Effect of Heat Stress During Meiosis on Grain Yield of Rice Cultivars Differing in Heat Tolerance and Its Physiological Mechanism

To understand the mechanism of grain yield in response to heat stress in rice (Oryza sativa L.), the development of anther and pollen, yield components, and some physiological parameters under the heat-stress during meiosis were investigated in a pot experiment. Two indica rice varieties differing in heat tolerance, Shuanggui 1 (heat-sensitive) and Huanghuazhan (heat-tolerant), were treated with heat stress (average temperature during the day above 35°C) and natural temperature (average temperature during the day below 33°C, control). The heat stress significantly reduced anther dehiscence and pollen fertility rate in Shuanggui 1, whereas, its effects were much smaller in Huanghuazhan. Number of spikelet per panicle, seed-setting rate, 1000-grain weight, and grain yield significantly decreased in both varieties, but the yield reduction in Shuanggui 1 was greater than that in Huanghuazhan. Moreover, significant narrowness of grain width and significant enlargement of grain length/width ratio were observed in Shuanggui 1. However, such effect was much smaller in Huanghuazhan. In physiological parameters that are associated with heat stress, the oxidation activity in roots and the RNA content in young panicles reduced significantly and the malondialdehyde (MDA) content in leaves and the ethylene evolution rate in young panicles increased significantly. These physiological characteristics were more sensitive to heat stress in Shuanggui 1 than in Huanghuazhan. The results indicate that the heat tolerant in rice is subject to high activity of roots, strong antioxidative defense system, high RNA content, little ethylene synthesis, and low MDA content in plants during meiosis.