Morphological Studies on the Mobilization of Reserves in Japanese Yam (Dioscorea japonica Thunb.) Seed Tuber and Eddo (Colocasia esculenta Schott var. antiquorum Hubbard & Rehder) Seed Corm on and after Sprouting

Abstract

We examined the mechanism of reserve mobilization in Japanese yam seed tubers and eddo seed corms on and after sprouting. The decomposition of starch in pith parenchyma cells of Japanese yam tubers and eddo corms progressed from the region distant from vascular bundles to that adjacent to vascular bundles. In eddo corms, the starch also decomposed from the proximal to the distal region adjacent to the sprout or regenerate plant body. In the yam tubers, the decomposition process was similar in the proximal, middle and distal regions. The first step in the reserve mobilization in pith parenchyma cells was the decomposition of the amyloplast envelope. Subsequently, starch granules decomposed. In Japanese yam tubers, the envelope and starch granules started to decompose from the peripheral regions of the amyloplasts. The observation of soluble polysaccharides, which was the decomposition product of starch granules, was made possible by the quick freezing-vacuum freeze-drying method. By this method, we demonstrated that the soluble polysaccharides in the parenchyma cells decomposed and decreased in density. In addition, the mucilage in the mucilage duct started to decompose and decreased in density from the proximal to the distal part of the corm and also from the periphery to the center of the duct. It was shown that not only starch mobilization but also mucilage mobilization mainly supported sprouting and the growth of the regenerate plant body during about the first half of the vegetative stage.