Effects of Cool Water Irrigation on Reduction of Grain Quality of Rice by High Air Temperatures during the Ripening Period

In order to mitigate the reduction of grain quality of rice by a high temperature during the ripening period of rice, the effects of irrigation with cool water was investigated. Field experiments were conducted using rice cultivar Koshihikari. Grain yield and apparent grain quality were examined under different air and soil temperatures regimes. Air temperature was changed by transplanting on different dates and soil temperature by setting the experimental plots at different distances from a cool water inlet. More chalky kernels, such as white back and basal white kernels, were found under a higher air temperature condition during 20 days after anthesis. The occurrence of these chalky kernels was reduced by irrigation with cool water. A significant correlation was found between the occurrence of chalky kernels and the average soil temperature during 20 days before anthesis, suggesting that a low soil temperature improved root activity and decreased the number of chalky kernels. In pot experiments, irrigation with cool water (17°C) reduced the occurrence of milky white kernels under high air temperatures to 2% compared to the 14% in the control by preventing the reduction of root activity and delaying leaf senescence.