Rice Megaenvironments and Climate Change

Rice, eaten by about three billion, directly supports more people than any other staple food (Maclean et al., 2002). Rice adaptation is affected by many environmental factors, including day length and temperature, and soil factors such as salinity, aluminium toxic-ity and iron toxicity. However, within broad bands of latitude, rice MEs tend to be defined in terms of hydrology, water availability and maximum water depth (Khush, 1984). Rice is grown in a much wider range of hydro-logical environments than other crop species, under conditions ranging from the basins of poorly drained watersheds where water accumulates to depths of 5 m or more, through transplanted paddy fields in which water levels are maintained at a constant 10-20 cm for most of the growing season, to upland environments in South-east Asia where direct-sown rice crops are grown under aerobic soil conditions on steep hillsides. Thousands of years of farmer selection have resulted in the local development of specific ecotypes that are adapted to each of these hydrological environments.

Climate change is likely to affect rice production in two principal ways: (i) higher temperatures, both night-time averages (Peng et al., 2004) and acute high-temperature stress during flowering, are already reducing yields in many areas (Wassman et al., 2009b); and (ii) water availability for irrigation is likely to be reduced, and variability in rainfall may increase the frequency of both drought and flooding in rainfed systems. Fortunately, a wide range of genetic variation for adaptation to both temperature and hydrological environments exists, and can be deployed to adapt production systems to climate change.

The most severe climate change effects are likely to be those affecting water availability. Rice environments are broadly characterized as irrigated or rainfed. Irrigated rice is generally grown in puddled, flooded fields in which a standing water layer is maintained. Because this water is constantly being lost due to seepage, percolation, evaporation and transpiration, irrigated rice production, which supports the bulk of the population of Asia, is one of the biggest users of the world's freshwater resources (Tuong and Bouman, 2003). Irrigated rice systems, although buffered against short-term variation in water availability, are extremely sensitive to climate change effects on surface water availability. Reduced availability of impounded water or river flows for irrigated rice production is likely to have a major impact on irrigated rice production. The most urgent area requiring adaptation is likely to be the Indo-Gangetic Plain and the Indus Basin, where irrigated rice based primarily on Himalayan snowmelt supports hundreds of millions of people in India, Pakistan, Nepal and Bangladesh (Wassman et al., 2009b). The expected melting of the Himalayan glaciers (IPCC, 2007) is likely to greatly reduce irrigation water available in this critical system, driving shifts to water-saving production systems (Bouman and Tuong, 2001) or, in many cases, rainfed production.

Rainfed rice production areas may also be affected as climate change increases rainfall variability, increasing the frequency of damaging rain-free periods of drought in some areas, and the frequency of flooding in others. However, because the current range of rice production environments already covers these extremes, adaptation strategies can be devised based on currently existing systems. Germplasm that can support these needed adaptations is available, and can be targeted at critically affected systems through the use of managed stress screening and extrapolation from METs. It should be noted, however, that the existence of adapted germplasm and accompanying management systems will not guarantee against productivity loss; rice systems in which drought or uncontrolled flooding occur are inherently less productive than those in which water availability is controlled and no stress occurs, even when adapted germplasm is used to mitigate losses.

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