Quality Response of Grains in Different Spikelet Positions to Temperature Stress During Grain Filling of Rice

High temperature after anthesis may degrade the grain quality of rice (Oryza sativa L.). To understand the effect of stress duration (initial and mid grain filling stages) and the influence to grain quality at different spikelet positions, an indica cultivar and a japonica cultivar, Yangdao 6 and Yangjing 9538, were used in a pot experiment under 35 °C during the initial (8–18 d after anthesis) and mid (18–28 d after anthesis) terms of grain filling. The treatment under normal atmospheric temperature (25–27 °C) served as the control. The high-temperature treatment at initial filling stage had larger influence on grain quality than the stress imposed at mid filling stage, and the grain quality varied with cultivars and spikelet positions. Both treatments with high temperature reduced milled rice rate and head milled rice rate, but increased crude protein content; grains on the primary branches received larger effect than those on the secondary branches. In the same branch, later-flowered grains were affected greater than early-flowered grains. When high temperature was imposed at initial grain filling stage, the chalkiness degree and chalky grain percentage were significantly increased in the early-flowered grains, whereas the amylose content was significantly decreased; such variations were larger in grains on the primary branches than those on the secondary branches. Besides, the 2 cultivars showed different responses of grain quality to high-temperature stress, which indicated the fact of phenotypic difference on temperature sensitivity. Generally, the influence of temperature on rice quality was greater when the stress imposed at the earlier stage of grain filling.