Abstract
The rice cultivars, Nipponbare and Hinohikari, were cultivated in the paddy field of Field Science Center, Okayama University in 2009 and 2010 (extremely hot summers). High-temperature treatment was given in the transparent chamber whose sides were opened when the temperature exceeded 36℃, and closed when it was lower than 25℃. Shading treatment was given by covering with a black cloth during the grain-filling period. The brown rice yield in 2009 and 2010 was decreased by high-temperature treatment by 10−21% and 30−33%, respectively, due to the decrease in the percentage of ripened grain and 1000-grains weight. In the 50% shading plot the yield was decreased by 16−24% and 30% in 2009 and 2010, respectively, mainly due to the decrease in the percentage of ripened grains. The distribution of grain thickness was shifted more to the thicker side in 2010 than in 2009. The 50% shading inhibited the grain thickening and shifted the distribution of the thickness to a thinner side, and the high-temperature in 2010 promoted the grain thickening growth despite the decrease in 1000-grain weight. In both cultivars, the lower the grain thickness group, the lower the percentage of perfect grain due to the increase in the percentage of chalky grain. In 2010, the grains were thicker, but the percentage of white-based grains was higher than in 2009. The palatability of cooked rice was higher in Hinohikari than in Nipponbare. The lower the grain thickness, the lower the palatability, and such tendency was strong in Nipponbare than in Hinohikari. The shading and high-temperature treatments decreased the palatability, especially in Nipponbare. A positive correlation was observed between the palatability and percentage of chalky grains in 2009, but negative in 2010 probably because white-based grains in thicker grains with good eating quality. Clearly, the appearance quality and palatability of grains can be controlled by changing the sieve size of rice grader, and the increase of white-based grains caused by the high-temperature may not affect the eating quality of rice.
