Improving wheat tolerance to post-flowering heat using matched development stages and field-based reaction norms

Post-flowering heatwaves impact individual grain weight (IGW) and grain quality of wheat. Here, the effect of post-flowering heatwaves on IGW, yield and grain protein content (GPC) were examined in 25 wheat genotypes grown in irrigated field trials across locations, seasons and sowing dates. The photoperiod-extension method (PEM) was employed with measurements on individual spikes to synchronize flowering among genotypes, ensuring heat impacts were assessed at similar developmental stages. Adjacent to most PEM trials, genotypes were cultivated in machine-harvested field plots to evaluate crop performance using a more conventional screening method. Heat stress significantly reduced IGW and increased GPC in PEM trials, with impacts not as clearly discernible in conventional plots. Across genotypes and traits, heat responses (slope of the reaction norms) were negatively correlated with trait values observed in non-stressed conditions (intercept). The level of heat stress each genotype could endure before reaching critical IGW (30 mg), yield (250 g m-2) or GPC (15%) thresholds revealed high genotypic variability and highly tolerant genotypes. The findings provide insights for improving heat responses in crop models and enhancing genotype selection for climate-resilient breeding.