DEVELOPMENT AND RAPID ADOPTION OF SUBMERGENCE-TOLERANT (SUB1) RICE VARIETIES

Flooding is a frequent hazard in lowland rice environments and is expected to intensify with climate change. Short-term submergence regularly affects over 20 million ha of rice in the tropics. Tolerant varieties like the Indian landrace FR13A can survive submergence for 2 weeks or more, whereas susceptible varieties are severely damaged or killed within a week. The discovery of the tolerance gene SUB1 enabled marker assisted backcrossing to transfer it into rice mega varieties, and eight "Sub1 varieties" were recently developed in the background of popular Asian cultivars. Five have been officially approved for distribution to farmers and the remaining three are under advanced evaluation. The Sub1 varieties have grain yields equal to their parent varieties under non-submerged conditions and yield advantages of 1t/ha or more when affected by submergence. They are tolerant at all growth stages from a week after seeding up to 2weeks before flowering. The yield advantages are observed with submergence durations of 4-20days or more, depending on environmental conditions. Farmers growing Sub1 varieties can avoid the costs of re-transplanting when submergence occurs in the nursery, and they can realize additional yield gains with improved crop management practices. Because the varieties have the same genetic background as popular cultivars, national and state governments have expedited their release. Intensive seed production and distribution programs have facilitated their rapid adoption by farmers. Further improvements to Sub1 varieties will include increasing the level of tolerance; incorporating additional traits, such as disease resistance and drought or salinity tolerance; and transferring the SUB1 gene into additional cultivars and promising breeding lines.