Abstract
We estimated cool tolerance in high yielding varieties bred in Hokuriku National Agricultural Experiment Station and their ancestral varieties, including indica varieties, for screening materials to breed high-yielding indica rice varieties suitable for cold weather. "Jou 344", "Hokuriku 193" and "GUIZHAO 2", which are indica varieties, had high cool tolerance at the booting stage. In the breeding process of "Jou 344", large transgressive segregation towards high cool tolerance occurred. This suggests that there is a combination of genetic factors, which is complementary and highly effective for cold tolerance at the booting stage in indica rice varieties. "Suweon 258" and "Jou 344", which are indica varieties, had high cool tolerance at the flowering stage. Because "Suweon 258" is the parental variety of "Jou 344", these varieties may have a genetic factor effective for cool tolerance at the flowering stage. It is useful to find the genetic factor for improving the cool tolerance at the flowering stage in indica varieties. No relationship was found between cool tolerance at the booting stage and cool tolerance at the flowering stage.
