Abstract
The starch accumulation process and the ultrastructure of the plastid-amyloplast system in potato tuber were observed by electron and light microscopes. Amyloplasts were slightly flat ovalshaped and contained a single starch grain. Even if they were lrge, their replications were observed. In the parenchyma cells around the phloem, the plastid-amyloplast system intensively replicated and grew larger with starch accumulation. As a result, much starch was accumulated in the parenchyma cells in the tuber's internal cortex and perimedulla. Structures assumed to be involved in starch synthesis were observed in the stroma of plastids and the localized stroma of amyloplasts. The possible functions of these structures were discussed. We obsrved that parts of the tubular complex extended around the starch grain. It was considered that the complex played a role in efficient assimilate transportation for starch synthesis into the area forming a starch grain. Osmiophilic granules were originated from the tubular complex. It was observed that the granular material in the membrane-bound body was carried to near the starch grain. Therefore it was suggested that the branching enzyme reported to be eixsting in the membrane-bound body was supplied into the area synthesizing starch. The aggregating granules, which had been regarded as phytoferritin, were indentified as polysaccharide by the bismuth oxynitrate staining method in this study. This polysaccharide might function as temporary storage material for regulation of the starch synthesis and accumulation. The eccentric rings were observed in each starch grain under light microscopy. From our observations, we concluded that the amyloplast intensively synthesized starch in the localized stroma.
