Abstract
The method of estimating the developmental stage necessary to prevent and analyze the cool damage in rice plants using meteorological elements was examined. We investigated the relationship between the elongation rate of the young panicles and the cumulative temperature from the panicle initiation using three Japonica cultivars, Akitakomachi, Koshihikari and Nipponbare. A logistic function was fitted to the relationship between the length of young panicle and the cumulative daily mean temperature. It was shown that the later the heading date, the higher the cumulative temperature until the panicle reached 50% of the final length. Although the young panicle length of Koshihikari in another area (Koriyama) could be roughly estimated by the logistic function fitted to the data obtained in Tsukuba, there was a difference between the estimated and measured length in some years. This was suggested to be due to the delay of panicle elongation by the temperature below about 20°C. The morphological observation of the pollen revealed that the sensitivity to a low temperature varied with the panicle length, and reached the maximum when the panicle became 70% of the final length. The developmental stage of the panicle varied with the tiller, but we obtained a formula showing the change in the low-temperature sensitivity considering the variation, based on the correlation between the cumulative temperature and young panicle length and that between the young panicle length and the sensitivity to cool temperature. From these results it is shown that the cumulative value, which is calculated from the degree of sensitivity and the degree of cooling (mean air temperature below 20°C) during panicle development, is useful for the estimation of the percentage of sterility caused by a low temperature.
