Climatic requirements of rice

Rice is grown from 53° N in northeastern China to 35° S in New South Wales, Australia. Major environmental factors that limit rice cultivation are air temperature and water supply. A seasonal total of at least 2400 growing degree-days for a period with a daily mean temperature above 10°C, is the climatological temperature limit required for rice cultivation (Ozawa, 1964). Since irrigated rice is grown under flooded conditions during most of the growing season, evapotranspiration for flooded rice is nearly equal to potential evaporation at a given location (Sakuratani and Horie, 1985). Thus, a water supply equivalent to the sum of seasonal potential evaporation is the minimum water requirement for irrigated rice culture. However, the actual amount of water required for an irrigated rice crop is the sum of evapotranspiration, water lost during land preparation (100-200 mm) and water lost due to percolation, which ranges from 0.2 to 15.6 mm day-1 (Yoshida, 1981), depending on field infrastructure and soil properties.

The air temperature range for vegetative growth of rice is generally from 12 to 38°C, with an optimum between 25 and 30°C. However, during reproductive development, rice is particularly sensitive to low temperature at boot stage, during which pollen grains are in the early microspore phase (Satake and Hayase, 1970), and to both low and high temperatures at flowering (Abe, 1969). Temperatures below 20°C during the early microspore stage inhibit pollen development (Nishiyama, 1984). Temperatures below 20°C or above 35°C at flowering generally result in increases in spikelet sterility (Terao et al, 1942; Abe, 1969; Satake and Hayase, 1970; Satake and Yoshida, 1976; Matsui et al., 1997a) due to anther indehiscence (Satake and Yoshida, 1976; Matsui et al., 1997a). Rice is more sensitive to cool temperatures and less sensitive to high temperatures during the early microspore stage than at flowering. For these reasons, low air temperature during the early microspore stage is the primary environmental factor that determines the northern limit of rice cultivation. Cool summers frequently result in severe yield reductions for rice grown in the northern parts of China, Korea and Japan.

High-temperature-induced spikelet sterility is typically found in tropical and subtropical areas, especially for rice crops grown during the dry season (Satake, 1995). Reductions in rice yields caused by high temperatures may become more severe under projected future global warming scenarios. In some regions of the world, potential future global warming could exacerbate this problem. However, relatively large genetic variability exists among rice cultivars with regard to tolerance of both high and low temperature damage.

The optimum temperature range for grain-fill in rice is between 20 and 25°C. This optimum is lower than that for vegetative growth (Yoshida, 1981). Temperatures above the optimum shorten the grain-filling period and reduce final yield.

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