An early-morning flowering trait in rice helps retain grain yield under heat stress field conditions at flowering stage

Early-morning flowering (EMF) trait is supposed to be effective in retaining grain yield due to mitigation of heat-induced spikelet sterility at flowering in rice. This study evaluated (i) phenotypic differences between a near-isogenic line carrying a QTL for EMF trait, designated as IR64+ qEMF3 , and a recurrent parent, IR64, under wide variation in climates and (ii) whether an EMF trait can retain grain yield under heat stress at flowering. IR64+ qEMF3 had significant earlier flower opening time (FOT) in diverse environmental conditions including temperate, subtropical, and tropical regions. Under normal temperatures at flowering, IR64+ qEMF3 had similar grain yield to IR64 with some significant changes in agronomic traits and yield components. Field trials in heat-vulnerable regions of central Myanmar for seven crop seasons showed that higher percentage of filled grains contributed to the significantly higher grain yield in IR64+ qEMF3 among yield components when plants were exposed to daily maximum air temperatures around 36.5 °C or higher. Lower spikelet sterility in IR64+ qEMF3 was attributed to the earlier FOT during cooler early morning hours. This is the first field study that clearly demonstrates the advantage of the EMF trait for retaining grain yield by stabilizing percentage of fertile grains under heat stress at flowering.