Abstract
Cool-weather tolerance at the flowering stage of 54 rice varieties in the area south of Tohoku district, Japan was examined in 1999. The plants were exposed to a cool temperature of 17.5°C in an air conditioned room for 15 days. The data on Hokkaido varieties reported previously were compared with the above results. Although the degree of cool tolerance at the flowering stage greatly varied with the district where the variety was bred, the higher the latitude of the breeding district, the higher the tolerance (r=0.497**, n=106). Moreover the variety with early heading date showed higher tolerance (r=-0.576**, n=106). Among the tested varieties, Chuubo 42 had an extremely high tolerance, 6 varieties had high tolerance, and 16 varieties had extremely low tolerance. Koshihikari, Hitomebore and Akitakomachi, the present major varieties of Japan, showed rather low tolerance at the flowering stage. Although a significant positive correlation was observed between the tolerance at the booting stage reported previously and that at the tlowering stage in the present study (r=0.462**, n=51), the coefficient of correlation was not high. Moreover among Tohoku district varieties bred after 1975, the varieties bred more recently had a higher cool tolerance at the booting stage, and no relation was found between the year of breeding and the tolerance to cool weather at the flowering stage. These results suggest that to breed a new rice variety with high cool tolerance at the flowering stage, direct selection for the tolerance is needed in the breeding program.
