Genetic analyses of low photoperiod sensitivity of rice cultivars from the northernmost regions of Japan

The genetic factors controlling the extremely low photoperiod sensitivity (PS) of rice culyibstd from the northernmost regions of Japan were studied using 26 cultivars from the Hokkaido district (42 N 45 N). They were grown under 10‐h and 24‐h daylengths, and their PS. expressed by the difference between the days to heading under these daylengths, was estimated. The genetic factors responsible for the intervarietal difference in PS were then analysed under 10‐h, 14‐h and 24‐h daylengths using the progenies from crosses between Hokkaido cultivars. These results showed that a recessive gene, se‐9(t), drastically reduced PS under a 24‐h daylength and that several genes, including se9(t), determined the extremely low PS of the cultivars from the northernmost regions.