The gradual increase in global average global temperatures, especially in tropical and subtropical regions, poses significant challenges to warm-season food crops (Jha et al., 2014), such as mung beans, compromising their yield and nutritional security (Bhardwaj et al., 2023). Crops thrive within specific temperature ranges (minimum and maximum), with those exceeding the maximum considered heat stress (Wahid et al., 2007; Hasanuzzaman et al., 2013; Kumari et al., 2021). Heat stress triggers various morphological, anatomical, physiological, biochemical, and molecular changes in leaves, flowers, and seeds, thereby curtailing the overall growth and yield (Bita and Gerats, 2013). Manifestations include leaf and stem scorching, leaf abscission, senescence of leaves, flowers, and fruiting structures, and reduced growth of leaves, shoots, roots, and seeds (Vollenweider and Günthardt-Goerg, 2005; Chaudhary et al., 2022b).

Heat stress profoundly affects the reproductive stage of plants, leading to flower and fruiting structure abortion, as observed in cereals (Niu et al., 2021) and legumes (Liu et al., 2019; Basu et al., 2022). Seed filling plays a crucial role in determining the seed weight and yield (Devasirvatham et al., 2012; Jiang et al., 2015). This process involves the transportation of precursor molecules and minerals from leaves to seeds to synthesize storage constituents, such as carbohydrates, fats, and proteins (Barnabás et al., 2008; Awasthi et al., 2014). Abiotic stresses, such as heat, negatively impact leaf function, flowering, seed development, and seed composition in legumes (Sarkar et al., 2021), causing significant yield losses (Kumar et al., 2021).

Mung bean (Vigna radiata (L.) R. Wilczek) is the second most cultivated leguminous crop after chickpea and is grown on over six million hectares globally. India contributes approximately 54% of the global mung bean production (HanumanthaRao et al., 2016; Nalajala et al., 2023). Mung bean seeds have high nutritional value, containing 22%–27% protein; essential vitamins such as B5, B6, thiamine, and niacin; minerals such as magnesium and iron; and dietary fiber. Mung bean seeds also contain beneficial compounds, such as alkaloids, coumarins, phytosterins, polyphenols, oligosaccharides, and antioxidants, which support physiological metabolism in animals and humans (Tang et al., 2014). Mung beans have low input requirements, are adaptable, and are short-duration grain legumes (65–90 days) (HanumanthaRao et al., 2016).

Mung beans are primarily grown in tropical areas characterized by dry or semiarid conditions, usually during fall and summer. Their ideal temperature range is 27°C–30°C (Pannu and Singh, 1993). However, as a warm-season crop, mung beans are susceptible to extreme heat (Sharma et al., 2016), impairing vegetative and reproductive growth (HanumanthaRao et al., 2016).

Although the adverse impact of heat stress on overall plant growth and yield is well established (Sharma et al., 2016), little is known about its influence on pod formation, seed filling, seed size, and seed quality in mung bean. Therefore, this study aimed to determine the detrimental effects of heat stress during the reproductive and seed-filling stages and evaluate its impact on mung bean seed-filling duration and seed quality. Furthermore, we sought to identify the genetic traits associated with heat tolerance and susceptibility among the contrasting mung bean genotypes. This information holds the potential to greatly enhance breeding programs aimed at developing heat-tolerant mung bean varieties for sustainable production in regions prone to high temperatures.

Tables 1 , 2 show the mean sum of squares and genotype × treatment interaction significance for various traits, respectively.

Our investigation focused on the impact of heat stress on seed filling in two contrasting mung bean genotypes. Studies have shown that heat stress adversely affects pod set and reduces seed number in mung bean (Sharma et al., 2016; Van Haeften et al., 2023). Our results showed that heat stress decreased seed number plant⁻¹ in the HT and HS genotypes, likely due to a reduction in filled pods, similar to observations in other crops such as wheat (Balla et al., 2019), maize (Niu et al., 2021), lentil (Choukri et al., 2020), pea (Mohapatra et al., 2020), urdbean (Chaudhary et al., 2022a), and chickpea (Devi et al., 2022). Heat stress also significantly reduced seed weight and size, possibly due to decreased seed-filling rate and duration (Sarkar et al., 2021). The accelerated maturation of legume plants under heat stress could also hinder seed filling, decreasing seed weight (Gaur et al., 2015).

Studies on various crops, including maize (Wilhelm et al., 1999), rice (Begcy et al., 2018), mung bean (Bhardwaj et al., 2023), field pea (Sharma et al., 2023), and chickpea (Devi et al., 2022), have documented the adverse effects of heat stress on seed size. Heat stress disrupts seed development by impeding the transport and utilization of molecules and minerals, including carbohydrates such as sucrose, which serve as precursors for protein and fat synthesis (Kaushal et al., 2013). The leaves of heat-stressed mung bean plants showed chlorosis and necrosis, resulting in poor photosynthetic efficiency in both the genotypes. These changes correlated with significant reductions in leaf water status (RLWC) and stomatal conductance, especially in HS genotypes. High temperatures and photooxidation can damage leaf tissues and membranes, leading to the loss of chlorophyll pigments (Ibrahim, 2011; Chen et al., 2012 ; Prasad et al., 2017). Consequently, sucrose production declines, limiting its availability and transport to the developing seeds (Weber et al., 1997; Weschke et al., 2000; Asthir et al., 2012). Reduced photosynthetic ability can impede seed filling in legumes, as has beeb reported in chickpea (Gaur et al., 2015). Moreover, heat stress adversely affects sucrose transport more than photosynthesis, further inhibiting sucrose synthesis (Asthir et al., 2012; Phan et al., 2013; Liu et al., 2019).

Seeds consist primarily of carbohydrates, proteins (including storage proteins), lipids, and minerals. These organic compounds originate from precursor molecules transported from the leaves to seeds. Invertases are essential for releasing glucose and fructose from imported leaf sucrose, with glucose involved in starch synthesis during seed development. However, under heat stress, the activities of the enzymes responsible for sucrose synthesis and utilization (sucrose synthase and acid invertases, respectively) and starch synthesis (soluble starch synthase) notably diminished in this study, decreasing carbohydrate accumulation in seeds. Similar adverse effects on enzymes involved in carbohydrate and protein synthesis have been reported in various crops, including chickpea (Kaushal et al., 2013), lentil (El Haddad et al., 2021), common bean (Soltani et al., 2019), and wheat (Zhao et al., 2008). We found that the HS genotype had more pronounced inhibition of sucrose synthase, acid invertase, and soluble starch synthase than the HT genotype, resulting in a greater reduction in seed size, possibly due to the decreased import of sucrose into seeds. Reduced starch synthesis in mung bean seeds further hindered their development, possibly due to the low acid invertase activity decreasing the availability of glucose precursors. Similar observations have been reported in heat-stressed maize (Wilhelm et al., 1999; Yang et al., 2018), wheat (Liu et al., 2011), and lentil (Sita et al., 2018). Sucrose synthase and starch synthase activities increased from 5 to 15 DAP in both mung bean genotypes but subsequently decreased, more so in the HS genotype than the HT genotype, similar to the patterns observed in the grain of heat-stressed maize (Yang et al., 2018) and ‘basmati’ rice (Ahmed et al., 2015).

Heat stress significantly reduced grain protein content, including storage proteins, in mung bean, indicating a lack of sufficient precursors and inhibition of biosynthetic enzymes (Triboi et al., 2000). Similar reductions in grain protein content in high-temperature environments have been reported in other crops such as wheat (Wilhelm et al., 1999) and lentil (Sita et al., 2018). Interestingly, heat-stressed maize experienced an increase in protein content due to the activation of related enzymes (Yang et al., 2018). Understanding the mechanisms underlying the decreased protein synthesis in heat-stressed mung bean seeds is essential for future studies. The decline in protein content can reduce grain nutritional quality, highlighting the need to explore strategies to mitigate the adverse effects of heat stress on crops.

The reduced fat synthesis in heat-stressed mung bean seeds is likely due to impaired photosynthesis, reducing acetyl-CoA production, as reported in the leaves of heat-stressed common bean (Phaseolus vulgaris L.) (Taiz and Zeiger, 2006). Similar studies have found that heat stress significantly decreases fat accumulation in mung bean seeds, consistent with observations in canola (Brassica napus L.) seeds exposed to high temperatures (Aksouh et al., 2001). However, the mechanism underlying the decrease in fat accumulation in heat-stressed mung bean seeds requires further investigation.

Heat-stress-induced leaf damage may have decreased the enzyme profiles in seeds, further disrupting photosynthesis due to the source–sink relationship interrupting seed filling. Positive correlations were observed between soluble starch synthase activity and carbohydrate content in mung bean seeds, indicating the significance of this enzyme in controlling carbohydrate synthesis during storage. Similarly, positive associations occurred between sucrose synthase activity and sucrose, sucrose synthase and soluble starch synthase, and sucrose synthase and acid invertase activity, suggesting that sucrose synthase regulates sucrose metabolism and starch synthesis in mung bean seeds. Our findings corroborate those of Fan et al. (2019), who emphasized the importance of these enzymes in carbohydrate storage. These correlations highlight the potential of targeting these enzymes to enhance mung bean yield and quality under heat stress conditions.

Incorporating genotypes with diverse heat tolerance in this study captured a broad range of responses to high-temperature stress and enhanced our understanding of the underlying mechanisms involved. The PCA revealed significant differences between HT and HS genotypes across 17 traits, including leaf traits, seed traits, yield traits, and enzyme activities. The strong negative impact of electrolyte leakage on PC1 highlighted its pivotal role in determining heat sensitivity in mung bean genotypes. Conversely, positive contributions to PC1, including RLWC, stomatal conductance, seed weight per plant, carbohydrates, proteins, soluble starch synthase, and acid invertase, likely play integral roles in determining heat tolerance. These findings underscore the significance of maintaining appropriate plant hydration, managing stomatal conductance, and ensuring adequate levels of carbohydrates, proteins, and enzymes, such as soluble starch synthase and acid invertase, in developing HT mung bean genotypes. This knowledge can aid in identifying potential markers for selecting heat-tolerant genotypes and developing heat-resistant mung bean varieties. Previous studies have demonstrated that heat-tolerant genotypes often exhibit superior leaf traits, including membrane integrity (chickpea; Kumar et al., 2013; Devi et al., 2022), leaf water content (urdbean; Chaudhary et al., 2022a), chlorophyll levels (lentil; Sita et al., 2017), photosynthetic activity (mung bean; Sharma et al., 2016), and yield traits (Sita et al., 2017; Chaudhary et al., 2022b; Devi et al., 2022; Devi et al., 2023).

This study revealed that heat stress significantly impacts the yield and quality of mung bean seeds during the seed-filling phase. Heat stress damages leaves and seeds, reducing seed numbers and weight and altering seed composition, encompassing carbohydrates, proteins, lipids, and minerals. Reductions in these crucial constituents can compromise seed nutritional value, posing potential risks to food security. Our investigations revealed that the two mung bean genotypes responded differently to high-temperature stress. The HT genotype (EC 693369) exhibited greater resilience to heat stress and maintained optimal leaf and seed functions at elevated temperatures, whereas the HS genotype (KPS1) experienced considerable damage to various cellular and enzymatic attributes. These findings suggest that the HT genotype could be the preferred choice for cultivation in regions susceptible to heat stress, as it can sustain higher yields and seed quality. These results underscore the detrimental effects of high-temperature stress on mung bean seed yield and quality, emphasizing the need to develop HT cultivars to ensure food security in areas prone to heat stress. Furthermore, this investigation emphasizes the importance of understanding the physiological mechanisms underlying heat stress responses in plants to develop effective strategies to alleviate the unfavorable effects of heat stress on crops.

The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding authors.

MP and AB: conducting the experiments, analysis, compilation of findings, writing, UJ: statistical analysis and interpretation, BH: provided seeds, editing inputs, PVVP, KDS, and KS: review and editing, HN and KS: conceptualization, final review, and editing. All authors contributed to the article and approved the submitted version.