Estimation of cool-weather tolerance at the flowering stage and its relationship with that of the booting stage in Hokkaido rice varieties.

Cool-weather tolerance at the flowering stage of 52 Hokkaido rice varieties was examined in 1998 and 1999. Eight plants having only the main culm were cultivated in small square pots, 15×5×10cm, and were tested by means of a 15-day treatment at 17.5°C, starting from the heading day of tested plants, in an air conditioned room with 50% shading. Among the varieties tested, Hayayuki and Hayakogane had the highest tolerance(estimated ranking was high to extremely high). Akage, Fukuyuki, Uryuu, Hoshinoyume, and Hatushizuku had high tolerance(high), and Fukoku, Wase nishiki, and Shimahikari had the lowest tolerance(low to extremely low). Between the tolerances at the booting stage and the flowering stage, a significant positive correlation was observed(r=0.54l). Although the variety or which the registered year as the recommmended variety was more recent had higher cool tolerance at the booting stage(r=-0.366), no correlation was found between the registered year and the cool tolerance at the flowering stage(r=-0.055ns), and it was concluded that to breed high-cool tolerant varieties at the flowering stage, direct selections for cool tolerance are needed in the breeding program. Moreover, 37 breeding lines, which had extremely high tolerance at the booting stage, were tested for tolerance at the flowering stage, and it was found that Eikei 88223 and Hokuiku mochi 87 had an extremely high tolerance at the flowering stage too. These lines are considered to be a desirable parent for cool tolerance.