Drought Adaptation in Rice

Anyone who has seen lush green rice crops growing in the sparkling floodwater of paddy fields might be surprised to find rice mentioned in a book on drought tolerance. More than 75 percent of the world&s;s annual rice production, some 430 million metric tons of rough grain, is produced in irrigated fields where the soil is saturated for most or all of the season ( Narciso and Hossain, 2002 ). In these situations, drought is not a constraint. The remaining 25 percent of global production is produced on about 45 percent of the land cropped to rice, in systems where water cannot be controlled. These areas include rice grown in upland systems, where the soil is aerobic, and rainfed lowland systems, where rainwater is impounded and soils range from saturated to aerobic within a season, depending on rainfall amount and distribution. Upland and rainfed lowland rice cropping systems are found primarily in tropical and subtropical regions. In these systems, drought is considered the most important abiotic constraint to production ( Evenson et al., 1996 ). The pattern and severity of drought varies widely among regions and across years, and considerable investment has been made in characterizing the rainfed rice environments from both biophysical and socioeconomic perspectives ( Tuong et al., 2000 ). Yields in these environments average less than 2 t·ha -1 , compared to 5.5 t·ha -1 in irrigated tropical and subtropical environments ( IRRI, 1997 ); rainfed rice regions have a higher incidence of poverty and poor human health. In addition, the availability of water for irrigated rice in Asia has declined dramatically in the past decade, due to increased urban water requirements. As the cost of water rises, some irrigated lands have been taken out of production or have been forced to restrict irrigation ( Tuong and Bouman, 2003 ), resulting in reduced production in some of the traditional rice bowls that feed burgeoning urban popula tions. These facts—the low yields of rainfed rice in regions of chronic poverty and the trends for reduced water availability in traditionally irrigated areas—provide ample justification for investment in improving our understanding of rice response to water deficit.