Studies on the Development of Endosperm in Rice : 11. Development of starch granules in endosperm tissue.

Abstract

Increase in size of the starch granules began to precede from the central portion of endosperm tissue and reached the maximum size (39μ in major diameter) by the 15th day after anthesis falling on around 10 days after begining date of the granule formation. According as a portion of the endosperm tissue was on the more outside, the begining date of the granule formation was delayed more, and the increase in size of the starch granules proceeded at a slower pace. Accordingly, starch granules found in the exterior cell layers were always smaller in size than those found in the interior ones. And the starch granules in the outermost cell layer facing the aleuron layer were delayed most. Their final size was as small as 6μ in major diameter on about the 35th day after anthesis, i.e., on around 25 days after begining of starch granule formation. Average size of the whole starch granules showed increase at the most remarkably rapid pace during the period from 10th to 18th day after anthesis, and continued to show an increase until the 35th day to the extent of 23×19μ, thus completing the increase in size. It was generally found that there was little or no significant difference in size of the starch granules contained in each cell. And that it seemed that there was no any appreciably great difference in the number of starch granules in each cell, irrespective of remarkably great differences in size of cells. Size of starch granules of the sample variety "Yoneshiro" (japonica) was generally larger than that of starch granules of such sample varieties as "T136" and "Mushakudanti" (indica). We inferred the following types of routes of translocation of the material substances composing of starch granules into the endosperm tissue. At the very early stage, direct translocation of substances into endosperm from surrounding nucellus (route a) might be fairly noticeable, besides the supply of substances from the vascular bundle on the outside of dorsal side (conducting strand). In and after milk-ripe stage, route (a) had no longer been noticed. Instead, the route (b) translocation "conducting strand→nucellar epidermis→surface cell layer of endosperm (aleuron layer)→starch cell layers" seemed to have probably played the important role in translocation. In and after dough-ripe stage, it seemed that translocation route has come to be increasingly confined to the route (c) translocation "conducting strand→nucellar projection→dorsal side of endosperm". In spite of the fact that the pathway of starch materials in the endorperm cell was becoming increasingly narrower due to the growing size in starch granules after dough-ripe stage, a considerably large amount of materials have been translocated through the endosperm tissue. From this fact, it is suggested that the starch materials have been translocated not only by the simple physical stream in the cell, but also with a help from a certain "carrier" which pocesses enzymatic or chemical activities.