Physiological Changes Associated with Submergence Tolerance in Genetically Diverse Lowland Rice Genotypes

Rainfed lowland rice ecosystem is affected not only by water deficit but also by excess water leading from partial to complete submergence. Although the rice plant is well adapted to aquatic environment, it is unable to survive if completely submerged in water for several weeks. Damage to rice plants due to excess water has been advocated to occur during submergence and also on entry of oxygen after the recession of flood water. Breeding rice varieties with ample submergence tolerance is one of the approaches to alleviate the adverse effects of submergence which requires incorporation of physiological traits linked to tolerance mechanisms. In a controlled pot culture experiment where 21 days old plants of six rice genotypes differing in submergence tolerance were subjected to 10 days complete submergence. Underwater shoot elongation, shoot carbohydrate status and the activity of enzymes catalase and superoxide dismutase (SOD). Submergence tolerant varieties Swarna Sub1, FR13A and NDR 9730018 had higher carbohydrate status and lower shoot elongation during submergence, consequently higher plant survival. Catalase and superoxide dismutase activity before submergence was almost at par in all the genotypes but increased after de-submergence. A time course study of SOD activity indicated that the increase continued until 4 hrs after de-submergence which showed declining trend subsequently until 24 hrs. Submergence tolerant genotypes showed much higher increase in SOD activity as compared to intolerant ones. This can be correlated with higher survival rates observed in tolerant genotypes possibly through mitigating the adverse effects of reactive oxygen species. Low under water shoot elongation, high shoot carbohydrate and higher post submergence SOD activity could be possible physiological markers for screening of rice germplasm for submergence tolerance. INTRODUCTION Rice (Oryza sativa L.) is the major staple food of millions of the people in the world especially in Asia. National food security stems primarily on the productivity of rice followed by wheat. In India, the annual production of 85.5 million tons of rice with an average productivity of 1.9 t/ha comes from an area of 44.5 million ha spread over several ecologies of which about 17 million ha is rainfed, drought and submergence prone (Singh, 2002). Rainfed lowlands contribute about 19% of the national rice production where 1 Centre of Advanced Studies in Plant Physiology, N.D. University of Agriculture and Technology, Kumarganj, Faizabad, India. 2 Crop Soil and Water Sciences Division, International Rice Research Institute, Philippines.