Abstract
We developed a method for evaluation of varietal differences in grain appearance (the occurrence of chalky grain) and of tolerance to high temperature and low solar radiation during the ripening period of rice (Oryza sativa L.) in warm regions of Japan. Average temperature of the 20 days following heading of each tested variety was kept over 27°C by transplantation one-month earlier than in a normal rice cropping season. Shading treatment using a semi-transparent film covering brought about a 15 to 20% reduction in solar radiation from that under the non-shaded condition. Under the shaded condition, the percentage of chalky grains of each tested variety increased and the varietal differences on percentages of chalky grains were greater than those under the normal condition without shading. Although a positive correlation was observed for several years between the percentage of white immature grains and the average temperature of the 20 days following heading, a practical evaluation of varietal differences in grain appearance is considered to be possible among the varieties with similar ripening temperature. The varietal differences in percentage of chalky grains observed in this study are consistent with evaluations on tolerance to high temperature based on the main characteristics of each tested variety by breeding site in previous reports．This method is considered to be a practical method for the evaluation of tolerance to high temperature and low solar radiation during the ripening period in rice breeding.
