Chapter 7 Challenges and prospects

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Keeping pace with population growth

The world population of 5 000 million in 1990 is expected to reach 8 000 million by 2020. Populations of less-developed countries, presently totalling 3 700 million, will reach 6 700 million by 2020. The present 2 100 million rice consumers in developing countries will reach 3 700 million in 2020 (IRRI, 1989).

To meet the projected growth in demand for rice (making allowances for its substitution by other foods) as incomes increase, the 1988 rice production of 490 million tonnes must increase to 556 million tonnes by 2000, and to 758 million tonnes by 2020-a 65 percent increase (1.7 percent per year), (IRRI, 1989). However, for the leading rice-growing countries of South and Southeast Asia, the needed increase in rice production by 2020 is about 100 percent (2.1 percent per year).

Environmental considerations

There has been increasing concern over the growth in aggregate rice output, which has peaked and is starting to decline, according to long-term experiments (IRRI, l990b). Comparison of data from farmers' fields and experiment stations in Indonesia, the Philippines and Thailand confirmed that yield potentials are stagnating and that there is a diminished gap between potential and actual farm yields. There is also strong but not conclusive evidence that rice yields and productivity are declining more than those of wheat in rice-wheat cropping systems. Zinc deficiency and yield response to phosphorus in addition to nitrogen are now more common.

Current production trends address the stability and ecological sustainability of rice production, its economic viability and equity. Thus, organic fertilizers such as Sesbania species and biological nitrogen fixation with organisms such as Azolla and Anabaena species are being pursued as partial substitutes for inorganic N fertilizers. Nitrogen fertilizer efficiency is only about 40 to 50 percent because of ammonia volatilization, nitrification and denitrification, leaching and surface drainage. Losses may be minimized by the use of slow-release and controlled-release fertilizers (De Datta, 1989). Deep placement either by hand or machine has also shown promising possibilities, but tests on machine deep placement have not given consistent results. Optimum time of split application of nitrogen fertilizer needs to be studied further with the current short-duration varieties. A recent review (Conway and Pretty, 1988) suggests that fertilizer use in developing countries presents very little actual hazard to health and contamination of the environment.

Integrated pest management is being introduced to minimize pesticide use and its concomitant pollution problem. The escalating use of insecticides in ricegrowing areas from the 1960s to the 1980s was not balanced by widespread improvements in insect pest control (IRRI, 1984b). Undesirable consequences have included pest resurgence, multiple insecticide resistance of major pests in high-use areas, destruction of communities of natural enemies, drastic reduction of fish as a local protein source and disturbing increases in human and farmanimal poisoning. A framework for long-term stable crop protection should be based on the primary control tactics of varietal resistance, cultural control and biological control. When such control tactics fail to provide adequate protection, insecticides may be applied in relation to pest populations and economic damage levels. The banning of persistent use of pesticides in most rice-producing countries has considerably reduced the pesticide residue and pollution problem. For example, carbofuran residues were found to be below the 0.2 ppm tolerance limit in rough rice from rice plants treated with 0.5 to 1.0 kg/ha active ingredient by various methods (Seiber et al., 1978). Levels of total organophosphate pesticides in irrigation water runoff from the IRRI farm in 1987-88 were low (at the ppb level) on the average, and no organochlorine pesticides were detected (IRRI, 1988b).

Environmental problems related to rice production include global climate changes: increases in atmospheric carbon dioxide, methane and nitrous oxide and a decrease in stratospheric ozone with a resultant increase in the ultraviolet-B radiation reaching the earth's surface, retention of solar radiation (the greenhouse effect) and global warming. Rice fields have been cited as the major generators of methane and nitrous oxide; studies are under way to verify these observations (IRRI, I 990a).

Soil loss in the 13 percent upland rice area ( 18 million ha) is estimated to be 2 to 4 cm per year or the equivalent of 200 to 400 t/ha per year in the open-field agricultural systems in Southeast Asia (IRRI, 1990b). Indicative of this problem is the fact that major rivers in Southeast Asia carry ten times more sediment out to sea than river systems in other parts of the world.

Water-induced land degradation includes waterlogging and salinity development from the intensive use of land in irrigated conditions. In addition, excessive sedimentation from mine tailings and industrial pollution affects land productivity (IRRI, 1990b). Irrigation management for sustainable production systems is imperative, including water management for acid sulphate soils and productivity enhancement for coastal saline areas. For non-irrigated farms, rainwater conservation in a reservoir of about 7 percent of the farm area can help stabilize yield and increase productivity in rain-fed, drought-prone lowland areas.

Malaria, schistosomiasis and Japanese encephalitis are important vector-borne diseases associated with rice production in developing countries (IRRI, 1988a). The causal agents are directly or indirectly associated with aquatic environments. Mosquitoes are the infective agents of malaria and of the encephalitis virus. Snail species act as intermediate hosts for the schistosome parasites, the cercariae, which swim about freely in contaminated water after they have been shed by the snails.

Increasing yield potential

More than 60 percent of the world's rice area is now planted to varieties of improved plant type. Little improvement in yield potential has occurred since the introduction of improved varieties in the mid- 1960s when efforts were directed toward incorporating disease and insect resistance, shortening growth duration and improving grain quality. Yield is a function of total dry matter and harvest index (panicle/panicle and straw). The semi-dwarf varieties have a harvest index of around 0.45 to 0.50, in contrast to about 0.3 to 0.4 for the traditional tall varieties (Yoshida, 1981). Efforts are being made to improve the harvest index to around 0.6 to increase yields. The modern varieties have 20 to 25 tillers, of which only about 15 to 16 produce small panicles with about 100 to 120 grains. Efforts are being made to breed rice with only four to five productive tillers but with large panicles of about 250 grains to give a maximum yield of 13 t/ha as compared to the maximum yield of 10 t/ha of the present varieties (IRRI, 1989). These rices must have sturdy stems to support large panicles, dark green erect and thick leaves and a vigorous root system, and they should be about 90 cm tall. The proposed plant type will have to be managed differently from the present high-tillering modern rices which have been bred for transplanted conditions. They will be more suitable for direct sowing. The genetic diversification of tropical rice is being increased by crossing with japonica rices and with wild species through wide hybridization.

Drought resistance is important, particularly in upland and rain-fed lowland rices. Factors such as rooting depth, extent of stomata! closure and cuticular resistance to water vapor are involved in varietal differences in response to water stress.

Deep-water or floating rices have a trait that enables their internodes to elongate to keep up with increases in water level. Deep-water conditions prevail in deltas, estuaries and river valleys in Bangladesh, Cambodia, India, Indonesia, Myanmar, Thailand and Viet Nam where flood waters rise annually to depths of from 0.5 to 5 m. Some of these rices also show drought resistance. Cold tolerance at the seedling, tillering or maturity stage is important in the mountains and hilly regions of countries such as Bangladesh, India, Indonesia, Nepal and the Philippines, where the semi-dwarf rices turn yellow and die or are stunted because of low ambient-air or irrigation-water temperatures. However, in the hot regions, such as southern Iran, Pakistan and Senegal, sterility is the problem, mainly because of disturbed pollen shedding and pollen viability.

No modern varieties have been bred to withstand completely the acid sulphate soils in parts of India, Viet Nam, etc.; the salty soils in inland desert areas in parts of India and Pakistan and the salt in brackish water in coastal regions; alkali soils; or organic soils (histosols). Iron toxicity and iron, zinc and phosphorus deficiencies are serious soil problems.

Losses due to insects, diseases and weeds in individual countries in the region range from 10 percent to more than 30 percent. Because of the rapid breakdown of single dominant gene resistance of rice plants to insect pests (about three years for resistance to brown planthopper, the Bphl gene), durable moderate resistance is a major focus. This type of resistance is sought to regulate the selection pressure on the insect pests so that insect strains resistant to the rice variety will not readily develop by mutation, genetic drift (the process by which smaller subpopulations hold random subsets of the total genetic variation), migration or selection. More insect population genetic studies are needed to determine how genetic variation in the pests'ability to feed on resistant varieties differs among subpopulations. To manage resistance, we need to understand how both populations and regions differ in genetic variation to overcome resistance. Approaches include pyramiding two or more resistance genes, the multiline approach and horizontal resistance. Wide crosses with wild Oryza species are being used to incorporate resistance genes from wild rices. The same strategy is required for all pest resistance genes.

Probably as important a factor as seed viability is seed vigour, which tends to deteriorate during the few months of seed storage after harvest, depending on the variety. Loss of seed vigour results in an uneven initial stand of the rice crop, particularly on direct seeding (Seshu, Krishnasamy and Siddique, 1988). This is particularly critical for direct-seeded irrigated rice, where the pregerminated grain is drilled at least an inch below the soil surface under water. To improve overall productivity not only rice but the whole Asian rice farming system should be considered to determine the best pattern for each region.

Rice biotechnology

The Rockefeller Foundation's International Programme on Rice Biotechnology, established in 1984, has the following goals: to assure that new techniques for crop genetic improvement based on advances in molecular and cellular biology are developed for rice; to facilitate the transfer of these biotechnologies to rice breeding programmes in the developing world to produce improved varieties that address priority needs; and to help build the scientific research capability necessary for the continued development and application of new rice genetic improvement technologies in selected developing countries (Toenniessen and Herdt, 1989). Activities include wide hybridization to transfer useful traits from wild relatives to cultivated rice and the development of a knowledge base and biotechnology tools. These include the development of genetic maps and markers based on cloned DNA sequences, protoplast techniques as a vehicle for various genetic manipulations, genetic transformation techniques, cloning and characterizing of rice genes, diagnostic tools for the study of host-pathogen interactions and novel genes for rice improvement. Novel genes being studied for rice improvement include viral genes such as a coat-protein gene conferring resistance to rice tungro virus, Bacillus thuringiensis toxin genes for resistance to yellow stem borer and other insect pests and wheat genes for inhibitors of rice weevil amylase. The objective is to produce transgenic rice plants containing these useful genes to confer resistance or tolerance to pests and diseases or to environmental stresses in order to ensure stable, high yields.

Efforts are being made to incorporate the maize Y1 gene (endosperm ▀carotene), (Buckner, Kelson and Robertson, 1990) or a provitamin A carotenoid (e.g. tomato phytonene) gene (Cheung and Kawata, 1990) into rice grain to reduce the incidence of vitamin A deficiency in Asia (see Chapter 2). However, a non-pigmented precursor, as in white as opposed to yellow maize, is preferred to avoid the consumer objection to yellow-endosperm rice. The genes for carotenoid synthesis are present in rice, as in all photosynthetic plants, but are expressed in photosynthetic tissue and not in the endosperm.

Maize and wheat inhibitors of a-amylase from insects, especially the rice weevil, are being examined for possible incorporation into rice grain to improve its shelf-life and reduce storage losses. Oryzacystatin also inhibits rice-weevil digestive enzymes (sulphhydryl protease) and is under study (Reeck, Muthukrishnan and Kramer, 1990).

The wheat glutelin gene involved in bread-baking quality (MacRitchie, du Cros and Wrigley, 1990) is being introduced into rice. The effect of the introduced gene on the grain protein content and quality of transgenic rice plants may be interesting. A wheat high-molecular-weight glutenin gene accumulates in transgenic tobacco endosperm at approximately 0.1 percent of total endosperm protein (Robert, Thompson and Flavell, 1989). Introduction of the barley aamylose gene may also improve the seed vigour and malting quality of rice grain.

Starch mutants

Starch mutants from Japanese rices have been induced by treatments of Sasanishiki with ethyl methane sulphonate, of Norin 8 with 32P beta rays, or of fertilized egg cells of Kinmaze with N-methyl-N-nitrosourea (Omura and Satoh, 1984; Juliano et al., 1990). They have been transferred to IR36 by two back-crosses. Sugary mutants contain phytoglycogen and have a high content of free sugars. Shrunken mutants have a low starch content. Both sugary and shrunken mutants have wrinkled brown rice, but the endosperm is hard in sugary and soft in shrunken mutants (Omura and Satoh, 1984). Floury grain has a chalky, soft endosperm. Dull mutants contain 2 to 14 percent amylose on a starch basis (as compared to 0 to 2 percent in waxy rice starch) and have a tombstone-white hard endosperm. Amylose extender(ae) mutants contain irregularly shaped starch granules characteristic of high-amylose maize starch. The IR36-based mutants had lighter, lower-density grains than IR36 and had a higher amylose content than the original Japanese rice mutants except for the dull mutants (Juliano et al., 1990).

The IR36-based ae mutants have a 40 to 42 percent apparent amylose content and a GT of 73 to 80░C (Juliano et al., 1990). Their protein lysine content is higher than that of IR36 by 0.8 percent in brown rice and 0.5 percent in milled rice (Juliano et al., 1990). The maize ae mutant also has a higher lysine content in its protein (Grover et al., 1975). This ae mutant and other endosperm starch mutants have SDS-polyacrylamide gel electrophoresis patterns identical to those of the parent varieties (IRRI, 1983b; IRRI, unpublished data, 1990).

Protein mutants

Higher lysine mutants produced by S-2-aminoethyl-L-cysteine treatment of United States rices (Schaeffer and Sharpe, 1983) had a higher percentage of lysine in the grain protein (by about 0.5 percent) and a higher percentage of protein in the grain, but they also had lighter grain and actually contained no more lysine than the parent (Juliano, 1985a). The 0.5 percent increase was also reflected in the screening of the rice germplasm bank for lysine content (Juliano, Antonio and Esmama, 1973). Amylose extender mutants of IR36 also had 0.5 percent more lysine in their protein than IR36 (Juliano et al., 1990).

A screening programme was initiated for mutants for the rice storage protein bodies PB-I and PB-II, which are located in the starchy endosperm. The crystalline PB-II is rich in glutelin and the large, spherical PB-I is rich in prolamin (see Chapter 3). Glutelin has a better amino acid score than prolamin except for that of a minor subunit of prolamin (see Table 17). Thus, the aim of the screening programme was to improve the nutritional quality of the rice protein by increasing the proportion of PB-II proteins, or reducing the proportion of PB-I proteins (Kumamaru et al., 1988). A number of mutants were identified which met these criteria, and their protein bodies were isolated and characterized (Ogawa et al., 1989).

Rice/sorghum and rice/wheat hybrids from the People's Republic of China were also rechecked for amino acid composition, particularly lysine, because rice protein is richer in lysine (3.5 to 4 percent) than sorghum (1 to 2 percent lysine and wheat (2 to 3 percent). The lysine content of four milled rice/sorghum hybrids of 3.1 to 3.6 g per 16.8 g N was closer to rice than to sorghum (IRRI, 1980). One milled sample of rice/wheat hybrid had 4.1 g lysine per 16.8 g N at 10.8 percent protein, with an SDS-polyacrylamide gel electrophoresis pattern characteristic of milled rice glutelin (IRRI, 1983a).

Studies on the biosynthesis of storage proteins in developing rice seeds (Yamagata et al., 1982) indicate that a rice glutelin and a soybean glycinin have evolved from a common ancestral gene (Higuchi and Fukazawa, 1987). Molecular biologists are studying protein biosynthesis in order to enhance the biosynthesis of glutelin, and thus to improve the nutritional quality of rice protein. An alternative approach is to suppress the biosynthesis of prolamin polypeptides that are low in lysine (see Table 17), such as the 10-kd prolamin subunit, which is probably involved in the indigestible BP-I of cooked rice.

Other mutants

Giant embryo mutants have an embryo two to three times the normal size and an increased brown rice lipid content (4 percent as compared to 2.5 percent), (Omura and Satoh, 1984). Some mutants have a thicker aleurone layer (>50 Ám as compared to the normal 30 Ám) which is seen as a possible means to increase the lipid content of the rice grain. The large embryo mutant variety Hokkai 269 has 14 percent bran as compared to 7 percent for common rice, but it has a lower oil content in the bran of 18 percent versus 21 to 22 percent for common bran (A. Nagao, personal communication, 1990).


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