Sustained assimilate supply determines grain development in wheat under post-anthesis heat.

Short episodes of post-anthesis heat restrict grain filling by accelerating leaf senescence, reducing photosynthetic capacity, and limiting assimilate supply to developing grains. We explored the contribution of assimilates, derived from both current photosynthesis and remobilisation, for sustaining wheat yield under post-anthesis heat. Two recombinant inbred lines, SB062 and SB003, were subjected to 7-day heat treatment during different post-anthesis phases. High temperature during the early to mid-grain filling phases (7-14 or 14-21 days after anthesis (DAA)) accelerated leaf senescence, with a greater impact on older leaves, primarily in SB003. Significant genotype × environment interactions were observed for grain yield components, with post-anthesis heat causing a 10-15% reduction in individual grain weight (IGW) and grain dimensions of SB003 only. In contrast, SB062 showed no significant reduction in leaf senescence or grain size in response to heat treatments. This genotypic response in IGW to heat (7-14 DAA) became non-significant when all green leaves were excised, indicating that post-anthesis assimilate supply contributed to the observed genotype × environment interactions. In a separate experiment, injection of the cytokinin 6-benzylaminopurine (100 μM) significantly increased stay-green and IGW in heat-stressed SB003 plants, suggesting a role of cytokinin-mediated processes in protecting the heat-sensitive genotype. In contrast, abscisic acid and anti-cytokinin 3-isobutyl-1-methylxanthine reduced IGW in heat-stressed SB062 plants, but not in SB003 that already had IGW substantially reduced by heat stress. These results indicate that under the tested conditions, the superior performance of SB062 under post-anthesis heat was associated with a sustained assimilate supply to developing grains through delayed senescence and enhanced remobilisation. Understanding these physiological processes offers targets for selecting heat-tolerant wheat lines for future breeding programs.