Abstract
In the extremely cool summer in 1980 (Fig. 1), rice plants grown in Touhoku District (the north-eastern parts of Honshu) suffered from the severe sterility due to the cool temperature at the booting and/or flowering time. The distribution pattern of coolness in Touhoku District at the extremely cool summer was estimated by the degree days of coolness, Σ(20-T), for five days from July 15 to 19, 1980, on which the high atmospheric pressure at 0khotsk Sea influenced (Fig. 2). Spikelets sterilities were recorded on some rice cultivars at 22 agricultural experiment stations located in main rice planting areas in Touhoku District. Daily changes of sterility were classified into five types (Fig. 3). Touhoku District was geographically divided into five areas according to the relative grades of danger caused by sterility type of cool injury of rice plants, which were estimated firstly from the types of the daily change of spikelets. sterility, and secondly from the degree days of coolness, altitude and topographical characteristics (Fig. 4). The relative grades of danger caused by sterility type of cool injury should be utilized for selecting the rice cultivars with the appropriate degree of resistance to cool-weather, though the relative grades of danger caused by delayed growth type of cool injury has been utilized for establishing the standard cultivation method and for choicing the rice cultivars with adapted maturity.
