Simulating the Effects of Climate Change on Rice Yield and Appearance Quality in Japan.

It is well documented that the increase of air temperature above 26-27 °C during 20-day period after the heading enhance the occurrence of chalky grains in rice (Morita et al., 2005). On the other hand, many authors reported that both decline of carbohydrate supply to single grain and reduced plant nitrogen concentration during the grain-filling period increased the occurrence of chalky grains in rice, depending on types of chalky grain. Utilizing a rice growth model which predicted a close negative correlation between the observed occurrence of chalky grains and the simulated carbohydrate and nitrogen contents available per single grain (Yoshida et al., 2016), we simulated the effects of climate change on not only rice yield but also the occurrence of chalky grains at several sites in Japan. We utilized weather datasets of 3 RCP scenarios (RCP2.6, RCP4.5 and RCP8.5) by ensemble of 6 climate models (Ishigooka et al., 2017). Averaging over 30 years from 2021 to 2050 under each climate scenario, our simulation suggested that the timing of top-dressing nitrogen fertilizer before heading would have significant influence on the occurrence of chalky grains, especially if the rice was grown under high air temperature conditions during the grain-filling period. The effect of the timing of nitrogen fertilization on the occurrence of the chalky grains was simulated through the changed spikelet number per unit area: the nitrogen fertilization during the spikelet differentiation period increased the spikelet number and yield but decreased the carbohydrate and nitrogen contents available per single spikelet and enhanced the occurrence of chalky grains. The N fertilization method to optimize the dynamics in the balance between carbon and N gains would be a central technology to avoid the decline in rice production and quality due to the increased occurrence of chalky grains under the global warming tendency.