Heat Stress during Flowering Affects Time of Day of Flowering, Seed Set, and Grain Quality in Spring Wheat

Flowering in wheat (Triticum aestivum L.) is highly sensitive to heat stress. Eleven spring wheat genotypes were exposed to heat stress (34/16 degrees C, day/night temperature) during flowering to investigate the impact on time of day of flowering, seed set, grain yield, and quality under controlled environment chambers. In general, all 11 wheat genotypes recorded peak flowering during cooler hours of the day (i.e. either early in the morning or late in the evening). The trend was more pronounced under heat stress, providing first evidence of an alternative mechanism (i.e. heat escape in wheat minimizing damage during flowering). On average, significant reduction in net CO 2 assimilation (17%), starch (25%), and protein content in grain (21%), seed set (7-19%), kernel weight (11-15%), and grain yield (22-38%), but with 17% increase in flag leaves proline concentration, was recorded under heat stress over control. The negative impact of heat stress on seed set was greater among the primary spikes than for the main spike. This differential impact is mainly attributed to limited plasticity of early reproductive processes such as gametogenesis to escape heat stress, unlike heat escape phenomena observed at flowering. KSG1203 and KSG41 exhibited heat escape strategy, whereas KSG1194 emerged as a true heat-tolerant line. Systematic characterization of time of day of flowering to introduce the heat escape trait and developing heat tolerance strategies for early reproductive stages in ongoing wheat breeding programs is an ideal strategy to minimize heat stress damage.