Abstract
Rice is one of the few submergence-resistant crops, but cannot grow under extended periods of complete submergence. The frequency of heavy rains and flooding is expected to increase due to global warming. Therefore, flood-control is extremely important in flood-prone rice cultivation areas. In rice plants, shoot elongation is generally accelerated by submergence to escape the submerged condition by photosynthesis under aerobic conditions on the water surface. When completely submerged by an abrupt rise in the water level at the seedling stage for a period of less than two weeks, the ethylene concentration is increased, foliage growth promoted, chlorophyll function in leaves fails, and energy consumption is increased by the rapid elongation, resulting in depletion of carbohydrates. The gene related to the escape from submergence through such internodal elongation is SNORKEL1, 2 (SK1, 2). However, some varieties of rice tolerate complete submergence by inhibiting the elongation of submerged stems and leaves, thus suppressing the consumption of carbohydrates instead. In such submergence-tolerant rice plants, elongation of stems and leaves is inhibited by suppression of the increase of ethylene concentration in the plant, which also decreases the sensitivity to gibberellins. Thus, the plant maintains a balance between carbohydrate supply and demand. The gene related to such submergence tolerance is SUBMERGENCE1 (Sub1). Physiological knowledge of these mechanisms of submergence tolerance will contribute to development of submergence-tolerant varieties of rice.
