Molecular Plant Breeding 2025, Vol.16, No.1, 82-92 http://genbreedpublisher.com/index.php/mpb 84 3 Genetic Basis of Heat Tolerance in Wheat 3.1 Quantitative trait loci (QTL) associated with heat tolerance QTL mapping has been instrumental in identifying genomic regions associated with heat tolerance in wheat. Several studies have pinpointed key QTLs that contribute to heat tolerance by influencing various phenotypic traits. For instance, a significant QTL hotspot on chromosome 4D was identified, which includes six major QTLs associated with traits such as shoot length, root length, and whole plant length under heat stress conditions (Lu et al., 2022). Another study identified a major QTL on chromosome 7AL, which was linked to heat tolerance traits such as SPAD chlorophyll content and grain yield (Lu et al., 2020). Additionally, QTLs on chromosomes 2B, 7B, and 7D were found to be associated with heat tolerance traits like heat susceptibility index and canopy temperature depression, explaining up to 20.34% of phenotypic variation (Paliwal et al., 2012). These QTLs are crucial for marker-assisted selection and breeding programs aimed at improving heat tolerance in wheat. 3.2 Identification of candidate genes for heat tolerance The identification of candidate genes within QTL regions has provided deeper insights into the genetic mechanisms underlying heat tolerance. For example, seven candidate genes linked to a major QTL on chromosome 7AL were identified, which are involved in critical processes and pathways in response to heat stress (Lu et al., 2020). Another study identified 12 potential candidate genes associated with Fv/Fm, a measure of photosynthetic efficiency under heat stress, located on chromosomes 1D and 3B (Sharma et al., 2017). These genes are involved in photosynthesis and heat stress response, making them valuable targets for genetic improvement. The integration of molecular markers and candidate gene discovery has facilitated the development of near-isogenic lines (NILs) and other genetic resources that are essential for fine-mapping and cloning of major heat tolerance genes (Sharma et al., 2017; Lu et al., 2020). 3.3 Gene-environment interaction and heat tolerance The expression of heat-tolerant genes in wheat is significantly affected by environmental factors, which determine the phenotypic outcomes. Studies have shown that QTLs for heat tolerance can exhibit varying degrees of expression depending on environmental conditions such as temperature and sowing dates. For instance, QTLs identified for traits like normalized difference vegetation index(NDVI) and grain yield showed different levels of expression under late and very late sown conditions, highlighting the importance of gene-environment interactions (Raveendran et al., 2020). Additionally, the stability of QTLs across different environments is crucial for their effective utilization in breeding programs. Some QTLs, such as those on chromosomes 2B and 7B, have been shown to be stable across multiple trials, indicating their robustness in different environmental contexts (Paliwal et al., 2012). Understanding these interactions is essential for developing wheat varieties that can maintain high levels of heat tolerance across diverse growing conditions. 4 Application of Traditional Breeding Methods in Wheat Heat Tolerance Improvement 4.1 Phenotypic selection Phenotypic selection remains a cornerstone in traditional breeding methods for improving heat tolerance in wheat. This approach involves selecting plants that exhibit desirable traits under high-temperature conditions. For instance, traits such as delayed leaf senescence, increased photosynthetic capacity, and extended grain filling periods have been identified as critical for heat tolerance. These traits were effectively utilized in a three-tiered phenotyping strategy to select heat-tolerant wheat genotypes derived from emmer wheat, demonstrating the potential of phenotypic selection in identifying and developing heat-tolerant varieties (Ullah et al., 2021). Additionally, the use of agro-physiological indices and multidimensional analyses has been shown to enhance the accuracy and credibility of phenotypic selection, allowing breeders to classify wheat genotypes based on their heat tolerance levels (Al-ashkar et al., 2023). 4.2 Hybrid breeding and trait improvement Hybrid breeding is another traditional method that has been effectively used to introduce heat tolerance genes into wheat. This method involves crossing genetically diverse parents to produce hybrids with superior traits. For example, the genetic distance between wheat genotypes has been correlated with heterosis, which can be exploited
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