Abstract
The juvenile phase and photoperiodic sensitivity of 13 Japanese paddy rice varieties were investigated. The duration of juvenile phase was determined by counting the days of insensitivity to a short-day rearing condition through observation of the panicle initiation, using rice plants grown under a long-day conditions which was altered to short-day conditions at different intervals. The photoperiodic sensitivity was estimated from the slope of the regression line of the growth periods and the periods from germination to panicle initiation, using the same rearing condition. The following results were obtained. (1) The duration of juvenile phase at 28°C was 4.3 days (2.8 plant age in leaf number) for the shortest variety and 19.0 days (6.3 plant age in leaf number) for the longest variety. (2) The varieties of each short, medium and long juvenile phases existed in all areas of cultivation throughout Japan. Specific patterns of latitudinal distributions of the varieties were not demonstrated. (3) As already confirmed previously, the degree of photoperiodic sensitivity in rice varieties was found to increase with latitudinal descent from cool Hokkaido to warmer districts. (4) Large differences were found among the varieties in the number of days required from panicle initiation until heading.
