Assessment of spring wheat responses to late season heat stress under Mediterranean conditions

1 Abstract In Mediterranean climatic regions, wheat yield is significantly influenced by sowing date and cultivar selection, particularly where production could be limited by heat waves during critical growth stages. This investigation was undertaken to determine the effect of shifting sowing date on a cohort of six high yielding spring wheat cultivars ( Triticum aestivum L.), by delaying sowing date 15 and 30 days in a two-year field trial, aiming to highlight agronomic traits contributing to sustained yield potential under heat stress induced by late sowing, with a focus on identifying tolerant cultivars. Heat stress during anthesis reduced grain set and yield by up to 42%, mainly via reduced grain number per spike and thousand grain weight Late sowing shifted the timing of heat stress, resulting in floral abortion and significantly reduced number of grains/spike. More importantly, some cultivars showed up to 10% significant reduction in thousand grain weight (TGW) following delaying sowing date by 30 days, together with the TGW strong correlation with yield especially at late sowing (r=0.74), imposing the yield penalty in the heat stressed late sowing conditions. Among the cohort of tested cultivars, Sids12 and Misr1 cultivars showed minimal yield reduction (10-14%) due to 30 days delayed sowing in the second season highlighted with hot spring. On the contrary, harvest index of sowing delayed by 30 days was significantly higher compared to both 15 days late sowing and the optimum sowing date, suggesting that decline in biological yield exceeded that of grain yield. Upon shifting sowing date, the strength of traits association is altered, grain yield associations showed the strongest positive correlation in the 30 days late sowing with plant height (r=0.86), in the 15 days late sowing with number of total tillers/m 2 (r=0.63) and in optimum sowing date with biological yield (r= 0.62), suggesting that grain yield enhancement through selected traits with direct or indirect contributions should be evaluated independently across different growing conditions.