Impact of high day and night temperatures on major food crops: A comprehensive review

Plant Science Today, ISSN 2348-1900 (online) Introduction

Global warming is accelerating, with human-induced temperatures reaching 1.37 °C above pre-industrial levels in 2025 and warming rates of 0.27 °C per decade (1). Extreme heat events now threaten over one billion people and cause half a trillion work hours lost annually. Major food crops such as rice, wheat, maize, soybean and barley are particularly vulnerable to heat stress, especially during sensitive developmental stages such as flowering and grain filling (FAO and WMO, 2026). By 2050, each 1 °C rise could reduce yields by

10–20 % (2). This review examines heat stress impacts across growth stages and explores mitigation strategies. The intensification of global warming has led to a marked increase in the frequency and severity of high-temperature events, both during the day and night, posing a significant threat to global food security (3).

Recent projections suggest that, under current adaptation levels, each 1 °C rise in global mean temperature could reduce wheat yields by 6.0 %, maize by 7.4 % and rice by 3.2 %. These losses are compounded by the dual impact of high day temperatures (HDT) and high night temperatures (HNT), which disrupt crop physiology and reproductive development through distinct but overlapping mechanisms (4). High temperatures during the day primarily impair photosynthetic efficiency, stomatal regulation and carbon assimilation, leading to reduced energy capture and utilisation in crops such as maize and wheat. Elevated daytime temperatures decrease the activity of ribulose diphosphate carboxylase/ oxygenase (rubisco), a key enzyme for CO₂ fixation, resulting in lower carbon assimilation rates. Heat stress also induces oxidative damage via the accumulation of reactive oxygen species (ROS), which disrupt chloroplast ultrastructure and electron transport chains, further compromising photosynthetic capacity (5). In addition, high temperatures accelerate chlorophyll breakdown, reducing green leaf area and overall photosynthetic efficiency in crops like maize and beans.

High night temperatures, on the other hand, significantly increase respiration rates, depleting non-structural carbohydrate reserves and disrupting assimilate partitioning between source and sink tissues. For instance, soybeans exposed to high night temperatures consume more photosynthetic products during respiration, reducing seed filling efficiency and ultimately lowering

PLANT SCIENCE TODAY Vol 13(sp5): 01–14 https://doi.org/10.14719/pst.14740 eISSN 2348-1900

REVIEW ARTICLE Impact of high day and night temperatures on major food crops: A comprehensive review

Devadharshini S¹, N K Sathyamoorthy¹, S Kokilavani¹, V Manivannan², Chandrakumar Kalichamy³ & K Vanitha⁴*

1Department of Agro Climate Research Centre, Tamil Nadu Agricultural University, Coimbatore 641 003, Tamil Nadu, India

2Department of Agronomy, Tamil Nadu Agricultural University, Coimbatore 641 003, Tamil Nadu, India

3Department of Renewable Energy Engineering, Tamil Nadu Agricultural University, Coimbatore 641 003, Tamil Nadu, India

4Department of Crop Physiology, Tamil Nadu Agricultural University, Coimbatore 641 003, Tamil Nadu, India

*Correspondence email - nks6@tnau.ac.in

Received: 27 March 2026; Accepted: 07 July 2026; Available online: Version 1.0: 30 September 2026

Cite this article: Devadharshini S, Sathyamoorthy NK, Kokilavani S, Manivannan V, Chandrakumar K, Vanitha K. Impact of high day and night temperatures on major food crops: A comprehensive review. Plant Science Today. 2026;13(sp5):01–14. https://doi.org/10.14719/pst.14740

Abstract Understanding plant responses to both day and night heat is essential for developing climate smart agriculture systems to sustain global food production. High daytime (> 35 °C) and nighttime (> 25 °C) temperatures are increasingly threatening global food production. Major crops such as rice, wheat, maize, soybean and barley are especially vulnerable during flowering and grain filling. However, the distinct and combined effects of day versus night heat on crop performance remain underexplored in a comparative manner. This review synthesises findings from recent physiological, biochemical and molecular studies on heat stress responses in staple food crops. It compares the impacts of high day temperature and high night temperature on photosynthesis, respiration, reproductive development, oxidative stress, antioxidant defenses, grain yield and quality. Data from field experiments, controlled environment studies and meta-analyses are integrated. Daytime heat impairs photosynthesis and carbon assimilation, while nighttime heat increases respiration rates, depletes carbohydrate reserves and disrupts nutrient partitioning. Both conditions reduce pollen viability and spikelet fertility, causing yield losses of 5–40 % depending on the crop and growth stage. Heat stress triggers reactive oxygen species accumulation, leading to cellular damage and membrane lipid peroxidation.

Antioxidant systems (superoxide dismutase, catalase, ascorbate peroxidase) provide only partial protection under prolonged heat.

Crop-specific vulnerabilities include grain chalkiness in rice, reduced kernel weight in maize and impaired nodulation in legumes. Breeding tools such as clustered regularly interspaced short palindromic repeats/ CRISPR-associated protein 9 (CRISPR/Cas9) and agronomic practices like adjusted sowing dates and optimised irrigation help mitigate these effects. This review uniquely compares daytime and nighttime heat effects across multiple staple crops, integrating physiological, molecular and agronomic perspectives with quantitative yield data.

Keywords: antioxidants; climate resilience; heat shock proteins; heat stress; oxidative stress; thermo-tolerance

DEVADHARSHINI ET AL 2 https://plantsciencetoday.online yields (6). In rice and wheat, elevated nighttime respiration leads to reductions in stem carbohydrate reserves, limiting energy availability for grain development. The combined effects of HDT and HNT exacerbate yield losses by further reducing stomatal conductance and water-use efficiency, making crops more susceptible to drought stress.

Reproductive development, particularly pollen viability and fertilisation, is acutely sensitive to high temperatures. Male gametophytes (pollen) are especially vulnerable, with elevated temperatures disrupting meiosis, degrading essential proteins and altering carbohydrate metabolism within the anther (7). For example, in canola, exposure to 42 °C during flowering can reduce yields by up to 70 % due to pollen sterility. Similar effects are observed in rice, wheat and tomatoes, where high temperatures during reproductive stages lead to reduced seed set, increased spikelet sterility and lower grain quality. Genetic studies highlight that thermotolerant cultivars, such as certain indica rice varieties, maintain reproductive function under heat stress, suggesting avenues for breeding heat-resilient crops.

The detrimental effects of high temperatures extend beyond yield reduction to the deterioration of grain quality. In rice, high night temperatures increase chalkiness and reduce head rice yield, while in wheat, heat stress during grain filling decreases starch content and increases the proportion of gliadin proteins associated with gluten intolerance (8). Barley exposed to HNT during grain number determination exhibits increased grain shriveling and reduced yield.

These quality losses are compounded by declines in protein content and mineral bioavailability, further threatening the nutritional value of staple crops. At the cellular level, heat-induced oxidative stress leads to the accumulation of ROS, causing membrane lipid peroxidation and protein denaturation. While antioxidant defense systems, such as superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX), offer some protection, prolonged exposure to high temperatures often overwhelms these mechanisms, resulting in cellular damage. Heat shock proteins (HSPs), phytohormonal signaling pathways (e.g. abscisic acid and ethylene) and epigenetic modifications play critical roles in regulating plant thermotolerance and stress memory.

Crop-specific vulnerabilities to heat stress have been identified, such as chalkiness in rice grains, reduced kernel weight in maize and impaired nodulation in legumes. These vulnerabilities underscore the need for targeted breeding and management strategies to enhance crop resilience under high-temperature regimes. Advances in molecular breeding, including clustered regularly interspaced short palindromic repeats/ CRISPR-associated protein 9 (CRISPR/Cas9) gene editing, quantitative trait locus (QTL) mapping and genomic selection are accelerating the development of heat-resilient cultivars (9). Agronomic interventions such as adjusting sowing dates, adopting conservation tillage, optimising irrigation and applying protective compounds, have also shown promise in mitigating heat-induced damage. The integration of physiological, genetic and agronomic approaches is essential for developing climate-smart agriculture systems capable of sustaining food production in the face of intensifying climate change. A deeper understanding of plant responses to HDT and HNT at the physiological, biochemical and molecular levels will inform the design of effective breeding and management strategies. Such multidisciplinary efforts are critical for safeguarding global food security and ensuring the resilience of agricultural systems under future climate scenarios.

In summary, the impact of high day and night temperatures on major food crops is multifaceted, affecting photosynthesis, reproductive development, yield and grain quality through complex physiological and molecular pathways. Continued research and innovation in crop breeding, agronomy and climate adaptation are imperative to address the challenges posed by rising temperatures and to secure the future of global food systems (10). This review aims to describe the physiological and molecular mechanisms of heat injury in major food crops, evaluate the impacts of high day and night temperatures on crop yield and grain quality and synthesise current adaptive strategies, including molecular breeding, agronomic practices and biotechnological approaches (Table 1).

High day temperatures (> 35 °C) primarily impair photosynthesis by reducing rubisco activity and carbon assimilation, whereas high night temperatures (> 25 °C) increase respiration rates, leading to depletion of carbohydrate reserves and more severe yield losses.

Effects on photosynthesis and carbon assimilation High temperatures, both during the day and night, significantly impair photosynthesis and carbon assimilation in major food crops, though through distinct mechanisms. These thermal stresses disrupt various physiological and biochemical processes, leading to reduced crop productivity. Understanding the specific mechanisms through which heat affects photosynthesis is crucial for developing strategies to mitigate yield losses in the face of global warming.

Elevated daytime temperatures reduce rubisco activity, a critical enzyme for CO₂ fixation, leading to lower carbon assimilation rates in maize and wheat (11, 12). Heat stress induces oxidative damage via

ROS, which disrupt chloroplast ultrastructure and electron transport chains, impairing photosynthesis in maize (13). High-temperatures accelerate chlorophyll breakdown, reducing photosynthetic capacity and green leaf area in crops like maize and beans (14). In beans, temperatures above 30 °C limit Calvin cycle regeneration and

Table 1. Comparison of high day vs high night temperature effects

Parameter High day temperature (HDT) High night temperature (HNT)

References Temperature range High day temperature refers to conditions exceeding

35 °C during daytime.

High night temperature refers to conditions exceeding 25 °C during nighttime. (1, 40)

Main process affected Photosynthesis is the primary process affected under high day temperature.

Respiration is the primary process affected under high night temperature. (8, 13)

Rubisco activity Rubisco activity decreases significantly under high day temperature.

Rubisco activity is indirectly affected under high night temperature conditions. (8, 17)

Carbon assimilation Carbon assimilation is reduced due to direct inhibition of photosynthesis.

Carbon assimilation is reduced due to depletion of carbohydrate reserves. (14, 13)

Respiration rate Respiration rate shows a slight increase under high day temperature.

Respiration rate increases strongly under high night temperature. (13, 69)

Carbohydrate status Carbohydrate production is reduced under high day temperature.

Stored carbohydrates are depleted under high night temperature. (13, 69)

Yield impact Yield reduction ranges from moderate to severe under high day temperature.

Yield reduction is generally severe under high night temperature. (39, 40)

3 Plant Science Today, ISSN 2348-1900 (online) starch synthesis, causing metabolic bottlenecks despite initial increases in CO₂ assimilation (15) (Fig. 1).

Soybeans exposed to high night temperatures exhibit increased respiration, consuming 260 % more photosynthetic products and reducing seed filling efficiency. Rice and wheat experience non-structural carbohydrates (NSCs) reductions in stems due to elevated nighttime respiration, limiting energy availability for grain development (16). Nighttime heat disrupts assimilate transport from leaves to seeds in soybeans, lowering yield by 4.6 % per °C increase (17). Combined high day and night temperatures exacerbate yield losses in maize by reducing stomatal conductance and water-use efficiency more severely than individual stresses (18).

While crops activate ROS-scavenging enzymes (e.g. superoxide dismutase) under heat, prolonged stress overwhelms these systems, worsening oxidative damage (19). Varietal differences exist; for example, some soybean cultivars maintain stable protein content under HNT, while others suffer significant carbohydrate losses.

Elevated temperatures can impair photosystem II (PSII) activity, leading to decreased electron transport rates and reduced adenosine triphosphate (ATP) generation, which are essential for carbon fixation in crops like maize and wheat (20). Heat stress leads to the dissociation of the oxygen-evolving complex in PSII, disrupting electron transport and ATP generation, which are essential for carbon fixation in crops like maize and wheat (21). Heat stress induces significant alterations in chloroplast structure, particularly affecting thylakoid membranes. For instance, in a study on Brassica campestris L. (wucai), heat-sensitive cultivars exhibited severe damage to chloroplasts, including swelling and displacement from the cell wall, as well as ruptured or disappeared thylakoids. These structural damages led to decreased chlorophyll content and reduced photosynthetic efficiency (22).

Research on maize (Zea mays L.) has demonstrated significant genotypic variation in physiological traits under high-temperature stress. A study evaluating 21 maize genotypes found that certain cultivars maintained higher net photosynthetic rates, stomatal conductance and grain yield under elevated temperatures, highlighting the importance of selecting heat-resistant varieties for breeding programs (23). Heat stress negatively affects physiological processes by inhibiting photosynthesis through Rubisco inactivation and PSII damage, increasing respiration, degrading chlorophyll and reducing stomatal conductance and water-use efficiency, ultimately leading to reduced crop productivity (Table 2). High temperatures during both day and night significantly impair photosynthesis and carbon assimilation in major food crops through various mechanisms, including enzyme deactivation, oxidative damage and structural alterations in the

Table 2. Physiological effects of heat stress Process

Effect Outcome References Photosynthesis Heat stress causes rubisco inhibition and damage to photosystem II.

This results in reduced carbon fixation efficiency. (8, 17, 18)

Respiration Respiration rate increases significantly under heat stress.

This leads to increased carbon loss. (13, 69) Chlorophyll

Chlorophyll degradation occurs under elevated temperature conditions.

This results in reduced effective leaf area and photosynthetic capacity. (11)

Stomatal conductance Stomatal conductance initially increases and later decreases under heat stress.

This ultimately reduces CO₂ uptake by the plant. (48, 50)

Water-use efficiency Water-use efficiency declines under heat stress conditions.

This increases plant susceptibility to drought stress. (49, 51)

Fig. 1. Physiological effects of heat stress in plants.

DEVADHARSHINI ET AL 4 https://plantsciencetoday.online photosynthetic apparatus. Understanding these processes is essential for developing heat-tolerant crop varieties and implementing effective agricultural practices to sustain food production in a warming climate.

Oxidative stress and antioxidant defense mechanisms

High day and night temperatures induce oxidative stress by accelerating ROS production, including superoxide (O₂•-), hydrogen peroxide (H₂O₂) and hydroxyl radicals (•OH), which damage lipids, proteins and nucleic acids. Heat stress triggers lipid peroxidation, leading to malondialdehyde (MDA;

C₃H₄O₂) accumulation (5–50 nmol g-1 fresh weight), a biomarker of oxidative damage in wheat (Triticum aestivum L.), maize (Zea mays L.) and chickpea (Cicer arietinum L.). ROS accumulation damages chloroplast membranes and photosystem II (PSII, P680 reaction centre), reducing quantum yield (Fv/Fm) and crop productivity. Enzymatic antioxidants include superoxide dismutase (SOD, EC 1.15.1.1; 2 O₂•- + 2 H+ → H₂O₂ + O₂), catalase (CAT, EC 1.11.1.6; 2 H₂O₂ → 2 H₂O + O₂), ascorbate peroxidase (APX, EC 1.11.1.11; H₂O₂ + 2 ascorbate → 2 H₂O + 2 dehydroascorbate) and glutathione reductase (GR, EC 1.8.1.7; GSSG + NADPH + H+ → 2 GSH

+ NADP+). Non-enzymatic antioxidants include ascorbic acid (vitamin

C; C₆H₈O₆; 1–25 µmol g-1 FW) and reduced glutathione (GSH; C₁₀H₁₇N₃O₆S; 0.5–5 mM), which neutralise free radicals and maintain redox homeostasis (23).

Rice (Oryza sativa L.) shows decreased enzymatic antioxidant activities under heat stress, leading to increased ROS and yield loss.

Maize (Zea mays L.) exhibits higher antioxidant enzyme activity than rice, contributing to better heat tolerance. Wheat (Triticum aestivum L.) shows increased MDA but reduced antioxidant enzyme efficiency under prolonged heat. Soybean (Glycine max L. Merr.) benefits from moderate heat pre-treatment, which improves antioxidant responses via acclimation.

Heat-tolerant genotypes exhibit 1.5–3 fold higher expression of SOD, CAT, APX and GR compared to susceptible lines. Heat shock proteins (HSPs) help stabilise antioxidant enzymes under extreme temperatures (> 40 °C). Heat-tolerant wheat genotypes maintained higher activities of (SOD, CAT, APX and glutathione reductase (GR), which were associated with improved growth and reduced oxidative damage (24). Selenium (Se) application in chickpea enhanced antioxidant enzyme activities and reduced oxidative damage.

Similarly, heat-tolerant genotypes of Indian mustard (Brassica juncea L.) maintain higher antioxidant activities under heat stress, while susceptible genotypes show decline (25). Breeding heat-tolerant cultivars with enhanced antioxidant systems, alongside agronomic practices such as adjusted planting dates and

Se supplementation, holds promise for improving crop resilience to heat stress (26) (Table 3).

Heat stress induces oxidative stress by increasing reactive oxygen species, which leads to lipid peroxidation, while antioxidant enzymes such as SOD, CAT and APX are upregulated to mitigate cellular damage.

Heat shock proteins and thermo-tolerance mechanisms

In the face of escalating global temperatures, understanding plant responses to heat stress is paramount for ensuring agricultural sustainability. Heat shock proteins (HSPs) serve as molecular chaperones, safeguarding cellular proteins from denaturation and aggregation under thermal stress. Their roles are pivotal in maintaining cellular homeostasis, facilitating proper protein folding and enhancing plant resilience. Beyond intrinsic plant mechanisms, interactions with soil microbes and nutrient dynamics significantly influence thermotolerance, affecting overall plant health and productivity (27). These HSPs are critical molecular chaperones that stabilize protein folding and mitigate thermal damage in plants under extreme temperatures (Table 4).

Biotechnological applications Overexpression of AtHSP101 in tobacco enhances recovery post-HS by reactivating metabolic enzymes (28). CRISPR-edited HSP23.8 in tomato alters cold tolerance, demonstrating dual roles in temperature stress (29). Inoculation with beneficial microbes, such as Enterobacter sp. SA187, has been shown to upregulate key heat-responsive genes in

Arabidopsis, including HSFA2, HSP101, HSP70 and HSP90.1. This microbial association enhances thermotolerance by modulating hormonal pathways and activating heat memory genes. Plant growth- promoting rhizobacteria (PGPR) consortia, like B3P, can solubilise essential nutrients such as phosphorus and zinc (Zn) at elevated temperatures (45–50 °C). This solubilisation improves nutrient availability and uptake, bolstering plant growth and stress resilience (30). Heat stress can impair nutrient uptake by affecting peroxisomal functions, leading to reduced expression of nutrient transport proteins.

This impairment underscores the importance of peroxisomes in maintaining nutrient homeostasis under thermal stress (31).

Small heat shock proteins (SHSPs) not only prevent protein aggregation but also play roles in nutrient transport and signal transduction during stress conditions. Their expression is crucial for maintaining cellular functions and supporting plant development under heat stress (32). Plants employ epigenetic mechanisms, such as histone modifications, to regulate the expression of HSPs and other stress-responsive genes. These modifications enable plants to

'remember' previous heat exposures, leading to a more robust response upon subsequent stresses (33). The intricate network of heat shock proteins, in concert with microbial interactions and nutrient dynamics, forms the cornerstone of plant thermotolerance mechanisms. Harnessing these insights through biotechnological interventions and sustainable agricultural practices can pave the

Table 3. Oxidative stress and antioxidant defense Component

Role Heat response References ROS Reactive oxygen species cause oxidative damage in plant cells.

Their concentration increases significantly under heat stress. (21, 22)

SOD Superoxide dismutase converts superoxide radicals into less harmful molecules.

Its activity increases under heat stress. (23) CAT

Catalase detoxifies hydrogen peroxide into water and oxygen.

Its activity increases under stress conditions. (23)

APX Ascorbate peroxidase plays a role in scavenging reactive oxygen species.

Its activity increases during heat stress. (23) MDA

Malondialdehyde serves as a marker for lipid peroxidation.

It accumulates under heat stress conditions. (22) Ascorbate

Ascorbate acts as a non-enzymatic antioxidant.

Its concentration shows a moderate increase under stress. (23)

Note: reactive oxygen species (ROS), superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), malondialdehyde (MDA).

5 Plant Science Today, ISSN 2348-1900 (online) way for developing heat-resilient crops. Continued research into these synergistic relationships will be vital for ensuring food security in the face of climate change.

Impact on reproductive development and pollen viability

High temperatures during both day and night critically impair reproductive development and pollen viability in major food crops, posing significant threats to global food security. Male gametophytes (pollen) are highly sensitive to heat stress. In Arabidopsis, elevated temperatures disrupt meiosis, causing irregular spindle orientation and cytokinesis, leading to aneuploid gametes. Heat stress (≥ 34 °C) accelerates meiosis but prolongs the recombination-dependent pachytene/diakinesis phase, increasing crossover frequency.

Proteins essential for pollen tube growth degrade at temperatures above 32 °C, causing tube bursting or failure in crops like canola, rice and tomatoes. For example, canola yields dropped by ~70 % when exposed to 42 °C during flowering. While less studied, heat stress also affects ovule development and fertilisation.

In rice and wheat, high night temperatures reduce pistil viability and disrupt synchrony between pollen release and stigma receptivity (34). Lipid metabolism genes in the tapetum, critical for pollen wall formation, are repressed under heat, further compromising pollen viability. Day temperatures above 32 °C cause pollen sterility, forcing growers to narrow planting windows. Heat during reproduction reduces seed yield by 30–40 % and degrades nutritional quality. Wheat and barley exhibit reduced grain set under sustained heat due to pollen and ovule abortion (35). Heat shock factors (HSFs), transcription factor families including NAM, ATAF1/2 and CUC2 (NAC) and WRKY regulate stress responses.

Thermotolerant cultivars like rice Indica-type 996 maintain lipid metabolism and anther dehiscence under heat, highlighting genetic targets for breeding. Integrating genetics, AI-driven breeding and agronomic practices (e.g. adjusted planting schedules) can enhance resilience. For instance, heat-induced haploid induction in

Arabidopsis demonstrates potential for accelerating breeding programs. A study comparing basmati and non-basmati rice varieties found that at 35.5 °C, the basmati variety Pusa Sugandh 2 exhibited 17 % pollen sterility and a 26 % reduction in pollen germination. The primary cause of sterility was reduced anther dehiscence and less pollen deposition on the stigma at higher temperatures. Increased temperature during the grain-filling period also increased spikelet sterility in rice, with the basmati variety being more affected. This study suggested that increasing temperature could limit rice yield by affecting pollen germination and grain formation (36).

Continuous exposure of tomato 'Trust' to high temperatures (32/26 °C) markedly reduced the number of pollen grains per flower and decreased viability. The effect of heat stress on pollen viability was associated with alterations in carbohydrate metabolism in various parts of the anther during its development. Under control conditions (28/22 °C), starch accumulated in the pollen grains, but under high- temperature regimes, this accumulation was prevented, leading to decreased sugar concentration in the mature pollen grains and possibly contributing to decreased pollen viability (37). Severe heat stress (33 °C day–30 °C night for 2 days) caused rapid abortion and abscission of reproductive organs in field pea. High night temperatures (24 °C day–30 °C night) caused 25 % yield loss, whereas high day temperatures (32 °C day–15 °C night) caused only 8 % loss.

Elevated day temperatures ranging from 24 °C to 33 °C did not affect the number of seeds per pod or the seed to ovule ratio, whereas severe heat stress significantly reduced these parameters when day temperatures increased from 33 °C to 36 °C. These findings highlight the sensitivity of pollen functionality to high-temperatures and the potential impact on crop yield (38). High temperatures during both day and night critically impair reproductive development and pollen viability in major food crops through various mechanisms, including disrupted meiosis, degradation of essential proteins and altered carbohydrate metabolism. Understanding these processes is essential for developing heat-tolerant crop varieties and implementing effective agricultural practices to sustain food production in a warming climate. Integrating genetic, physiological and agronomic approaches will be key to enhancing crop resilience to heat stress (Table 5).

Reproductive processes are highly sensitive to heat stress, resulting in reduced pollen viability, disrupted meiosis, poor fertilisation and ultimately decreased seed set and grain formation.

Yield reduction and grain quality deterioration High day and night temperatures significantly reduce yields and degrade grain quality in major food crops, with impacts varying by crop and temperature regime. Each 1 °C rise in global mean temperature reduces wheat yields by 6.0 %, maize by 7.4 % and rice by 3.2 % under current adaptation levels (39). High nighttime temperatures are particularly damaging. Rice yields decline by 7 %

Table 4. HSP families, regulatory networks and crop-specific adaptations

Category HSP/Gene Crop Function References HSP100 SlHSP100

Tomato SlHSP100 enhances acquired thermotolerance (ATT) and helps maintain cell viability under heat stress conditions. (28)

HSP100 OsHSP101 Rice OsHSP101 improves basal thermotolerance (BTT) by facilitating the disaggregation of denatured proteins. (29)

HSP90 GmHSP90A2 Soybean Knockout of GmHSP90A2 using CRISPR-Cas9 reduces heat tolerance, indicating its essential role in stress adaptation. (28)

HSP70 CaHSP70 Pepper CaHSP70 prevents protein aggregation and enhances antioxidant activity during heat stress. (28) sHSP sHSP18.1

Potato sHSP18.1 prolongs protein stability and protects cellular structures under heat stress conditions. (28)

Regulatory HSFA2 Tomato HSFA2 activates the transcription of heat shock proteins, particularly in anthers during heat stress. (28)

Regulatory CHIP E3 ligase Tomato CHIP E3 ligase degrades misfolded proteins and maintains protein quality control during heat stress. (28)

Crop-specific OsHSP18.6 Rice OsHSP18.6 mitigates oxidative stress and enhances seedling survival under elevated temperatures. (28)

Crop-specific ZmHSP16.9 Maize ZmHSP16.9 stabilises chloroplast membranes and protects photosynthetic machinery under heat stress. (28)

Crop-specific SlHSP20 Tomato SlHSP20 is upregulated in thermotolerant lines and shows a strong correlation with acquired thermotolerance. (28)

DEVADHARSHINI ET AL 6 https://plantsciencetoday.online per 1 °C above the optimal 22 °C nighttime threshold due to reduced seed set (40). In maize, HNT shortens grain-fill period by 4–7 days and reduces kernel weight by 15–20 %. Soybean yields decrease by 3.1 % per 1 °C, with pollination failure as a key driver (41). Elevated temperatures increase rice chalkiness by 18–25 % and reduce head rice yield by 12 %. In wheat, heat stress during grain filling reduces starch content by 9–14 % and increases gliadin proteins linked to gluten intolerance. Barley HNT exposure reduces yield by 12–15 % and increases grain shrivelling (42, 43).

Research in the Western Guanzhong Plain showed that high temperatures during soybean flowering-podding stage reduced grains per plant and overall yield (44). Heat stress during wheat anthesis and grain filling damages photosystem II (PSII), increases

ROS production and reduces seed quality (45). A meta-analysis confirmed that each 1 °C increase in night temperature reduces wheat yield by 7 % (anthesis) and rice yield by 4 % (grain filling) (46).

Vernalisation (cold seed exposure) enhances barley resilience to heat stress. The Photoperiod-H1 (PPD-H1) gene interaction with vernalisation optimises flowering time in spring barley varieties, mitigating heat-induced yield losses (47). Understanding heat stress mechanisms-including disrupted meiosis, protein degradation and altered carbohydrate metabolism-is essential for developing heat- tolerant varieties through integrated genetic, physiological and agronomic approaches (Table 6). Yield reduction under heat stress varies among crops, with rice experiencing approximately 7 % loss per °C increase in nighttime temperature, wheat about 6 %, maize

7.4 % and soybean up to 4.6 %, mainly due to impaired reproductive development and increased respiration.

Water use efficiency and stomatal regulation Heat stress disrupts the balance between CO₂ uptake and water loss by altering stomatal behaviour. Elevated temperatures initially increase stomatal conductance (g+) for cooling, but prolonged heat reduces stomatal density (SD) and pore size, limiting CO₂ uptake (48). Rapid stomatal closure in C₄ crops (e.g. maize) improves water use efficiency (WUE) by up to 16 % compared to C₃ crops due to smaller stomatal size and faster kinetic responses (49). Heat-induced damage to guard cell chloroplasts and mitochondria leads to erratic stomatal behaviour.

Temperatures > 35 °C reduce chlorophyll fluorescence, ribulose-1,5-bisphosphate (RuBP) regeneration and electron transport rates in wheat, cotton and tomato by damaging PSII oxygen-evolving complexes (50). Heat stress also reduces root hydraulic conductivity and leaf water potential, exacerbating drought effects (51). In cotton, temperatures > 86 °F during flowering cause pollen sterility and boll abscission, reducing yields by

98 lbs·acre-1 per 1.8 °F increase.

Crop-specific adaptations: Rice OsEPF1oe mutants maintain

WUE under heat but suffer reduced cooling capacity. Exogenous melatonin stabilises stomatal conductance and improves WUE in tomato under combined heat and drought (52). Maize and sorghum outperform C₃ crops in fluctuating light due to faster stomatal closure. Natural variations in stomatal behaviour affect heat tolerance and intrinsic WUE in Vitis vinifera (53). Some drought- stressed plants open stomata during heatwaves to prevent lethal overheating despite water loss risk (54). Heat stress induces abscisic acid (ABA), promoting stomatal closure and enhancing antioxidant defenses in rice (55). Crops with higher stomatal density possess greater gas exchange potential for heat mitigation (56). Stomatal development is influenced by temperature and CO₂ levels (57).

Targeting stomatal development genes (e.g., EPF1) or optimising nitrogen supply can enhance heat resilience. Critical research gaps include interactions between heat, humidity and CO₂ on stomatal kinetics, long-term field studies on stomatal plasticity and root-shoot signalling roles (58). Understanding stomatal regulation is vital for breeding heat-resilient crops (Fig. 2).

Impact on nutrient uptake and soil-microbe interactions

Severe heat stress (40–42 °C) decreases nutrient-uptake and assimilation proteins in tomato roots, impairing nutrient acquisition and plant growth (59). Heat stress also reduces nitrogen (N), phosphorus (P) and potassium (K) concentrations in cool season turfgrass. In lentil (Lens culinaris L.), zinc and iron concentrations decline at 37 °C, while manganese (Mn) increases in wheat shoots during grain filling. Elevated temperatures reduce soil microbial diversity, with Firmicutes, Acidobacteria and Proteobacteria showing varied responses that affect nutrient cycling (60). Heat alters rhizosphere microbial community structure, influencing plant growth and stress responses (61). Soil temperature modifies root architecture and microbial interactions, affecting both mutualistic and pathogenic relationships (62).

High temperatures disrupt symbiotic relationships between plant roots and nitrogen-fixing microbes, decreasing nitrogen fixation rates and compromising plant nutrition (63). Elevated temperatures increase microbial metabolism and enzyme activity, accelerating soil organic matter decomposition and altering N

Table 5. Reproductive sensitivity to heat stress Parameter

Effect References Pollen viability Pollen viability decreases significantly under heat stress conditions. (34, 36)

Meiosis Meiosis is disrupted due to elevated temperature stress. (3, 34)

Anther dehiscence Anther dehiscence is reduced, limiting pollen release. (36)

Fertilisation Fertilisation efficiency decreases under heat stress. (34)

Seed set Seed set is reduced due to reproductive failure. (38)

Grain formation Grain formation is impaired under high temperature conditions. (9)

Table 6. Crop-wise yield reduction under heat stress

Crop Temperature type Yield reduction Key reason References

Rice High night temperature Yield decreases by approximately

7 % per °C increase.

This is mainly due to spikelet sterility. (40, 46)

Wheat High day and night temperature Yield decreases by approximately

6 % per °C increase.

This is due to reduced grain filling duration. (39, 46)

Maize High day temperature Yield decreases by about 7.4 % per °C increase.

This is mainly due to kernel weight loss. (39) Soybean

High night temperature Yield decreases by about 3.1–4.6

% per °C increase.

This is due to increased respiration losses. (41, 69)

Barley High night temperature Yield reduction ranges between

12–15 %.

This is due to grain shrivelling. (43) 7 Plant Science Today, ISSN 2348-1900 (online) availability (64). Heat stress affects soil carbon and N sources and enzyme activities, influencing nutrient cycling (65). Certain microbes mitigate heat stress by enhancing antioxidant enzyme activity and modulating hormonal balances in plants (66). Biochar-induced changes in soil microbial communities influence nutrient availability, depending on feedstock type and pyrolysis temperature.

Understanding heat-induced disruptions to nutrient uptake and soil- microbe interactions is essential for developing resilient agricultural systems through beneficial microbes, biochar amendments and optimised soil management.

Nutrient uptake and soil–microbe interactions under high temperatures

Heat stress significantly affects nutrient uptake and metabolism in crops. It reduces N, P and K levels, while micronutrients such as Zn and Fe decline under elevated temperatures, although Mn may increase in some cases. The HNT disrupts carbon allocation, accelerates senescence and reduces grain filling in crops like rice and wheat, while soybean experiences carbohydrate depletion due to increased respiration (67). Prolonged heat stress also alters starch metabolism and reduces seed weight and grain quality (68).

Soil–microbe interactions are also influenced by temperature stress.

Mycorrhizal colonisation may increase with temperature, but microbial community structure shifts under combined heat and drought conditions, affecting plant growth (69). Heat stress alters nutrient mobilisation pathways and reduces nutrient uptake efficiency, partly through changes in uptake proteins and metabolic processes. Overall, elevated temperatures disrupt nutrient dynamics and soil–microbe interactions, negatively impacting crop productivity. However, strategies such as utilising beneficial microbes, biochar application and improved soil management can enhance resilience under heat stress conditions (Fig. 3).

Genetic and molecular breeding for heat tolerance The HNT and daytime temperatures significantly threaten global food security by impairing physiological and molecular processes in crops. Recent advances in genetic and molecular breeding aim to address these challenges through innovative strategies. A structured review of their impacts and breeding solutions across major crops are given below. Gene editing using CRISPR/Cas9 has been shown to improve heat tolerance in crops by enhancing antioxidant enzyme activities, thereby mitigating oxidative damage under heat stress (70). Marker-assisted backcross breeding has been effectively utilised in bread wheat to incorporate heat-tolerant traits, accelerating the development of resilient varieties (71). Combining quantitative trait loci (QTLs) for early morning flowering and heat tolerance in rice has proven effective in maintaining spikelet fertility under high- temperature stress during flowering (72). Development of molecular markers linked to heat shock proteins, such as GhHSP70-26 in cotton, facilitates the selection of heat-tolerant genotypes through marker-assisted breeding (73).

Precise editing of the SlHyPRP1 gene in tomato using

Fig. 2. Water use efficiency and stomatal regulation.

DEVADHARSHINI ET AL 8 https://plantsciencetoday.online

CRISPR/Cas9 has resulted in enhanced tolerance to multiple stresses, including heat, by modifying specific protein domains (74).

High-throughput genotyping and phenotyping have been employed in pepper to implement genomic selection, identifying genotypes with superior heat tolerance. Advancements in CRISPR/Cas9 technology have been applied to legume crops, enhancing their resilience to heat stress and improving overall productivity (75).

Combining transcriptomics, metabolomics and genomics provides a comprehensive approach to identify novel targets for breeding heat-tolerant crops (76). Genome editing prospects using CRISPR/

Cas9 in cereal crops like rice and wheat offer potential for developing climate-smart varieties with enhanced heat stress tolerance (77).

Marker-assisted selection has been utilised in cole crops to introgress heat-tolerant QTLs linked to photosynthetic efficiency and HSP70 expression, improving thermotolerance (78).

Impact of high nighttime temperatures HNT reduces spikelet fertility and induces grain chalkiness in rice, causing yield losses up to 50 % (79). It accelerates leaf senescence, disrupts carbohydrate metabolism and increases respiration rates, leading to carbon deficits. Hormonal imbalances occur under HNT, with ABA rising and auxins declining. Tolerant rice genotypes like Oryza glaberrima (CG14) exhibit resilience via thermostable proteins such as

TT1 (80). Gene editing targeting HTG3 and SLG1 enhances thermotolerance by regulating heat shock proteins and tRNA thiolation.

Wheat genotypes Raj-4037 and PBW590 maintain photosynthetic efficiency under HNT through stabilised stomatal conductance (81).

Daytime heat stress (> 35 °C) reduces photosynthetic efficiency in wheat and chickpea, lowering biomass by 22–30 %. In maize, temperatures above 38 °C during flowering disrupt pollen viability, reducing kernel set.

Breeding breakthroughs of marker-assisted selection (MAS) identifies QTLs linked to membrane stability and antioxidant production in wheat. Heat-tolerant chickpea genotype ICC15614 shows enhanced stomatal conductance and seed yield under stress.

Tomato Hybrid 61 maintains photosynthesis under combined heat and drought via robust stomatal regulation.

MAS accelerates introgression of heat-tolerant QTLs linked to photosynthetic efficiency and HSP70 expression in cole crops. Rice genomic selection leverages natural variants like TTL1 and GSA1 to improve grain quality under heat. CRISPR/Cas9 edits negative regulators of heat tolerance (e.g. TT2) in rice, reducing wax synthesis and cooling leaves. Overexpression of HSP70 in Brassica napus improves thermotolerance and photosynthesis. Limited genetic diversity in modern cultivars necessitates exploring wild relatives and landraces.

Integrating multi-omics data (transcriptomics, metabolomics) can identify novel targets like flavonoid glycosides and ROS scavengers.

Field validation of gene-edited lines under realistic climate scenarios remains critical. Integrating genetic and molecular breeding techniques, including CRISPR/Cas9 and MAS, has advanced heat- tolerant crop development. Continued research is essential for food security under climate change (82).

Physiological acclimation and epigenetic regulation

As global temperatures rise, crops are increasingly exposed to elevated daytime and nighttime temperatures, challenging their growth and productivity. Plants have evolved intricate physiological and epigenetic mechanisms to acclimate to these thermal stresses.

Understanding these adaptations is crucial for developing resilient crop varieties in the face of climate change. In Arabidopsis, H3K4me3 correlates with active gene expression, while H3K4me2 shows a negative correlation in rice. Alterations in these marks misrelate genes related to development. Genes like HSP18.2, HSP21 and

HSP22.0 exhibit increased H3K4me3 and H3K4me2 modifications under continuous heat stress, enhancing their expression during

Fig. 3. Nutrient uptake and soil–microbe interactions under high temperatures.

9 Plant Science Today, ISSN 2348-1900 (online) subsequent stress events (83). These enzymes remove H3K27me3 marks from small HSP genes, maintaining their expression during heat stress and contributing to stress memory (84). Heat stress activates elements like the LTR-copia-type retrotransposon ONSEN in Arabidopsis, contributing to the plant's stress response. DNA methylases and demethylases regulate the dynamic status of DNA methylation under heat stress, ensuring rapid and precise genomic responses (85). Identified as essential for the sustained induction of heat stress memory genes, FGT1 interacts with chromatin remodelling complexes like SWI/SNF (86). These modifications play a crucial role in epigenetic regulation under heat stress, altering chromatin structure and gene expression (87). Histone modifications, such as H3K4me3, can serve as transgenerational epigenetic signals, affecting gene expression in subsequent generations.

Epigenetic mechanisms, including histone modifications and chromatin remodelling are essential for regulating thermomorphogenesis, allowing plants to fine-tune their growth responses-such as stem elongation and leaf angle adjustments- under elevated temperatures. These processes involve dynamic interactions between histone deacetylases (e.g., HDA9), histone chaperones (e.g. FACT complex) and transcription factors like PIF4, which together modulate chromatin accessibility and gene activation in a temperature-dependent manner.

Histone methylation is a critical epigenetic modification that regulates gene expression and cellular differentiation in plants, playing a significant role in plant regeneration. By influencing chromatin structure, histone methylation can either activate or repress genes essential for developmental pathways, often in conjunction with DNA methylation to form a complex regulatory network.

Recent research highlights the importance of understanding these mechanisms to enhance plant regeneration techniques and develop resilient crop varieties. As scientists continue to unravel the specific pathways involved, the potential for innovative applications in plant biotechnology and agriculture grows, offering new strategies for improving plant traits and stress responses (88).

Epigenetic regulation of heat responses 1. Chromatin remodelling

•Histone modifications: Heat stress induces H3K4me3 activation marks at heat shock protein (HSP) loci (e.g. HSP70, HSP90), enhancing thermotolerance. Conversely, H3K27me3 repressive marks silence growth-related genes under stress (89).

DNA methylation: Dynamic cytosine methylation at transposable elements (TEs) stabilises genomes during stress. Demethylase ROS1 activity is critical for maintaining epigenetic plasticity in tomatoes under fluctuating temperatures (89, 90).

2. Stress memory and priming •Somatic memory: Pre-exposure to sublethal heat primes plants through HSFA2-mediated histone acetylation, enabling faster HSP induction during recurrent stress (90).

Transgenerational inheritance: Heat-induced DNA methylation changes in parental gametes enhance offspring stress tolerance in cereals, though stability varies across generations.

3. Thermosensory epigenetics •Thermo-morphogenesis: PIF4 activation under warm temperatures requires histone deacetylase (HDA9) and the SWR1 complex, which evicts H2A.Z nucleosomes to depress growth genes.

Circadian Coordination: Evening complex proteins (e.g. ELF3) interact with histone modifiers to align heat responses with circadian rhythms, optimising resource allocation (90).

Crop- specific adaptive strategies In Arabidopsis, H3K4me3 correlates with active gene expression, while H3K4me2 shows a negative correlation in rice. Alterations in these marks misrelate genes related to development. Genes like

HSP18.2, HSP21 and HSP22.0 exhibit increased H3K4me3 and

H3K4me2 modifications under continuous heat stress, enhancing their expression during subsequent stress events.

Future directions Precision epigenome editing: Tools like CRISPR-dead Cas9 (CRISPR- dCas9) could target stress-responsive loci to enhance climate resilience without altering genome sequences.

Diurnal stress modelling: Integrating day-night temperature fluctuations into crop models will improve yield predictions under realistic warming scenarios (90, 91). The dual impact of high daytime and nighttime temperatures presents a formidable challenge to global food production, especially under the looming threat of climate change. However, crops exhibit remarkable resilience through finely tuned physiological acclimation mechanisms—such as thermomorphogenesis, respiration adjustment and altered resource allocation-as well as epigenetic regulatory networks that mediate gene expression in response to thermal stress.

Advances in understanding epigenetic modifications- including DNA methylation, histone acetylation/methylation and chromatin remodelling-highlight how plants establish stress memory and even transgenerational inheritance of thermotolerance. These insights open avenues for epigenome engineering, wherein tools like CRISPR-dCas9 can precisely modify regulatory regions without altering the genome, offering new strategies for climate-resilient crop breeding. Moving forward, integrating multi-omics data, circadian biology and diurnal thermal modelling will be essential to design crops that are not only thermotolerant but also optimised for future agroecological conditions. Strategic investments in epigenetics-informed breeding, coupled with field-level validation, will play a critical role in achieving sustainable food security in an increasingly warming world.

Modelling climate change impacts on crop productivity

Climate change poses a significant threat to global agriculture, with rising temperatures-particularly during nighttime-adversely affecting crop productivity. The intergovernmental panel on climate change (IPCC) projects potential yield losses of 10–30 % by 2050 due to these climatic shifts. According to IPCC understanding the multifaceted impacts of both nocturnal and diurnal warming on various crops and regions is crucial for developing adaptive strategies to ensure future food security. A study found that elevated nighttime temperatures reduced nocturnal stomatal conductance in wheat, leading to decreased grain yields without altering daytime physiological responses. This suggests that nocturnal warming independently affects yield components (92). Contrary to many findings, research in

North China indicated that a moderate increase in nighttime temperatures improved winter wheat yields by enhancing plant development and grain growth, highlighting regional variability in responses(93). Another study demonstrated that winter night- warming enhanced post-anthesis physiological activities and sink

DEVADHARSHINI ET AL 10 https://plantsciencetoday.online strength in winter wheat, contributing to better grain filling and yield (94). Increased night temperatures adversely affected grain yield, biomass and grain number in Chilean quinoa, emphasising the sensitivity of certain crops to nocturnal warming(95).

The multifaceted impacts of climate change on agriculture, encompassing both nocturnal and diurnal temperature increases, underscore the urgency for comprehensive modelling and adaptive strategies. While some regions may experience temporary benefits, the overarching trend indicates significant risks to global food security. Addressing these challenges requires integrating scientific insights into policy-making, investing in resilient agricultural practices and ensuring inclusive support for vulnerable populations, particularly in the Global South.

Mitigation strategies High day and night temperatures, exacerbated by climate change, significantly threaten global food security by impairing the growth, development and yield of major food crops. Agronomic mitigation strategies are essential to counteract these adverse effects. This review outlines key agronomic practices aimed at enhancing crop resilience to elevated temperatures (Table 7). Various mitigation strategies, including agronomic practices, irrigation management, breeding approaches, molecular techniques such as CRISPR/Cas9, chemical treatments and microbial inoculation, have been developed to enhance crop resilience to heat stress.

Future perspectives and biotechnology approaches Global climate change has increased the occurrence of high day and night temperatures, adversely affecting crop growth, physiology and yield.

Addressing these challenges requires advanced biotechnological and breeding strategies to develop heat-resilient crops. Key approaches include functional genomics, which identifies heat-responsive genes and pathways using GWAS, transcriptomics and proteomics (96); genome editing technologies such as CRISPR/

Cas9 that enable precise modification of stress-responsive genes (97); and exploitation of natural genetic variation through marker- assisted selection and QTL mapping to develop tolerant cultivars (98). The application of plant growth regulators (PGRs), including salicylic acid, enhances antioxidant defense and improves yield under heat stress (99). Furthermore, integration of multi-omics approaches (genomics, transcriptomics, proteomics and metabolomics) provides a comprehensive understanding of stress- response mechanisms and accelerates the development of climate- resilient crops.

Conclusion High day and night temperatures present a formidable threat to the sustainability of global food systems, especially under ongoing climate change. These thermal stresses disrupt critical physiological functions including photosynthesis, reproductive development, WUE and nutrient dynamics, ultimately leading to significant yield reductions and deteriorated grain quality in major food crops. The interplay between HDT and HNT exacerbates stress responses, compounding the challenges faced by modern agriculture. However, plants exhibit a range of defense mechanisms, such as enhanced antioxidant activity, expression of heat shock proteins and epigenetic regulation, which can be leveraged to develop heat-resilient cultivars. Emerging genetic and molecular breeding tools, including CRISPR/Cas9 and genomic selection, alongside targeted agronomic practices like optimised sowing schedules and foliar treatments, offer promising solutions to counteract heat stress effects. Moving forward, integrating physiological, molecular and ecological insights will be critical for designing climate-smart agriculture systems capable of sustaining productivity under extreme temperature regimes.

Acknowledgements The authors express their sincere gratitude to the Tamil Nadu

Agricultural University for providing academic support and necessary facilities to carry out this review work. The authors also acknowledge the support and encouragement provided by the faculty members of the Agro Climate Research Centre for their valuable suggestions and guidance during the preparation of this manuscript. The first author expresses heartfelt thanks to the advisory committee members for their continuous guidance, constructive criticism and encouragement throughout the preparation of this review article. The authors also acknowledge the researchers and scientists whose published works contributed significantly to the development of this comprehensive review.

Authors' contributions DS conceptualized the review topic, conducted an extensive literature survey, compiled and analysed the information and prepared the original draft of the manuscript. NKS provided overall guidance, supervised the work, critically reviewed the manuscript and contributed to technical corrections and improvement of the scientific content. SK assisted in manuscript review, provided valuable suggestions and contributed to refining the scientific

Table 7. Mitigation strategies Strategy Approach Example

References Agronomic Adjusting sowing dates helps crops avoid heat stress during sensitive stages.

Sowing is timed to avoid heat during flowering. (6)

Irrigation Irrigation provides a cooling effect and reduces canopy temperature.

Timely watering helps mitigate heat stress. (51) Breeding

Development of heat-tolerant varieties improves crop resilience.

Indica rice varieties show better heat tolerance. (6, 73)

Molecular Gene editing techniques enhance stress tolerance at the molecular level.

CRISPR/Cas9 is used for developing heat- tolerant crops. (71, 78)

Chemical Plant growth regulators help improve stress tolerance.

Application of salicylic acid enhances heat tolerance. (99)

Microbial Beneficial microbes improve plant stress resilience.

PGPR such as Enterobacter species enhance tolerance. (30)

11 Plant Science Today, ISSN 2348-1900 (online) interpretation and structure of the article. VM contributed to manuscript editing, validated the technical information and improved the discussion section. CK reviewed the manuscript and provided constructive comments to improve its clarity and scientific accuracy. KV assisted in manuscript proofreading, formatting and final corrections before submission. All authors read and approved the final manuscript.

Compliance with ethical standards Conflict of interest: Authors do not have any conflict of interest to declare.

Ethical issues: None

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