Physiological Studies on the Mechanism of Ripening in Rice plant.

Abstract

Chemical, Physico-chemical, and enzymatic changes that occur in the seed of rice plant as it grows from fertilization to maturity were studied. 1, There is little synthesis of starch up to the 4th day after flowering. A very rapid synthesis of starch takes place between the 7th and the 20th day. Storage of starch reaches to a maximum about at the 30th day. 2, The contents of sugars, including reducing sugars, increase untill around the 15th day after flowering, and decrease rapidly thereafter. 3, A lot of inorganic phosphorus is present in the seed in an earlier period of its development. After about the middle milk-ripe stage, its content in endosperm falls continuously. In the fluoride stage it is practically negligible. On the contrary it increases in the aleuron layer and embryo. The fact that inorganic phosphorus is not accumulated in the storage organ of starch is suitable for the accumulation of starch, because the starch synthesis by phosphorylase is inhibited by the presence of inorganic phosphorus. 4, The optimal pH for synthesis of starch by phosphorylase is 6.5-6.7. The optimal temperature is around 37°C. The amount of phosphorylase in endosperm rapidly increases untill the 18th day after flowering, hereafter it decreases continuously. After about the 30th day its activity is lost almost completely. The period of the highest activity of phosphorylase closely relates to that of the rapid accumulation of starch. 5. The optimal pH of rice-amylase is 4.1. The amount of amylase is very small in the early period of seed development. It increases from about the 10th day after flowering and reaches the maximum value about the 20th day. 6, The pH value of endosperm at the early stage is around 5 or less. However, it rises to more than 6 after the milk-ripe stage. On the contrary the pH value of aleuron layer and embryo is 5 or less. As the favorable pH value for synthesis of starch by phosphorylase is in the range from 6 to 7, it may be said that the pH condition within the endosperm of the milk-ripe stage later is suitatable for synthesis of starch. As a final conclusion, chemical, physico-chemical, and enzymatic conditions of endosperm are favorable for synthesis or accumulation of starch by the action of phosphorylase as far as the present author has examined.