bio-stimulants can provide short-term protection, although their reliability varies. Salicylic acid, epibrassinolide, methyl jasmonate and proline have all shown stress-mitigating physiological benefits in pepper experiments but not all studies isolated heat as the only stress factor (Masoumi et [50,27,28,51] . Their proposed mechanisms include improved antioxidant capacity, membrane protection, osmotic adjustment and altered stress signalling. Basic crop scheduling remains important. Adjusting sowing and transplanting dates can reduce overlap between peak flowering and the hottest part of the season where local climate permits. Protected structures can use ventilation, evaporative cooling, reflective materials or shading while mulches can influence root-zone temperature (Saeed et al., 2023; Shah et al., 2024) [16,17] . These measures should be treated as components of an integrated heat-management system. No single intervention is likely to fully compensate for a highly susceptible cultivar exposed to repeated heat waves. In practice, resilience is most likely to come from combining genetics with crop management. A cultivar that maintains reproductive function under heat reduces dependence on expensive climate control; a tolerant rootstock can add physiological buffering; moderate shade can reduce radiation load; and well-managed irrigation can sustain evaporative cooling. In high-value protected cultivation, these layers may be economically justified. In lower-input open-field systems, heat-tolerant cultivars, planting-time adjustment and efficient water management are generally more practical and scalable. Climate-smart recommendations should therefore be differentiated by production environment rather than presented as a single package (Raza et al., 2025; Saeed et al., 2023; Shah et al., 2024) [15,16,17] .

Conclusion

Heat stress poses a major threat to chilli production, particularly during flowering and early fruit development, by disrupting pollen development, germination, fertilization and fruit retention. Genetic, physiological, anatomical and genomic studies indicate that heat tolerance involves multiple mechanisms beyond pollen viability, including maintenance of photosynthesis, membrane stability, redox balance, osmotic adjustment and hormonal regulation. Heat stress can also reduce fruit quality by affecting capsaicin and ascorbic acid accumulation, although genotype × environment interactions require further study. Short-term adaptation can be achieved through thermo-tolerant rootstocks, moderate shading, irrigation management and validated bio-regulators, while long-term resilience requires breeding for reproductive success and marketable fruit quality under realistic heat conditions. Integrating reproductive phenotypic, genomics and climatesmart management can helps to achieve stable chilli production under rising temperature.

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