Pre- and post-flowering impacts of natural heat events on yield components in wheat

O_LICONTEXT Wheat crops are highly sensitive to elevated temperatures and experience significant yield losses when short periods of heat occur at sensitive developmental phases. C_LIO_LIOBJECTIVE This research aimed at quantifying wheat responses of grain yield and yield components to heat indicators in fluctuating field conditions. C_LIO_LIMETHODS The impacts of high temperature on yield and its components were assessed for 20-35 wheat lines in irrigated multi-environment trials over three years. Genotypes were cultivated using a novel photoperiod extension method adjacent to some conventional yield plots with different sowing dates. In the photoperiod-extension method, either single stems or plant quadrates were tagged at specific growth stages and hand-harvested at maturity, while conventional plots were mechanically harvested at maturity. The impact of heat was estimated for events occurring at different developmental stages and different temperature thresholds (26 to 35{circ}C). C_LIO_LIRESULTS The closest correlation between heat events and grain number was observed for a threshold temperature of 28{circ}C pre-flowering. For individual grain weight, the correlation was statiscally the closest for threshold temperatures above 32{degrees}C post-flowering. In the tested environments, grain number was most sensitive to heat between 300 and 200{circ}Cd before flowering. For each hot hour (T > 28{circ}C) during this period, wheat genotypes lost an extra 0.25 grains spike-1 or 90 grains m-2 on average. Post-flowering heat reduced individual grain weight by 0.26 mg (at spike harvest) and grain yield by 2.44 g m-2 (at plot harvest) for every hot hour (T > 32{circ}C) between 0 and 500{circ}Cd after flowering. Heat impacts were the clearest for plants with synchronised phenology. In addition, results suggest that heat impacts can be quantified more reliably using finer time units (i.e. hot hours rather than days). C_LIO_LIONCLUSIONS Natural heat events strongly impacted grain number and individual grain weight of wheat for temperatures above 28-30{degrees}C in well-watered conditions. Reduction in grain number and individual grain weight were strongly associated with accumulated hot hours that occurred during 200-300{degrees}Cd before and 0-500{degrees}Cd after flowering, respectively. C_LIO_LIIMPLICATIONS The findings from this study will assist improvement for crop modelling in response to heat, development of relevant phenotyping methods and selection of cultivars with better adaptation to warmer environments. C_LI