Integrating Phenotypic Selection, Population Structure, and SNP-Based Genetic Diversity to Improve Heat Tolerance in Spring Wheat

Heat stress is a major abiotic constraint limiting wheat production across its different growth stages, as it drastically impairs growth and reduces grain yield. A set of 103 highly diverse wheat genotypes was evaluated under normal (N) and heat stress (HS) conditions. The measured traits included plant height (PH), days to heading (DTH), days to maturity (DTM), grain number per spike (GNPS), grain yield per spike (GYPS), thousand-kernel weight (TKW), spike length (SL), and number of spikelets per spike (NSPS). The analysis of variance showed high genetic variation among genotypes under both conditions. High heritability estimates were observed for all traits in each condition. Grain yield per spike (GYPS) was the trait most affected by heat stress, with reductions of 57.8% and 46.6% in the first and second seasons, respectively, whereas spike length (SL) was the least affected trait, with reductions of 11.3% and 10.9% in the first and second seasons, respectively. Grain yield per spike (GYPS) showed a strong negative correlation with days to heading (DTH) and days to maturity (DTM), and a strong positive correlation with grain number per spike (GNPS), number of spikelets per spike (NSPS), and thousand-kernel weight (TKW) across all four environments. Selection results revealed 11 consistently low-yielding genotypes, while 13 high-yielding genotypes repeatedly ranked among the top 20 in both seasons. The genetic distance among the selected genotypes ranged from 0.236 to 0.627, with the greatest distance between WAS_140 (Morocco) and WAS_160 (Saudi Arabia) and the smallest between WAS_139 and WAS_140 (both from Morocco). The genotype WAS_016 (Egypt) was genetically distinct from the other selected genotypes. In breeding programs, crossbreeding between genotypes WAS_160 and WAS_140 appears promising for developing heat-tolerant, high-yielding lines.