Effects of High Temperature during Ripening Period on the Qualities of Indica Rice.

Abstract

Though varietal differences exist in the extent of effects, generally speaking, high tempeatures during ripening period accelerate the respiratiion of rice plant and hasten the maturation of kernel, consequently, the development of kernel and accumulation of starch into kernel are accelerated in the early period of maturation, yet they are depressed in the late period. Thus, high temperatures have various influences upon the grain development and the grain qualities. In this experiment, 8 varieties of Indica and one variety of Japonica were subjected to the temperatures of 23° and 30°C during ripening, and the following fesults were obtained: 1) Effects on the dimensions of kernel: The high temperatures lessened the length and breadth of kernel and the 100-grain weight in almost all varieties. The ventral radius was lengthened because of the acceleration of development in early period and the dorsal radius was shortened on accounts of the depression of development in the late period, so that the dorsoventral ratio was lessened at high temperature. Such varieties as Fujisaka-5, Prelude and Magnolia which showed remarkable decrease in dorsoventral ratio were assumed to be less adaptable to high temperature. (table 1, 2) 2) Effect on the thickness of bran: Number of layers of aleurone cells, thickness of aleurone layer and thickness of pericarp and testa were measured on the dorsal ridge and flank region as representatives of bran layer of kernel. In general, number of layers of aleurone cells and thickness of aleurone layer on the dorsal ridge and thickness of aleurone layer on the flank region increased on account of the acceleration of development in early period, and thickness of pericarp and testa on the flank region was thickened according as the depression of starch accumulation in the late period. High temperature had no marked effect on the thickness of pericarp and testa on the dorsal ridge and number of layers of aleurone cells on the flank region. (table 3) 3) Effect on the occurrence of the white-ridge kernel: Occurrence of the white-ridge kernel is due to the acceleration of developmemt in the early period and the depression of starch accumulation in the late period, accordingly, occurrence of them is closely connected with decrease of dordo-ventral ratio as reported previously⁵⁾. This relation was recognized in this experiment and the white-ridge kernels occurred numerously in such varieties as Fujisaka-5, Prelude and Magnolia of which dorso-ventral ratios markedly decreased at high temperature. (table 4, 2) 4) Effect on the occurrence of the milky-white kernel: The milky-white kernels (chalky rice) caused by high temperature occurred on account of deficient accumulation of starch in the early period of maturation, because of the competition for absorption of nutrients as a result of rapid maturation. Therefore, they had linear or spindle-shaped opaque part on the middle of cross section. As shown in table 4, the milky-white kernels occurred numerously at high temperature in all varieties, especialy in Tadukan, Tepa-I and Ubaisen, and these vaeieties are thought to be undesirable in the tropics. 5) Effect on the occurrence of the basal-white kernel: Basal wuite kernels are produced by the depression of starch accmulation in the last period of maturation, therefore, the occurrence of them has a connection with the increase of thickness of pericarp and testa on the flank regin of kernel. In this study, basal-white kernels noticeably increased at high temperature in all varieties, especially in Prelude, Tadukan, Tepa-I, Ubaisen and Fujisaka-5 assumed to be less adaptable to high temperature. (Table 3, 4) 6) Effect on the cooking qualities: Water-uptake ratio and total solids content were examined at cooking temperatures ranging from 65° to 75°C, because these characteristics of different kind of rice were mast distiguishable in this range of temperatures. [the rest omitted]