When rice plants are submerged under water in the case of flooding, the oxygen content in the internal atmosphere of rice plants is remarkably decreased due to the insufficient supply of oxygen to the plants. Under reduced oxygen tension, aerobic respiration is reduced and at the same time anaerobic respiration is promoted, both resulting in the increased consumption of hexoses with decreased evolution of energy. Accordingly the amount of available carbohydrates in the plants is exhausted rapidly and protein is exposed to hydrolysis. Extent of protein hydrolysis depends on the oxygen tension; low oxygen tension inhibits hydrolysis. Bubbling of water with air or exposure of submerged plants to light favour protein hydrolysis. Initial increase of soluble nitrogen fractions such as amino acids and final exhaustion of these fractions indicates the production of soluble nitrogen fractions by protein hydrolysis as the utilization of these fractions as respiration-substrate. Therefore the resistance of rice plants for submersion is determined by the following three factors-(a) amount of available carbohydrates contained in the plants, (b) velocity of subsrrate consumption; rate of respiration, and (c) oxygen tension. The higher oxygen tension results in the less consumption of carbohydrates and longer lasted supply of respiration-substrate by protein hydrolysis.