Abstract
In Chiba Prefecture, we cultivated rice cv. Koshihikari and Hitomebore under the conditions with wide levels of planting density and amount of nitrogen fertilizer applied, and investigated the rice quality–deteriorating factors and the varietal difference in the quality of rice (1000 grain weight, immature grain including milky white rice kernel and protein content of rice). In 2000 and 2001, the heading times were around July 15 to 20, and ripening periods were the season with a high temperature. Neither variety was lodged markedly. In cv. Hitomebore the number of unhulled rice was larger on equal fertilization, the percentage of ripened grains was lower, the quality of rice was lower, and the number of unhulled rice was higher than cv. Koshihikari. In addition, the percentage of ripened grain was lower in cv. Hitomebore than in cv. Koshihikari on equal number of unhulled rice. This is, because in 2001, the range of the grain thickness on the two branches overlapped in the range of 1.95–2.10 mm in Hitomebore, and the grain thickness fluctuated largely in the latter. We estimated that the decrease in the percentage of ripened grains in cv. Hitomebore was caused by the competition for carbohydrate between the spikelets on primary and secondary rachis branches. It is important to keep a proper number of unhulled rice per square meter, and avoid the excess in the number of grains per head, in early season culture in warm resions such as Chiba Prefecture.
