Panicle emergence pattern and grain yield of rice plants in response to high temperature stress

The timing of panicle emergence in rice (Oryza sativa L.) is an important trait. We studied the effects of increasing temperatures during growing seasons on the emergence of first panicle, consequent emerging panicle numbers, and grain yields in rice plant using temperature gradient chambers (TGCs) located in a part of Japan with a humid subtropical climate. Four japonica varieties, Koshihikari, Akitakomachi, Hitomebore, and Nipponbare, were exposed to different temperatures in the TGCs. The number of days from transplanting to heading (DTH) differed among temperatures. Increasing temperatures both accelerated and delayed the timing of heading. Extreme temperatures (>40 degrees C daily maximum) increased the DTH, whereas moderately high temperatures decreased or did not change the DTH. Increasing temperatures after heading affected the number of panicles emerging. The final panicle number increased with temperature in Koshihikari and Akitakomachi. In these varieties, spikelet fertility and mean grain weight of early-emerging panicles were lower at higher temperatures because of the occurrence of grain sterility. However, the grain numbers and grain weights of late-emerging panicles were greater under higher temperatures, and total grain yields were not significantly different among the temperature treatments. We concluded that late-emerging panicles recouped the yield of rice plants, compensating for grain sterility.