- Email: [email protected]
Rice
Rice RS Zeigler, International Rice Research Institute, Los Baños, Philippines Ó 2017 Elsevier Ltd. All rights reserved.
Rice: A Global Staple Rice (Oryza sativa L.) is the most important cereal food crop for people living in developing countries and is arguably the most important single human food in the world. Production is concentrated in Asia, but it has been taken up by farmers worldwide (Figure 1). Unlike other staple cereals, rice typically is consumed directly by humans and as a whole grain. Global per capita consumption has stabilized at around 65 kg annually increasing from almost 52 kg in 1960 before the beginning of what has been called the Green Revolution. The highest levels of consumption are concentrated in Asia, particularly
in South and Southeast Asia (Table 1). This coincides with the world’s greatest population concentrations and with the largest numbers of poor people (Figure 1). Consumption rates declined in East Asian countries, such as Japan and Korea, as their economies rapidly expanded; however, they remain high across Southeast Asia and expected large declines in China have failed to occur, despite its rapidly growing economy. Consumption is growing rapidly in sub-Saharan Africa albeit from a low baseline. It is difficult to overstate the cultural significance of rice in Asia. Its origins are interwoven within virtually all creation stories of Asian cultures, and rice forms an important part of
Figure 1 (a) Global distribution of rice related to poverty and as irrigated (Source: International Rice Research Institute). (b) Global distribution of rice related to poverty and as rainfed (Source: International Rice Research Institute).
Encyclopedia of Applied Plant Sciences, 2nd edition, Volume 3
http://dx.doi.org/10.1016/B978-0-12-394807-6.00211-2
45
46
Arable Crops j Rice
Table 1 Area, yield, and per capita consumption – 1960, 1985, 2010. (United States Department of Agriculture (USDA), Production, Supply, and Distribution (PSD) database.) 1960
Country/region East Asia China Japan Korea South and Southeast Asia South Asia Southeast Asia India Bangladesh Myanmar Thailand Vietnam Indonesia All of Asia Africa South America World
1985
2010
Area (103 ha)
Yield (t ha1)
Kilogram per capita
Area (103 ha)
Yield (t ha1)
Kilogram per capita
Area (103 ha)
Yield (t ha1)
Kilogram per capita
37 209 31 500 3308 1621 74 144
2.26 1.9 4.86 3.51 1.61
81.93 71.97 128.7 113.5 97.89
36 858 32 070 2342 1882 92 196
5.34 5.26 6.22 5.82 2.5
105 106.3 84.59 133.7 99.42
33 232 29 873 1643 1472 105 960
6.5 6.55 6.51 5.62 3.6
96.28 100.7 65.28 93.53 99.84
46 053 28 091 34 128 8857 4055 5643 4602 7285 111 353 2769 3866 120 138
1.55 1.71 1.52 1.64 1.69 1.68 1.99 2.05 1.83 1.33 1.82 1.84
82.64 140.3 78.89 209.1 125.2 171.5 177.2 127.6 90.01 12.02 30 51.73
55 747 36 449 41 137 10 403 4660 9833 5825 9896 129 054 5100 7027 144 728
2.31 2.79 2.33 2.17 2.47 2.06 2.74 4.01 3.31 1.76 2.13 3.23
78.57 154.4 79.38 163.4 162.8 165.7 175.7 162 102 15.29 35.18 63.22
59 728 46 232 42 860 11 700 7050 10 667 7607 12 075 139 192 10 216 5029 158 423
3.47 3.78 3.36 4.06 2.45 2.88 5.55 4.67 4.3 2.52 5.06 4.24
76.97 165.1 73.28 213.7 195.2 154.4 219.6 157.5 98.39 24.31 36.33 63.97
Source: International Rice Research Institute.
many milestone rituals and plays a central role in daily life. Societal demands of rice cultivation even shape cultural values. Thus, even as other foods become increasingly available via global trade, we will continue to see strong demand in Asia for the foreseeable future. Rice yields worldwide have been increasing since the 1960s. A concerted effort to improve the yield of rice-based systems through public sector investment in research, infrastructure, and farmer education resulted in a ‘Green Revolution’ in rice beginning around 1970. Average yields in Asia increased from around 1.8 t ha1 to over 4.2 t ha1. This remarkable achievement contributed to explosive growth of Asian economies.
The Rice Plant The specifics of rice domestication events remain topics of lively scientific debate. But, there is consensus that it was domesticated beginning possibly as early as 10 000–13 000 BCE from Oryza rufipogon Griffiths, a wild species common across tropical and subtropical Asia. While rice is mostly self-pollinated, a significant amount of outcrossing (2% or more) occurs, and there is good reason to believe that crossing between domesticated rice and its wild progenitor continues today. The early dissemination of rice across Asia led to extraordinary adaptation to local environments, with flowering triggered by day lengths assuring that the crop would receive adequate rainfall in the largely monsoonal environments. Communities also selected for a very wide range of culinary preferences, such as chewing texture, digestibility, and fragrance. Systematic efforts to collect the full genetic diversity by the International Rice Research Institute resulted in today’s collection of over 124 000 distinct accessions in its International Rice Gene Bank.
A distinct domestication process took place in sub-Saharan Africa that yielded an African rice, Oryza glaberrima Steud. Its production had been restricted to Sahelian regions of the continent and has largely been replaced by O. sativa introductions from Asia over the last 500 years. Analyses of similarities among their genomes suggest that all major cereals grown for food today share a common ancestor. Among cultivated grasses O. sativa has the smallest genome (400 mega base pairs (Mbp), 2n ¼ 24 chromosome count) relative to other major cereals (e.g., maize 2500 Mbp, 2n ¼ 20; wheat 16 000 Mbp, 2n ¼ 42). It was the first major food crop to have its genome fully sequenced, and it has served as a reference genome for the others. There is some variation in genome size among the subgroups of rice. Rice is a semiaquatic, weakly perennial grass (Figure 2). The grain is produced in single florets formed on an open, branched terminal panicle carrying many florets. Each plant produces one main stem and multiple secondary stems, called tillers, each of which can produce a panicle under optimal conditions. The achievements of the Green Revolution were built on the creation of a high yielding plant type for rice. Traditional varieties had a high ratio of straw: grain skewed heavily towardstraw. Under optimum conditions modern varieties can produce more grain than straw. This was achieved by genetically reducing the plant’s height (creating a ‘semidwarf’ rice plant) such that additional growth stimulated by, for example, fertilizer inducing the plant to produce more grains rather than leaves and stems. Other aspects of research-driven Green Revolution included shortening the growing season and decoupling grain production from day length which combined to allow farmers to grow two to three crops per year, regardless of planting date. Work continues to develop plant types amenable to changing production environments.
Arable Crops j Rice
47
Figure 2 (a) Illustration of rice plant showing general plant type of traditional variety compared to modern semidwarf plant types. (b) Drawing of rice plant showing roots, leaves, main and secondary tillers (stem), and panicle with grains. (c) Cross section of stem and roots showing aerenchyma (Courtesy of Amelia Henry). (d) Drawing of rice grain showing hull, pericarp and endosperm, and embryo. Reproduced from International Rice Research Institute.
The edible part of the grain, the endosperm plus the pericarp, is enclosed in an inedible, silica-rich husk which must be removed by milling before consumption. The milled, unpolished rice is called ‘brown rice,’ and polishing removes the oil- and protein-rich endosperm to yield white rice. The resulting bran is often used as animal feed or for other industrial processes such as oil extraction. Consumers generally prefer white rice to brown rice because it takes much less time to cook, it is easier to digest, especially for very young children whose digestive tract may be seriously irritated by the fiber in brown rice, and, perhaps most importantly, it has a much longer shelf life. Milled brown rice may be inedible after a relatively short time as the oils in the pericarp oxidize and become rancid.
How Rice Is Grown Rice was domesticated in monsoonal environments of Asia where torrential rainfall accompanied by flooding in flat or low-lying areas is the norm during a large part of the year.
Its ability to withstand saturated to flooded soils, while producing its maximum yield, is in stark contrast to almost all other major crop species. It is thus uniquely adapted to Asian environments, particularly river deltas, where Asian societies first established themselves and populations continue to be concentrated. Its ability to withstand the very-low-oxygen environment in its root zone is due to its unique stem and root structure (aerenchyma, Figure 2) that transports oxygen-rich gas from above-ground leaves to the root system. Rice is grown from 50 N (Japan) to 35 S (Chile) latitude, from sea level to 3000 m in mountainous regions of Nepal and as a rainfed or irrigated crop (Figure 1). Optimum temperatures for rice production are between 25 and 35 C, while temperatures below 18 C or exceeding 35 C during flowering can cause almost complete sterility. It can be cultivated as a rainfed, dry land cereal, much like wheat or maize; but it is most productive when there is standing water during most of the growing season. By area, about half the world’s rice is grown as a rainfed crop, but 75% of global production comes from irrigated rice.
48
Arable Crops j Rice
The most widespread production practice has been for farmers to plow water-saturated fields to produce a mud slurry. Seedlings raised in special beds for two or more weeks are then transplanted, usually by hand, directly into the mud at a spacing that will remain until harvest. Farmers with access to drainage and additional irrigation water will allow the fields to drain for the seedlings to become established, then reapply flood water until shortly before harvest time. Nitrogen fertilizer is generally applied several periods during the crop growth, while phosphorus and potassium, if applied at all, are included in the first applications. Wet plowing and transplanting seedlings are almost certainly developed as an effective way to control weeds. Weeds in the tropics are extremely difficult to control, but almost none that commonly infest rice fields produce seeds that can germinate underwater. Wet plowing kills weed seedlings that have already begun to establish themselves, and the flooded soil into which the rice seedlings are transplanted prevents a new flush of weed seeds from germinating. If weed seeds do eventually germinate, they cannot establish themselves before the rice plants’ leaves coalesce to close the canopy and shade out their competitors. The anaerobic chemistry of rice soils also improves the availability of a number of nutrients for the rice plant. Farmers, whether or not they have access to reliable drainage and irrigation water, will take measures to keep standing water in their fields. Almost all rice fields are, therefore, surrounded by some kind of small dike, or bund. Leveling fields as best they can improves the uniformity and beneficial effects of irrigation water. Where farmers must depend solely on rainfall for irrigation, yields are typically much lower because of water stress and low fertilizer input. Up until late in the last century many farmers were isolated from markets and could only obtain their rice by growing it under dryland conditions (‘upland rice’). As markets penetrated remote areas, farmers increasingly purchased rice to meet most of their needs and have continued to grow their old rice varieties only for ceremonial uses. Thus millions of hectares of upland rice, particularly on hillsides in mountainous regions, have been abandoned over the last 50 years. Coastal areas, river deltas, and inland river valleys remain as the most important rice-growing topographies. These are areas with very high population densities in Asia. In many very poor areas, particularly eastern India, Bangladesh, Myanmar, Laos, and Cambodia, farmers are not able to control their water. They frequently suffer droughts and catastrophic flooding. Rice, while tolerant of a shallow layer of water in the fields, dies when it is completely submerged for only a few days. So flood-prone areas of Asia suffer frequent severe losses to their rice crops. In a cruel twist, regions that are prone to flooding also may experience droughts in the same year. Because of the extremely risky nature of farming in these areas farmers are unwilling to invest in modern varieties or fertilizers. The great inland valleys of sub-Saharan Africa offer significant production potential but are plagued by similar water control problems as well as very poor infrastructure and market access. The revolution in plant molecular biology and genetics – exemplified by the sequencing of the rice genome – bodes well for the future of rice production by poor farmers living in marginal water – variable environments. Rice breeders
working closely with physiologists and geneticists have recently developed rice varieties that are tolerant of extreme flooding and drought events. Areas that were prone to near complete crop loss now support rice crops that will give a very good yield even if exposed to one or more serious floods. Similar success has been achieved for drought, and varieties are nearly complete that carry tolerance to drought and flooding combined. Thus areas that for millennia have been only marginally productive are poised to become reliably highly productive rice granaries for the world. The stresses that routinely confront rainfed rice – floods, drought, and extreme temperatures – mimic those that are expected to become commonplace for most rice production systems as global climates change. Thus, advances in developing stress tolerance for rainfed rice will be relevant for future global rice systems.
Trade For such an important crop rice is rather thinly traded internationally. In 2011–15 on average, 8.6% of global rice production entered formal international trade, compared to 12% and 22% for maize and wheat, respectively. Governments feel compelled to make sure they meet their internal demand with internal production before allowing rice to be exported. Certainly that has been explicit rice policy for a number of key countries. However, urban areas are encroaching on the best rice lands, and water previously used for rice production is being diverted to urban and industrial uses. And, as labor moves out of agriculture, there will be increasing pressure to access rice needs on the international markets. Rice will continue to be a staple food for the world for the foreseeable future. It will remain to be important in Asia for at least two reasons. First its cultural significance, and therefore its political importance, will persist for at least several generations; second, the environment and geography dictate that on existing rice lands no other crop can be grown profitably for roughly half the year. However, the way rice is grown will change dramatically. Heretofore labor-intensive rice production will give way to increasing mechanization. Today’s small land holdings averaging less than 2 ha in Asia will almost certainly be merged into larger management units. Most importantly some large rice-producing countries that have been selfsufficient as a matter of policy will move toward importing more rice. Combined with increasing demand from subSaharan Africa, this will have dramatic impact on the global rice trade. The extraordinary growth of rice yields and production over the last 50 years has been the fruit of public sector investments in research to develop new plant types and in the infrastructure, input supplies, and farmer training needed for the potential of the new varieties to be fully expressed. The public sector has been scaling back its investments in agriculture for almost two decades as the private sector sees more opportunity in developing country markets such as rice seed and marketing. It is likely that the world will see major transformations in the rice sector in the coming decades. First will be a much more aggressive engagement of the private sector in the
Arable Crops j Rice research and seed sector. Second, rice will become a much more widely traded commodity. From under 7% of global production just a few years ago, global trade will reach the levels of maize in a relatively short time. This will probably be accompanied by short-term disruptions in supply and demand with associated price fluctuations.
Further Reading Civán, P., et al., 2015. Three geographically separate domestications of Asian rice. Nat. Plants 1, 15164. http://dx.doi.org/10.1038/NPLANTS.2015.164. Gross, B., Zhao, Z., 2014. Archaeological and genetic insights into the origins of domesticated rice. Proc. Natl. Acad. Sci. U.S.A. 111 (17), 6190–6197. http:// dx.doi.org/10.1073/pnas.1308942110. International Rice Genome Sequencing Project, 2005. The map-based sequence of the rice genome. Nature 436, 793–800.
49
Ismail, A.M., Singh, U.S., Dar, M.H., Mackill, D.J., 2013. The contribution of submergence-tolerant (Sub1) rice varieties to food security in flood-prone rainfed lowland areas in Asia. Field Crops Res. 152, 83–93. Li, J.Y., Wang, J., Zeigler, R.S., 2014. The 3,000 rice genomes project: new opportunities and challenges for future rice research. GigaScience 3, 8. Talhelm, T., Zhang, X., Oishi, S., Shimin, C., Duan, D., Lan, X., Kitayama, S., 2014. Large-scale psychological differences within China explained by rice versus wheat agriculture. Science 244, 603–608.
Relevant Websites http://www.knowledgebank.irri.org/ – Further Information on Rice Crop (last accessed on 29.05.16.). http://irri.org/our-work/seeds – Further Information on Rice Genetic Resources (last accessed on 29.05.16.). http://irri.org/resources/publications/books/rice-almanac-4th-edition – Further Information on Rice Economy (last accessed on 29.05.16.).
Rice: A Global Staple Rice (Oryza sativa L.) is the most important cereal food crop for people living in developing countries and is arguably the most important single human food in the world. Production is concentrated in Asia, but it has been taken up by farmers worldwide (Figure 1). Unlike other staple cereals, rice typically is consumed directly by humans and as a whole grain. Global per capita consumption has stabilized at around 65 kg annually increasing from almost 52 kg in 1960 before the beginning of what has been called the Green Revolution. The highest levels of consumption are concentrated in Asia, particularly
in South and Southeast Asia (Table 1). This coincides with the world’s greatest population concentrations and with the largest numbers of poor people (Figure 1). Consumption rates declined in East Asian countries, such as Japan and Korea, as their economies rapidly expanded; however, they remain high across Southeast Asia and expected large declines in China have failed to occur, despite its rapidly growing economy. Consumption is growing rapidly in sub-Saharan Africa albeit from a low baseline. It is difficult to overstate the cultural significance of rice in Asia. Its origins are interwoven within virtually all creation stories of Asian cultures, and rice forms an important part of
Figure 1 (a) Global distribution of rice related to poverty and as irrigated (Source: International Rice Research Institute). (b) Global distribution of rice related to poverty and as rainfed (Source: International Rice Research Institute).
Encyclopedia of Applied Plant Sciences, 2nd edition, Volume 3
http://dx.doi.org/10.1016/B978-0-12-394807-6.00211-2
45
46
Arable Crops j Rice
Table 1 Area, yield, and per capita consumption – 1960, 1985, 2010. (United States Department of Agriculture (USDA), Production, Supply, and Distribution (PSD) database.) 1960
Country/region East Asia China Japan Korea South and Southeast Asia South Asia Southeast Asia India Bangladesh Myanmar Thailand Vietnam Indonesia All of Asia Africa South America World
1985
2010
Area (103 ha)
Yield (t ha1)
Kilogram per capita
Area (103 ha)
Yield (t ha1)
Kilogram per capita
Area (103 ha)
Yield (t ha1)
Kilogram per capita
37 209 31 500 3308 1621 74 144
2.26 1.9 4.86 3.51 1.61
81.93 71.97 128.7 113.5 97.89
36 858 32 070 2342 1882 92 196
5.34 5.26 6.22 5.82 2.5
105 106.3 84.59 133.7 99.42
33 232 29 873 1643 1472 105 960
6.5 6.55 6.51 5.62 3.6
96.28 100.7 65.28 93.53 99.84
46 053 28 091 34 128 8857 4055 5643 4602 7285 111 353 2769 3866 120 138
1.55 1.71 1.52 1.64 1.69 1.68 1.99 2.05 1.83 1.33 1.82 1.84
82.64 140.3 78.89 209.1 125.2 171.5 177.2 127.6 90.01 12.02 30 51.73
55 747 36 449 41 137 10 403 4660 9833 5825 9896 129 054 5100 7027 144 728
2.31 2.79 2.33 2.17 2.47 2.06 2.74 4.01 3.31 1.76 2.13 3.23
78.57 154.4 79.38 163.4 162.8 165.7 175.7 162 102 15.29 35.18 63.22
59 728 46 232 42 860 11 700 7050 10 667 7607 12 075 139 192 10 216 5029 158 423
3.47 3.78 3.36 4.06 2.45 2.88 5.55 4.67 4.3 2.52 5.06 4.24
76.97 165.1 73.28 213.7 195.2 154.4 219.6 157.5 98.39 24.31 36.33 63.97
Source: International Rice Research Institute.
many milestone rituals and plays a central role in daily life. Societal demands of rice cultivation even shape cultural values. Thus, even as other foods become increasingly available via global trade, we will continue to see strong demand in Asia for the foreseeable future. Rice yields worldwide have been increasing since the 1960s. A concerted effort to improve the yield of rice-based systems through public sector investment in research, infrastructure, and farmer education resulted in a ‘Green Revolution’ in rice beginning around 1970. Average yields in Asia increased from around 1.8 t ha1 to over 4.2 t ha1. This remarkable achievement contributed to explosive growth of Asian economies.
The Rice Plant The specifics of rice domestication events remain topics of lively scientific debate. But, there is consensus that it was domesticated beginning possibly as early as 10 000–13 000 BCE from Oryza rufipogon Griffiths, a wild species common across tropical and subtropical Asia. While rice is mostly self-pollinated, a significant amount of outcrossing (2% or more) occurs, and there is good reason to believe that crossing between domesticated rice and its wild progenitor continues today. The early dissemination of rice across Asia led to extraordinary adaptation to local environments, with flowering triggered by day lengths assuring that the crop would receive adequate rainfall in the largely monsoonal environments. Communities also selected for a very wide range of culinary preferences, such as chewing texture, digestibility, and fragrance. Systematic efforts to collect the full genetic diversity by the International Rice Research Institute resulted in today’s collection of over 124 000 distinct accessions in its International Rice Gene Bank.
A distinct domestication process took place in sub-Saharan Africa that yielded an African rice, Oryza glaberrima Steud. Its production had been restricted to Sahelian regions of the continent and has largely been replaced by O. sativa introductions from Asia over the last 500 years. Analyses of similarities among their genomes suggest that all major cereals grown for food today share a common ancestor. Among cultivated grasses O. sativa has the smallest genome (400 mega base pairs (Mbp), 2n ¼ 24 chromosome count) relative to other major cereals (e.g., maize 2500 Mbp, 2n ¼ 20; wheat 16 000 Mbp, 2n ¼ 42). It was the first major food crop to have its genome fully sequenced, and it has served as a reference genome for the others. There is some variation in genome size among the subgroups of rice. Rice is a semiaquatic, weakly perennial grass (Figure 2). The grain is produced in single florets formed on an open, branched terminal panicle carrying many florets. Each plant produces one main stem and multiple secondary stems, called tillers, each of which can produce a panicle under optimal conditions. The achievements of the Green Revolution were built on the creation of a high yielding plant type for rice. Traditional varieties had a high ratio of straw: grain skewed heavily towardstraw. Under optimum conditions modern varieties can produce more grain than straw. This was achieved by genetically reducing the plant’s height (creating a ‘semidwarf’ rice plant) such that additional growth stimulated by, for example, fertilizer inducing the plant to produce more grains rather than leaves and stems. Other aspects of research-driven Green Revolution included shortening the growing season and decoupling grain production from day length which combined to allow farmers to grow two to three crops per year, regardless of planting date. Work continues to develop plant types amenable to changing production environments.
Arable Crops j Rice
47
Figure 2 (a) Illustration of rice plant showing general plant type of traditional variety compared to modern semidwarf plant types. (b) Drawing of rice plant showing roots, leaves, main and secondary tillers (stem), and panicle with grains. (c) Cross section of stem and roots showing aerenchyma (Courtesy of Amelia Henry). (d) Drawing of rice grain showing hull, pericarp and endosperm, and embryo. Reproduced from International Rice Research Institute.
The edible part of the grain, the endosperm plus the pericarp, is enclosed in an inedible, silica-rich husk which must be removed by milling before consumption. The milled, unpolished rice is called ‘brown rice,’ and polishing removes the oil- and protein-rich endosperm to yield white rice. The resulting bran is often used as animal feed or for other industrial processes such as oil extraction. Consumers generally prefer white rice to brown rice because it takes much less time to cook, it is easier to digest, especially for very young children whose digestive tract may be seriously irritated by the fiber in brown rice, and, perhaps most importantly, it has a much longer shelf life. Milled brown rice may be inedible after a relatively short time as the oils in the pericarp oxidize and become rancid.
How Rice Is Grown Rice was domesticated in monsoonal environments of Asia where torrential rainfall accompanied by flooding in flat or low-lying areas is the norm during a large part of the year.
Its ability to withstand saturated to flooded soils, while producing its maximum yield, is in stark contrast to almost all other major crop species. It is thus uniquely adapted to Asian environments, particularly river deltas, where Asian societies first established themselves and populations continue to be concentrated. Its ability to withstand the very-low-oxygen environment in its root zone is due to its unique stem and root structure (aerenchyma, Figure 2) that transports oxygen-rich gas from above-ground leaves to the root system. Rice is grown from 50 N (Japan) to 35 S (Chile) latitude, from sea level to 3000 m in mountainous regions of Nepal and as a rainfed or irrigated crop (Figure 1). Optimum temperatures for rice production are between 25 and 35 C, while temperatures below 18 C or exceeding 35 C during flowering can cause almost complete sterility. It can be cultivated as a rainfed, dry land cereal, much like wheat or maize; but it is most productive when there is standing water during most of the growing season. By area, about half the world’s rice is grown as a rainfed crop, but 75% of global production comes from irrigated rice.
48
Arable Crops j Rice
The most widespread production practice has been for farmers to plow water-saturated fields to produce a mud slurry. Seedlings raised in special beds for two or more weeks are then transplanted, usually by hand, directly into the mud at a spacing that will remain until harvest. Farmers with access to drainage and additional irrigation water will allow the fields to drain for the seedlings to become established, then reapply flood water until shortly before harvest time. Nitrogen fertilizer is generally applied several periods during the crop growth, while phosphorus and potassium, if applied at all, are included in the first applications. Wet plowing and transplanting seedlings are almost certainly developed as an effective way to control weeds. Weeds in the tropics are extremely difficult to control, but almost none that commonly infest rice fields produce seeds that can germinate underwater. Wet plowing kills weed seedlings that have already begun to establish themselves, and the flooded soil into which the rice seedlings are transplanted prevents a new flush of weed seeds from germinating. If weed seeds do eventually germinate, they cannot establish themselves before the rice plants’ leaves coalesce to close the canopy and shade out their competitors. The anaerobic chemistry of rice soils also improves the availability of a number of nutrients for the rice plant. Farmers, whether or not they have access to reliable drainage and irrigation water, will take measures to keep standing water in their fields. Almost all rice fields are, therefore, surrounded by some kind of small dike, or bund. Leveling fields as best they can improves the uniformity and beneficial effects of irrigation water. Where farmers must depend solely on rainfall for irrigation, yields are typically much lower because of water stress and low fertilizer input. Up until late in the last century many farmers were isolated from markets and could only obtain their rice by growing it under dryland conditions (‘upland rice’). As markets penetrated remote areas, farmers increasingly purchased rice to meet most of their needs and have continued to grow their old rice varieties only for ceremonial uses. Thus millions of hectares of upland rice, particularly on hillsides in mountainous regions, have been abandoned over the last 50 years. Coastal areas, river deltas, and inland river valleys remain as the most important rice-growing topographies. These are areas with very high population densities in Asia. In many very poor areas, particularly eastern India, Bangladesh, Myanmar, Laos, and Cambodia, farmers are not able to control their water. They frequently suffer droughts and catastrophic flooding. Rice, while tolerant of a shallow layer of water in the fields, dies when it is completely submerged for only a few days. So flood-prone areas of Asia suffer frequent severe losses to their rice crops. In a cruel twist, regions that are prone to flooding also may experience droughts in the same year. Because of the extremely risky nature of farming in these areas farmers are unwilling to invest in modern varieties or fertilizers. The great inland valleys of sub-Saharan Africa offer significant production potential but are plagued by similar water control problems as well as very poor infrastructure and market access. The revolution in plant molecular biology and genetics – exemplified by the sequencing of the rice genome – bodes well for the future of rice production by poor farmers living in marginal water – variable environments. Rice breeders
working closely with physiologists and geneticists have recently developed rice varieties that are tolerant of extreme flooding and drought events. Areas that were prone to near complete crop loss now support rice crops that will give a very good yield even if exposed to one or more serious floods. Similar success has been achieved for drought, and varieties are nearly complete that carry tolerance to drought and flooding combined. Thus areas that for millennia have been only marginally productive are poised to become reliably highly productive rice granaries for the world. The stresses that routinely confront rainfed rice – floods, drought, and extreme temperatures – mimic those that are expected to become commonplace for most rice production systems as global climates change. Thus, advances in developing stress tolerance for rainfed rice will be relevant for future global rice systems.
Trade For such an important crop rice is rather thinly traded internationally. In 2011–15 on average, 8.6% of global rice production entered formal international trade, compared to 12% and 22% for maize and wheat, respectively. Governments feel compelled to make sure they meet their internal demand with internal production before allowing rice to be exported. Certainly that has been explicit rice policy for a number of key countries. However, urban areas are encroaching on the best rice lands, and water previously used for rice production is being diverted to urban and industrial uses. And, as labor moves out of agriculture, there will be increasing pressure to access rice needs on the international markets. Rice will continue to be a staple food for the world for the foreseeable future. It will remain to be important in Asia for at least two reasons. First its cultural significance, and therefore its political importance, will persist for at least several generations; second, the environment and geography dictate that on existing rice lands no other crop can be grown profitably for roughly half the year. However, the way rice is grown will change dramatically. Heretofore labor-intensive rice production will give way to increasing mechanization. Today’s small land holdings averaging less than 2 ha in Asia will almost certainly be merged into larger management units. Most importantly some large rice-producing countries that have been selfsufficient as a matter of policy will move toward importing more rice. Combined with increasing demand from subSaharan Africa, this will have dramatic impact on the global rice trade. The extraordinary growth of rice yields and production over the last 50 years has been the fruit of public sector investments in research to develop new plant types and in the infrastructure, input supplies, and farmer training needed for the potential of the new varieties to be fully expressed. The public sector has been scaling back its investments in agriculture for almost two decades as the private sector sees more opportunity in developing country markets such as rice seed and marketing. It is likely that the world will see major transformations in the rice sector in the coming decades. First will be a much more aggressive engagement of the private sector in the
Arable Crops j Rice research and seed sector. Second, rice will become a much more widely traded commodity. From under 7% of global production just a few years ago, global trade will reach the levels of maize in a relatively short time. This will probably be accompanied by short-term disruptions in supply and demand with associated price fluctuations.
Further Reading Civán, P., et al., 2015. Three geographically separate domestications of Asian rice. Nat. Plants 1, 15164. http://dx.doi.org/10.1038/NPLANTS.2015.164. Gross, B., Zhao, Z., 2014. Archaeological and genetic insights into the origins of domesticated rice. Proc. Natl. Acad. Sci. U.S.A. 111 (17), 6190–6197. http:// dx.doi.org/10.1073/pnas.1308942110. International Rice Genome Sequencing Project, 2005. The map-based sequence of the rice genome. Nature 436, 793–800.
49
Ismail, A.M., Singh, U.S., Dar, M.H., Mackill, D.J., 2013. The contribution of submergence-tolerant (Sub1) rice varieties to food security in flood-prone rainfed lowland areas in Asia. Field Crops Res. 152, 83–93. Li, J.Y., Wang, J., Zeigler, R.S., 2014. The 3,000 rice genomes project: new opportunities and challenges for future rice research. GigaScience 3, 8. Talhelm, T., Zhang, X., Oishi, S., Shimin, C., Duan, D., Lan, X., Kitayama, S., 2014. Large-scale psychological differences within China explained by rice versus wheat agriculture. Science 244, 603–608.
Relevant Websites http://www.knowledgebank.irri.org/ – Further Information on Rice Crop (last accessed on 29.05.16.). http://irri.org/our-work/seeds – Further Information on Rice Genetic Resources (last accessed on 29.05.16.). http://irri.org/resources/publications/books/rice-almanac-4th-edition – Further Information on Rice Economy (last accessed on 29.05.16.).