Current regional rice hectarage and yields of the world are shown in Fig. 5.2. Over 90% of the world's rice is produced and consumed in South, Southeast and East Asia, where monsoonal climates dominate. In these areas, rice is not only the staple food but also provides the livelihood of most people. Unlike wheat and maize, rice is produced mainly by subsistence farmers in developing countries.
Cultivated rice consists of two distinct species of Oryza: O. sativa L. and O. glaberrima L. The dominant species in world rice production is O. sativa; O. glaberrima is limited mainly to west Africa. O. sativa consists of three subspecies: indica, japonica and javanica, which are genetically distant from each other. Crosses among these subspecies generally result in extensive or partial spikelet sterility. These three subspecies are also different physiologically and morphologically and occupy different ecological niches (cf. review by Takahashi, 1997). Compared with indica cultivars, many japonica cultivars exhibit higher tolerance to cool temperatures, while the reverse is true for drought resistance (Oka, 1953). These physiological differences define major production regions for these two subspecies. The Yangtze River in
China forms an approximate border between these two subspecies, with japonica cultivars predominating north of the river and indica cultivars south of the river. The javanica subspecies originated in island areas of Southeast Asia, including Indonesia. The three subspecies of O. sativa are further ecologically differentiated into upland, lowland or deep-water ecotypes, depending mainly on their adaptability to different water environments.
The large ecophysiological diversity of genotypes permits rice to be grown in widely different environments. Rice culture is generally classified into one of four main types, depending on water environment: irrigated rice, rainfed lowland rice, rainfed upland rice and deep-water rice cultures. Of the total 140-145 Mha of land area planted to rice annually, roughly half (about 53%) is grown as irrigated flooded-paddy rice, one-quarter (about 27%) as rainfed lowland rice, 12% as rainfed upland rice and 8% as deep-water rice (IRRI, 1993).
World rice production in the past three decades since the Green Revolution increased at a higher rate than did world population (Fig. 5.1) mainly due to yield increases. However, total rice production will need to increase by 46% by the year 2025 (IRRI, 1993) to keep pace with projected population growth, especially in Asia. Attainment of these production goals is likely to be very difficult since current increases in world rice production are slowing. For example, increases in world rice production during the 1985-1995 period declined to 1.7% year-1 which is lower than the population growth rate of 1.8% year-1 (Hossain, 1998). This stagnation in rice production is largely due to yield levelling for irrigated-rice cultures that use advanced technologies. Furthermore, rice produced under rainfed cultures has not benefited from the advances of the Green Revolution, due to environmental and infrastructural barriers to the adoption of new technologies. Clearly, technological innovations are needed to break through the current yield barriers in both irrigated and rainfed rice cultures.
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