Abstract
The effects of climatic factors on heading of early-planting rice cultivars were discussed. Growing period from transplantation to heading was divided into three phases, i.e.phase I ; from the day of transplantation to the first day of tillering, phase II ; the first tillering day to the first day of panicle formation, and phase III, the first day of panicle formation to first day of heading. The heading time of Koshihikari was affected by climatic factors in phase II, and that of Kirarimiyazaki by climactic factors in both phase I and II. Heading days was highly correlated with the mean air temperature during the first half of phase II in Koshihikari, and throughout phase II in Kirarimiyazaki. In l998, temperature in phase I and II was very high, which reduced growing days, and decreased the cumulative temperature necessary for rice growth. The reduction in growing days was especially great in Koshihikari, and this might cause the reversed heading date between Koshihikari and Kirarimiyazaki in 1998.
