Molecular Plant Breeding 2025, Vol.16, No.1, 82-92 http://genbreedpublisher.com/index.php/mpb 83 tolerance (Kamara et al., 2021; Langridge and Reynolds, 2021; Al-ashkar et al., 2023). These efforts include the identification and selection of key physiological traits, such as membrane stability and photosynthetic efficiency, as well as the incorporation of heat tolerance genes through genetic engineering and marker-assisted selection (MAS) (Farooq et al., 2011; Kamara et al., 2021; Al-ashkar et al., 2023). The development of heat-tolerant wheat varieties not only helps mitigate the adverse effects of climate change but also contributes to the stability and resilience of wheat production systems (Langridge and Reynolds, 2021; Al-ashkar et al., 2023). This study will summarize the current status and methods of breeding heat-tolerant wheat varieties, providing an overview of the physiological, molecular, and genetic strategies applied in heat tolerance breeding. It will highlight the most promising approaches and identify key areas for future research. This study serves as a valuable resource for researchers, breeders, and policymakers, promoting the development of heat-tolerant wheat varieties to ensure sustainable wheat production in the context of global climate change. 2 Effects of Heat Stress on Wheat Physiology and Yield 2.1 Impact on photosynthesis Heat stress significantly impairs the photosynthetic efficiency of wheat, primarily by causing metabolic limitations and oxidative damage to chloroplasts, which in turn reduces dry matter accumulation and grain yield (Farooq et al., 2011). High temperatures inhibit photosynthesis by damaging the photosystem II (PSII), a critical component of the photosynthetic machinery. The critical temperature at which PSII begins to incur damage varies among genotypes, suggesting that genetic variation can be exploited to improve photosynthetic heat tolerance (Coast et al., 2022). Additionally, heat stress leads to the deactivation of Rubisco, a key enzyme in the photosynthetic process, further reducing the production of photoassimilates necessary for grain filling and yield (Tao and Han, 2024). Enhanced photosynthesis under heat stress conditions has been observed in genotypes that exhibit delayed senescence and improved assimilate remobilization, indicating that these traits could be targeted in breeding programs to improve heat tolerance (Kumar et al., 2023). 2.2 Sensitivity during flowering and grain filling The reproductive and grain-filling phases of wheat are particularly sensitive to heat stress, which can lead to significant yield reductions. High temperatures during these stages can shorten the grain filling duration, limit resource allocation to grains, and induce early senescence, resulting in lower productivity (Bergkamp et al., 2018). Terminal heat stress, which occurs towards the end of the growing season, is especially detrimental as it affects phenological traits such as biomass and crop duration, both of which are positively correlated with seed yield (Kumar et al., 2023). The grain filling stage is also critical, as heat stress during this period can decrease single seed weight and overall grain yield. Genetic improvements in heat tolerance have shown that certain genotypes can maintain higher chlorophyll content and shoot dry weight under heat stress, which are indicative of better heat tolerance mechanisms (Fu et al., 2023). Moreover, breeding efforts have demonstrated that selecting for traits such as membrane stability, photosynthetic rate, and grain weight under heat stress can lead to the development of more heat-tolerant wheat varieties (Farooq et al., 2011). 2.3 Impact on grain quality Heat stress not only affects the yield but also the quality of wheat grains. Elevated temperatures can alter the composition of starch and protein in the grains, which are critical determinants of grain quality. For instance, heat stress can accelerate the conversion of soluble sugars to starch in wheat grains, resulting in faster grain filling but potentially compromising the quality (Zhang et al., 2022). The biochemical activity of enzymes such as superoxide dismutase (SOD), peroxidase (POX), and ascorbate peroxidase (APX) is induced in heat-tolerant genotypes, which helps mitigate oxidative damage and maintain grain quality under heat stress (Kumar et al., 2023). Additionally, the interplay of various systems comprising antioxidants and hormones plays a crucial role in maintaining cellular homeostasis and grain quality under heat stress conditions (Lal et al., 2021). Therefore, breeding programs that focus on improving both yield and quality under heat stress conditions are essential for developing wheat varieties that can withstand the challenges posed by global warming (Yadav et al., 2022).
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