Abstract
White back kernel and milky white rice kernel are caused by a high temperature at the ripening stage. The recent tendency of high temperature is not only at the ripening stage but also during the vegetative growth stage. In this study we examined the effect of a high temperature during the vegetative growth stage on shoot and root growth of rice plant, and also on the brown rice quality. Three temperature sets 32/27, 27/22 and 22/17°C of day/night were given to soil grown rice cultivar Koshihikari during 50 days from 1 week after transplanting. Under a high temperature treatment, the growth enhancement was greater in the shoot than in the root, causing the reduction of root/shoot (R/S) ratio. The concentration of ammonium nitrogen in the soil solution was higher in the higher temperature treatment. In a water culture, with the same nutrient concentration, R/S ratio was also reduced by the higher temperature treatment. The number of degraded brown rice, especially white back kernel and milky white rice kernel was increased by the higher temperature during 28 days before anthesis. These plants had a low plant growth rate at the ripening period, mainly caused by a low net assimilation rate. Low breeding rates and accelerated leaf senescence in the high temperature treatment were associated with low R/S ratio caused by high temperature before anthesis.
