Effects of High Temperature during Ripening Period on the Development and the Quality of Rice Kernels

Abstract

With the purpose of making clear the effects of temperature on the development and the quality of kernels, plants of seven varieties, representing a wide range in adaptability to high temperatures, were subjected to temperatures of 23° and 30°C. at various stages of ripening. (Table 1) The results are summarized as follows. 1) Effect on ripening period, High temperatures throughout the ripening period accelerate the starch accumulation into kernel and the kernel development during the early period of ripening, but depress them in the late period. As a result, the length of ripening period is remarkably shortened and the weight of matured kernel is somewhat reduced. (Fig. 1, 2) However, the extent of reduction in length of ripening period and kernel weight are varied with varieties and more in the varieties seemed to be less adaptable to high temprature. 2) Effect on the dorso-ventral ratio. The ventral radius which grows in the early period of ripening is lengthened and the dorsal radius which grows until the late period is shortened by high temperature, with the consequence that the ratio of the dorsal radius to the ventral radius (dorso-ventral ratio) is lessened. (Fig. 3) The variations of dorso-ventral ratios caused by high temperature are greater in the varieties assumed to be less adaptable to high temperature than in the more adaptable varieties. Accordingly, the rates of variations in dorso-ventral ratios will be able to indicate the varietal differences in adaptability to high temperature. (Table 2) 3) Effect on the occurence of white-ridge kernels. When the growth of cells and the starch accumulation in the dorsal region of kernel are depressed by high temperature, the accumulation of starch into the outermost layers of starch cells along the dorsal ridge becomes markedly insufficient and these layers remain as opaque with the result that the dorsal ridge of the kernel is white colour in external look. Therefore, occurence of white-ridge kernels is closely connected with the decrease of dorso-ventral ratio and is abundant in less adaptable varieties of which dorso-ventral ratios greatly decrease by high temperature. (Fig. 4) The results of measurement of Vickers hardness indicating the density of starch accumulation show that the white-ridge kernel is softer than the normal kernel, especially, on the outer part of basal and dorsal regions of kernel. (Fig. 6) Then, the milling-loss is markedly more in white-ridge kernels owing to the softness of the outer part of kernel as well as the increase of thickness of bran. (Table 4, 5) 4) Effect on the occurence of basal-white kernel. Under high temperature, in the inferior kernels on a panicle, starch becomes insufficient to fill up starch cells on the outermost part of basal region on account of depression of starch accumulation in the late period of ripening, and the basal part becomes opaque and white colour in appearance. Then, the basal-white kernels occur numerously in the inferior kernels of lees adaptable varities by high temperature. (Fig. 5, 9) Therefore, the basal-white kernel is soft on the outer part of basal region as shown in Fig. 6, then the milling-loss increases as compared with the normal kernel. (Table 5) 5) Effect on the occurence of milky white kernel. The milky white kernel is milky white in external appearance owing to the opaque part at the central or middle region on the cross-section of kernel and becomes chalky rice by milling. (Fig. 7) This kernel nccurs in such a case that the accumulation of starch into the kernel is transitorily checked for a few days during ripening. [the rest omitted]