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L.J. Wade*
IRRI, P.O. Box 933, Manila, Philippines
Introduction
Rice, a semi-aquatic species well-adapted to flooded soils, is grown on 150 million ha worldwide, providing 20 percent of global per capita energy and 15 percent of global per capita protein requirements. The irrigated area of 79.2 million ha provides 75 percent of total production annually, at an average yield of 4.9 t/ha. In contrast, rainfed ecosystems yield only 1.9 t/ha on average, comprising 1.2 t/ha in the uplands, 2.3 t/ha in rainfed lowland, and 1.5 t/ha in flood-prone. The rainfed ecosystems occupy 17.2, 40.6 and 11.5 million ha, respectively. These ecosystems for rice are defined by their agrohydrologic conditions. This paper examines characteristics of these diverse ecosystems, and the implications for rice root growth and function.
Ecosystem Definition
Toposequence position and agrohydrologic condition define the ecosystems for rice. Irrigated rice is grown in bunded fields with water control, thereby stabilizing crop performance and expected returns, permitting the farmer to invest in inputs such as fertilizer. Rainfed lowland rice is also grown in bunded fields, but is reliant on seasonal rainfall. Consequently, the water regime is variable during the life cycle in rainfed lowland, and water may be in excess during the peak of the wet season, or in deficit when the rains fail. In other rainfed environments, these trends are taken to further extremes. In low-lying areas, the crop may be fully submerged, perhaps more than once in the life cycle. In contrast, upland rice does not encounter flooding, but may encounter water deficit.
Irrigated Rice
Traditional irrigated systems involve transplanted rice on puddled soils. Besides minimising weed competition for the well-established crop, nutrient availability is generally improved, and the crop flowers and matures evenly. Crop improvement programs have had greatest impact in this reliable system. Yield potential has been raised by increasing the allocation of dry matter to grain at the expense of stems and roots. Lodging may be a problem, however, especially with heavy rains and winds. Recently, yield decline has been observed in intensive irrigated systems, as a result of reduced nitrogen release from recalcitrant soil organic matter. Lodging resistance, improved nutrient capture, and further increases in yield potential are needed to ensure food security. At the same time, the resource base must be safeguarded against declining supply of irrigation water, leaching and percolation losses, and salinity. Integrated management for nutrients and pests are essential.
Upland Rice
Production of upland rice ranges from the shifting "slash and burn" cultivation on sloping uplands in southeast Asia, to the acid uplands of south America, and more stable production on level lands in south Asia. The defining feature of this ecosystem is the lack of ponded water at any time during the life cyle, so soils remain aerobic throughout. In the uplands, crop establishment is risky, competition from weeds is severe, soil fertility is often low, drought is a regular feature, and roots may encounter both chemical (low pH) and physical (hardpan) barriers. Absence of flooding does broaden the range of crops grown in the system, with intercropping and agroforestry being practiced in some areas.
Flood-Prone Rice
The strategy the plant requires to survive in flood depends on the depth and duration of submergence. Research has revealed the advantages of a non-elongation strategy (tolerance) for short-term submergence, such as occurs in rainfed lowland. For prolonged submergence, the plant needs to elongate with the floodwater to maintain oxygen supply to the roots (avoidance). Further challenges are posed by the nature of the floodwater: stagnant or flash flood, clear or turbid, fresh or saline. Direct seeding is common in flood-prone environments, as establishment need not be delayed until soils are puddled. Larger seedlings are better able to cope with submergence, because of larger reserves. Improved lines with submergence tolerance (rainfed lowland), and elongating types with semi-dwarf characters above the floodwater (deepwater), are raising yield potentials in flood-prone environments.
Rainfed Lowland Rice
By growing the crop in bunded fields to impound rainwater, farmers sought to capture the advantages of the irrigated system for weed control and nutrient availability. But lack of control of the water regime subjects the crop to extreme contrasts. Traditionally, photoperiod sensitive cultivars were transplanted onto puddled soils, flowering towards the end of the wet season to reliabily produce some yield each season. Shortage of labour and availability of improved, photoperiod insensitive cultivars is resulting in a shift to direct seeding, with increased exposure to establishment problems and weed competition. Exposure to late season drought is reduced, however, and in some areas, a short duration legume such as chickpea may now be grown after the rice. The root system of rainfed lowland rice may be developed in anaerobic conditions, but in subsequent drought, the shallow roots may face the challenge of extracting water and nutrients from deeper layers later in the season. These contrasting soil conditions provide a special challenge to root systems in rainfed lowland and flood-prone, where drought-submergence-drought may occur within the one season. These challenges are considered in greater detail in the second paper (Session 2).