Physiological basis of submergence tolerance in rice and implications for crop improvement

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 (similar to 22 m ha). In eastern India, similar to 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 location is known. This review focuses on current physiological 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 facilitated by the recent application of chlorophyll fluorescence spectrophotometry as a rapid and non-destructive technique to screen submergence-tolerant cultivars.