Cell-Physiological Studies on the Synthesis and Accumulation of Starch in Sweet Potato

Abstract

Distribution, and daily change in the contents of starch, reducing sugar and inorganic phosphate, in isoelectric point and in the activity of phosphorylase and amylase were examined at various parts of the plant body of sweet potato by means of histochemical tests. The following conclusions may be derived from those experiments. 1. Phosphorylase is located in the tissues where starch is synthesized naturally. 2. There is a tendency in the plant body of the sweet potato that the greater the activity of phosphorylase than amylase the higher is the activity of deposition of natural starch in the tissues. 3. Many factors responsible for activities of those enzymes may be considered. Amongst them pH and the content of inorganic phosphorus relating in starch synthesis should be taken into consideration. But in this experiment, the isoelectric point was measured instead of pH, assuming the value of isoelectric point indicates approximately the pH value of the protoplasm. Tissues of high starch contents usually have somewhat a higher value of isoelectric point (pHi is mostly 6 or higher). This is probably due to the fact that starch synthesis by phosphorylase becomes maximum at pH 6 approrimately. 4. In the tissues of sweet potato tuber, for instance, in parenchyma of the cortex and in the pith, large amounts starch are accumulated, but on the other hand only a negligible amount of phosphorus can be detected. On the contrary, a high amount of phosphorus can be found in the primary cambium and secondary vascular bundles, where the amount of starch is negligible. From these facts, there seems to exist a mechanism by which the liberated inorganic phosphorus of starch synthesis is removed from the site of enzyme reaction in order to promote the starch accumulation.