REVIEW ARTICLES CURRENT SCIENCE, VOL. 91, NO. 7, 10 OCTOBER 2006 899 *For correspondence. (e-mail: rksarkar_99@yahoo.com) Physiological basis of submergence tolerance in rice and implications for crop improvement R. K. Sarkar 1, *, J. N. Reddy 1 , S. G. Sharma 1 and Abdelbagi M. Ismail 2 1 Central Rice Research Institute, Cuttack 753 006, India 2 International Rice Research Institute, DAPO 7777, Metro Manila, Philippines The environments of rainfed lowland rice are highly variable both over time and location. Flash-flooding and submergence adversely affect at least 16% of the rice lands of the world (~22 m ha). In eastern India, ~13 m ha of rice lands are unfavourably affected by excess water and periodically suffer from flash-floods and complete submergence. Improvement of germplasm is likely the best option to withstand submergence and stabilize productivity in these environments. However, progress in germplasm improvement has been slow but can substantially be enhanced if the physiological and genetic bases of submergence tolerance are well understood and extent of damages over time and loca- tion is known. This review focuses on current physio- logical understanding of tolerance to submergence in rice with greater emphasis on floodwater environments, new genetic resources and potential of DNA marker technology for incorporating multiple traits associated with tolerance, to enhance and speed progress through breeding. Research on the aspect has been further facili- tated by the recent application of chlorophyll fluorescence spectrophotometry as a rapid and non-destructive technique to screen submergence-tolerant cultivars. Keywords: Crop improvement, physiology, rice, sub- mergence. IN India, the area under rice cultivation is 44.5 m ha (data of 2000–01) 1 with an annual production of 85.5 million tons and an average productivity of 1.9 t ha –1 . Rice is grown in a wide range of ecologies ranging from irrigated to uplands, rainfed lowland, deep water and tidal wetlands. About 29% of India’s total rice area, i.e. ~13 m ha is rain- fed lowland, which contributes only 19% to national rice production. In a normal year, about 4 m ha is drought-prone, while 3 m ha is favourable, another 3 m ha is medium deep waterlogged with water standing for up to 50 cm. The remaining 3 m ha is submergence or flood-prone, where plants are completely submerged for 1–2 weeks or so, resulting in partial or even complete crop failure. Rainfed lowlands constitute highly fragile ecosystems, always prone to flash-floods (submergence) with an aver- age productivity of only 1.2 t ha –1 in normal years and hardly 0.5 t ha –1 in case of submergence. Among the 42 biotic and abiotic stresses affecting rice production, sub- mergence has been identified as the third most important constraint for higher rice productivity in eastern India 2 , because it sometimes resulted in near total yield loss. Be- sides India, flooding is widespread in other South and Southeast Asian countries such as Bangladesh, Thailand, Vietnam, Myanmar and Indonesia. Suitable germplasm as well as management technologies are therefore needed to enhance and stabilize rice productivity in these areas. The effects of submergence on rice as well as physio- logical base of tolerance were recently reviewed 3–7 . The present article focuses on the multifaceted problems to which rice crop is exposed during submergence. It also emphasizes various tolerant traits or mechanisms that are necessary for high and stable productivity in submergence- prone areas, such as tolerance during germination and seedling emergence, tolerance to repeated floods and fast regeneration after submergence, all of which could con- tribute to better and stable productivity. Due consideration is also given to the use of innovative research approaches to build on the progress achieved so far for germplasm improvement. Inference from field studies There are many environmental and biophysical stresses to which plants are exposed during complete submergence in the field, which include reduced movement of gases to and away from plant surfaces. Gases are known to diffuse 10,000 times slower in water than air 8 . Hence, poor plant growth and survival during submergence or waterlogging is often considered a consequence of the decreased diffu- sion of gases which affects plant growth and metabolism. Reduced O 2 supply limits respiration, reduced CO 2 sup- ply limits photosynthesis and reduced ethylene diffusion away from the plant triggers chlorosis and excessive elongation of leaves of intolerant cultivars 9,10 . Measurement of the concentration of gases in floodwater during submergence gave important clues as to the causes of reduced growth and survival. Studies at NDUAT, Fai- zabad, eight locations in Bihar and Uttar Pradesh and by Central Rice Research Institute (CRRI), Cuttack also helped to understand the variations observed in plant survival at different locations of eastern India, when rice genotypes are exposed to submergence for similar durations. Oxy-