Cleistogamy Decreases the Effect of High Temperature Stress at Flowering in Rice

Rice sterility due to a high temperature at flowering is a serious agricultural problem that has been associated with global warming. The flowering stage in rice plants is most vulnerable to high temperature stress. Closed flowering rice plants may better withstand high temperature stress. The aim of this study was to determine the role of cleistogamy (closed flowering) in avoiding high temperature-induced sterility. Cleistogamy was induced by moderate heat treatment at 30 degrees C during the panicle development stage. Both cleistogamous and chasmogamous (ordinary open flowering) rice plants, which possess the same genetic background, were subjected to 38 degrees C or 36 degrees C for 4 h just before flowering, and the percentage of fertility, number of pollen grains on a stigma, number of germinated pollen grains on a stigma, and temperatures inside and outside of the closed spikelets were examined. The cleistogamous rice plants showed a higher fertility percentage and a larger number of germinated pollen grains on a stigma than the chasmogamous rice plants. The temperature inside the closed flowering spikelets was 1.8 degrees C lower than that outside the spikelets. The cleistogamous rice plants thus showed avoidance to high temperature stress at 38 degrees C at flowering. On the basis of these results, we concluded that cleistogamy was advantageous to rice pollination and fertilization at high temperatures because of glume cooling.