Varietal variations in basic vegetative phase and photosensitive phase among Japanese native cultivars of rice

The days to heading of 93 native rice cultivars collected from various locations in Japan weret examined under a short-day (30°C phytotoron, in winter, natural day-length), and a long-day (a paddy field, in summer, natural day-length and temperature) condition. From the clata, varietal variations in the periods of basic vegetative phase (BVP) as shown, by days to heading in shortday condition and in photosensitive phase (PSP) as shown by the difference in days to heading between longday and shortday conclitions were surveyed, so as to learn about the mode of differentiation of early-maturing cultivars in Japan. The cultivars tested were divisible into two groups with a long (75-90r days) and a short (38-75 days) BVP. Among the cultivars with short BVPs, the distribution of PSP, varying from O to 72 days, was trimodal. Among those from areas north of 32N in latitude, PSP decreasecl with increasing latitude of collection site. But no cor-relation between PSP and latitude was found among cultivars from the southwest islands. The early-maturing cultivars (with short BVP and short PSP) distributed throughout Japan alt*nough they were infrequent. The pattern of distribution of native rice cultivars vvith varying BVP and PSP periods seems to reflect the variations in environrnental condition anc'i cropping system under which they have been cultivated so far.