Modeling and Prediction of the Occurrence of Chalky Grains in Rice

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Reported data were collected on the occurrence of chalky grains in rice production in Hokuriku Area and analyzed to reveal that the percentage of chalky grain increases with an increase in temperature and agricultural practices and the types of chalky grains affect the temperature responses.Clipping treatments of leaf blades and panicles, a thinning treatment were given to field grown rice (cv. Koshihikari and Hatsubosi) in order to alter the amount of assimilate supply per one grain (ASPG). ASPG affected the occurrence of milky white grain, but not white-belly and white-based grains. An increase of 1 mg in ASPG reduced the percentage of milky white grain by 1%. Nitrogen topdressing at the full heading stage decreased the percentage of milky white, white-belly and white-based grains. High temperature treatments at various stages during the grain-filling phase with varied durations were given to potted rice plants (cv. Koshihikari), of which leaf blades and panicles were clipped at the full heading stage to alter ASPG.Based on those experimental data, we have developed a simple model to predict the percentage of milky white grain. The model uses an accumulated temperature concept with some modifications and includes the effects of ASPG. Our model well explained the occurrence of milky white grains in rice grown under various conditions. We estimated the best cropping seasons and the optimum grain numbers for rice crops in Kanazawa, Wajima, Toyama and Fukui by simulations with our model.