Effects of early planting on growth and yield of rice cultivars under a cool climate

The increasing temperatures accompanying global warming are expected to allow the early planting of rice in areas with a cool climate and extended the growing-season duration, thereby increasing productivity. In northern Japan, one of the coolest rice growing regions, transplanting time advanced from the 1960s to the 1980s, but not thereafter. We tested the hypothesis that early planting (ca. 10 days earlier than normal) can extend the growth duration and increase biomass and yield in a 2-year field trial using two cultivars ('Hitomebore', with normal maturity, and 'Koshihikari', with late maturity). Early planting extended the period from transplanting to heading and maturity of both cultivars by 8-10 days in both years. However, final dry weight tended to decrease, especially in 'Hitomebore', with a marginally significant planting time × cultivar interaction. Slower leaf expansion and reduced canopy radiation capture during early growth caused the dry weight decrease in both cultivars, but the longer growth duration of 'Koshihikari' compensated for this reduction until the heading stage. Grain yield was not significantly increased by early planting in either cultivar, and there was no significant planting time × cultivar interaction. For rice productivity to benefit from global warming through the extended growth period, it will be necessary to find cultivars with better early leaf expansion at low temperatures.