Effects of transplanting time on dry matter production, yield and grain appearance in rice cultivar [Oryza sativa] Koshihikari during high temperature years

Summary To decrease the occurrence of the degradation grain appearance during high temperature years, the effects of the transplanting time were investigated for 4 years by changing transplanting time from late April to May a change of about 10 days. With the transplanting time delayed, the days from transplanting to heading became shorter. From the viewpoints of dry matter production and translocation, the following characteristics were observed in late transplanted rice; 1. The crop growth rate (CGR) during panicle formation period increased. 2. During the ripening period, the leaf area remained large. Also the amount of leaves which died off were few. 3. During the former half of the ripening period, the rate at which the panicle weight increased was high. 4. The re-accumulation of carbohydrate nutrition to the culm was high. In the data from the 4 years, the number of ears had the highest positive correlation to the grain yield among yield components. And the late transplanted rice showed a decrease in the number of ears and spikelets. The difference of the yield between the different times was comparatively small because of the compensation in the increase of the percentage of ripened grain. In grain appearance, the percentage of milky grain decreased and that of perfect grain increased in late transplanting. The grain appearance showed a close negative relationship to the average air temperature and the vapor saturation deficit during the 15 days after the heading time. When the average air temperature rose over 28 deg C and the vapor saturation deficit followed, the grain appearance degraded severely. In relation to dry matter production, the high ear growth rate during the former half of ripening period showed high grain appearance. And in the cases where the nitrogen concentration of rice plant remained high during the ripening period, good grain quality on the same spikelet level was maintained.