Molecular Plant Breeding 2025, Vol.16, No.1, 82-92 http://genbreedpublisher.com/index.php/mpb 85 to improve hybrid performance under heat stress. In a study evaluating 16 wheat genotypes, hybrids with greater genetic diversity showed significant improvements in grain yield and other heat tolerance traits, such as grain filling duration and canopy temperature (Al-ashkar et al., 2020). Marker-assisted backcross breeding (MABB) is another strategy that has been employed to introgress specific QTLs associated with heat tolerance into popular wheat varieties. This method has successfully improved traits like early anthesis and high kernel weight, leading to the development of heat-tolerant lines with higher grain yields (Bellundagi et al., 2022). 4.3 Successful cases of classical breeding Several successful cases of classical breeding for heat tolerance in wheat have been documented. For instance, recurrent selection has been shown to be an effective strategy for improving heat tolerance. Studies conducted in different growing seasons demonstrated significant genetic gains and variability, indicating the potential for successful recurrent selection in developing heat-tolerant wheat varieties (Machado et al., 2010). Another notable example is the identification of heat-tolerant genotypes based on pollen viability under heat stress. Genotypes such as Chenab-70, Pari-73, and Pak-81 have been found to maintain high pollen viability under elevated temperatures, making them valuable genetic resources for breeding heat-tolerant cultivars. Khan et al. (2022b) found that the heat-tolerant line (Chenab-70) maintained better pollen morphology under heat stress, with even distribution and good staining, demonstrating a higher level of heat tolerance. In contrast, the susceptible lines (Meraj-08 and Gomal-08) showed significantly damaged pollen under heat stress, with poor staining and reduced quantity, indicating decreased pollen viability (Figure 1). Additionally, the use of QTL mapping has identified significant genomic regions associated with heat tolerance, which can be utilized in marker-assisted selection to develop heat-tolerant wheat varieties (Paliwal et al., 2012). Figure 1 Microscopic observation of pollen from heat-tolerant and susceptible wheat lines under normal and heat stress conditions (Adapted from Khan et al., 2022b) Image caption: Microscopic images (10x magnification) of pollen from heat-tolerant wheat line (A, B: Chenab-70) and heat-susceptible wheat lines (C: Meraj-08, D: Gomal-08) under normal (left) and heat stress conditions (right). The images display pollen conditions for the two types of wheat lines over two consecutive years (2020-2021 and 2021-2022) (Adapted from Khan et al., 2022b) 5 Application of Molecular Breeding Techniques in Wheat Heat Tolerance Improvement 5.1 Marker-assisted selection (MAS) MAS has become a pivotal tool in accelerating the breeding of heat-tolerant wheat varieties. MAS leverages molecular markers linked to desirable traits, enabling breeders to select plants with these traits more efficiently than traditional phenotypic selection methods. For instance, the introgression of QTLs associated with early anthesis and high kernel weight into the heat-sensitive wheat variety HD2877 through marker-assisted backcross breeding (MABB) has shown promising results (Figure 2). This approach utilized markers Xbarc186 and
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