Rice - biology.

Rice - biology. I INTRODUCTION Rice, plant that produces an edible grain; the name is also used for the grain itself. Rice is the primary food for half the people in the world. In many regions it is eaten with every meal and provides more calories than any other single food. According to the United Nations Food and Agricultural Organization (FAO), rice supplies an average of 889 calories per day per person in China. In contrast, rice provides an average of only 82 calories per day per person in the United States. Rice is a nutritious food, providing about 90 percent of calories from carbohydrates and as much as 13 percent of calories from protein. Of the 20 known species of rice, only two are cultivated--the widely grown Asian rice and the hardier African rice. Asian rice, if managed with modern techniques such as fertilizers, irrigation, and chemical pesticides, produces significantly more grain per plant than African rice, and for this reason is the preferred type in the majority of rice-growing countries. African rice, however, is more productive than Asian rice in traditional farming systems where modern techniques are not used or poor growing conditions are present. About 50,000 varieties exist within these two species, only a few hundred of which are cultivated. II THE RICE GRAIN AND PLANT A kernel or grain of rice is a seed that contains an embryonic rice plant. The hull, a hard protective covering, surrounds the bran, which consists of layers of fibrous tissue that contain protein, vitamins, minerals, and oil. Beneath the bran is the endosperm, which makes up most of the rice grain. The endosperm contains starch, the energy source used by the germinating seed. The bran and endosperm are the edible portions of the grain. A rice plant, a type of grass, has narrow, tapered leaves and grows from about 60 to 180 cm (about 2 to 6 ft) tall. Several flower stalks emerge from the plant, and in most varieties, a loose cluster of branching stems, called a panicle, radiates from the top of each stalk with small green flowers hanging from each stem. When the grain has developed, the panicle droops under the weight of the ripened kernels. Depending on the variety, one panicle provides about two handfuls of rice. III GROWING RICE Although rice originally flourished in the dry climate of Central Asia, it spread to the flood plains of tropical regions, resulting in evolution of varieties with the capacity to grow with roots submerged in water. The African and Asian varieties that are flooded during the growing season are more productive than the varieties that are not flooded, partly because the submerged roots easily extract needed nutrients from the water. Flooded rice, also known as lowland rice, is grown in paddies, which are fields that contain water enclosed by low walls of earth called bunds. Paddy rice fed by rainfall alone accounts for about 50 percent of all rice grown worldwide, while paddies flooded by a combination of rainfall and irrigation provide about 35 percent of rice produced. The major rice-producing countries, including China, India, and Vietnam, primarily cultivate paddy rice. Other African and Asian rice varieties, known as upland rice, are grown in regions of low rainfall that do not provide enough moisture for paddies, or in tropical rain forests where high humidity prevents the ripening of other edible grains. Upland rice is less productive than paddy rice, partly because it depends on nutrients that are dissolved in the soil moisture. When soil moisture is low, few nutrients are available, compared to the storehouse of nutrients typically found in paddy waters. Upland rice accounts for about 15 percent of world rice production, and is particularly important in Laos, where it accounts for between 20 and 22 percent of the rice harvest. Rice, grown in more than 100 countries, is particularly productive in tropical regions with abundant moisture, but it also grows successfully under widely different climate conditions. Rice farmers choose varieties adapted to the region's length of growing season, soil, altitude, and, for paddy farmers, the depth of water in the fields. Paddy rice farmers in developing countries usually sow seeds in small seedbeds, then hand-transplant the seedlings into flooded fields that have been leveled by water buffalo or oxen-drawn plows. One advantage of transplanting seedlings instead of planting from seed is that the young plants help limit weeds by shading them from needed sun. In industrialized countries, seed is sown with a planting drill or cast from an airplane into machine-leveled fields that are then flooded. Herbicides are the primary method of weed control. Depending on the rice variety and the climate, rice grains are ready for harvest in three to six months. In developing countries, farmers harvest rice with sickles or knives, tie it in bundles, and let it dry in the field. They then remove the grain from the plant, a process called threshing, by hitting the plant against a slatted screen or walking animals over it. Farmers in industrialized countries use combines, which are machines that move through fields and harvest, thresh, and clean the grains. The grain is then dried in sheds with heated air. Rice is susceptible to a range of diseases and pests, which annually destroy about 55 percent of rice crops. The most common diseases are caused by the fungi sheath blight and rice blast, and the stalk borer is a common insect pest. Weeds compete with rice for nutrients and water and are a serious problem, especially in upland rice farming. Rodents and birds also feed on rice grains before they are harvested. Disease-causing fungi, insects, and a variety of other pests infest rice during storage and transport. When rice is processed, the hull is removed, exposing the bran. Rice at this stage is brown rice. The fibrous bran of brown rice is rich in oil; protein; the B vitamins thiamin, riboflavin, and niacin; and the minerals iron, phosphorus, and potassium. To make white rice, the bran is removed. White rice is less nutritious than brown rice and, when feasible, is enriched with the addition of vitamins and minerals to increase its nutritive value. Without the tough bran layer, white rice cooks faster and stores longer than brown rice, so it is often preferred in regions where fuel is limited and refrigeration is not readily available. Polished rice is made by passing white rice kernels through a machine with a brush that smoothes and shines them. IV PRODUCTION AND USES Asian countries produced about 90 percent of the 576 million tons of rice grown worldwide in 2002. Typically, China and India together produce about 50 percent of the world's rice, and it is a significant agricultural crop in more than 50 other countries. About 96 percent of the rice grown worldwide is consumed in the countries where it is produced, with some exceptions. The United States, for example, exported about 37 percent of the 8.7 million tons it produced in 2000, and Pakistan exported about 28 percent of its 7.2 million tons, according to the FAO. In the same year, Thailand exported significantly more rice than any other country--6.6 million tons, or about 26 percent of its total, while India exported 1.5 million tons, or about 1.1 percent of total production. Major rice-importing countries include Côte d'Ivoire, Nigeria, Philippines, Iran, Saudi Arabia, Brazil, Senegal, Japan, and Indonesia. Some rice-importing countries buy rice on a regular basis, others buy when drought, floods, or other conditions reduce the yield of their own rice crop. Rice is used for a variety of food and nonfood products. Foods include cooked rice, breakfast cereals, desserts, and rice flour. Rice is also used in beer and in sake, a Japanese fermented brew. The inedible rice hull is used as fuel, fertilizer, and insulation, while the bran is a source of cooking oil. Straw from the leaves and stems is used as bedding for animals and for weaving roofs, hats, baskets, and sandals. V HISTORY According to the most widely accepted theory, rice cultivation originated as early as 10,000 early as 4000 BC, and over the centuries spread to China, Japan, and Indonesia. By 400 BC BC in Asia. Archaeological evidence shows that rice was grown in Thailand as rice was cultivated in the Middle East and Africa. The invading armies of Alexander the Great probably introduced rice to Greece and nearby Mediterranean countries around 330 BC. Rice was brought to the American colonies in the early 1600s, and commercial production began in 1685. Rice cultivation, a very demanding process, has shaped values and changed history. For example, rice encouraged populations to crowd together to take advantage of a reliable food supply. The labor-intensive process of growing paddy rice requires large numbers of people to work together to level fields, build and maintain bunds, and care for the crop. Where paddy-rice cultivation has been introduced, hard work, organization, persistence, and above all, cooperation, have been encouraged. In the United States, rice played an important role in establishing slavery in the coastal Southeast--the Carolinas, Georgia, and north Florida. For instance, rice exportation was deemed necessary for economic survival in Georgia, and as a result, slavery was legalized in that state to create a work force to clear swamps, install dikes, and plant, grow, harvest, and thresh the rice. VI RICE RESEARCH From the 1950s to 1970s, in an effort to combat world hunger, plant breeders at the International Rice Research Institute (IRRI) in the Philippines developed new rice varieties that were, when fertilized, higher yielding than traditional varieties. The new varieties were shorter and less likely to fall over, which made them easier to harvest mechanically. They also ripened sooner, reducing the risk of poor weather affecting yield, and enabling farmers to harvest and replant several times during the growing season. While successful in many areas, the new varieties required more money for fertilizer and chemical pesticides, and in some cases, machines for sowing and harvesting--tools often too costly for peasant farmers. In some areas a single new rice variety replaced diverse, centuries-old varieties adapted to thrive in a particular climate and soil type and with some resistance to local insects and diseases. The new variety was not able to thrive in these areas, and the crop yields were not always greater. Rice breeders at IRRI and other research facilities are now trying to increase yields through genetic engineering. They hope to create rice varieties that are genetically designed to require less fertilizer, resist insects and diseases, tolerate poor soil, require less irrigation, and photosynthesize more efficiently. Scientific classification: Rice is an annual grass in the grass family, Poaceae (formerly Gramineae). Asian rice is classified as Oryza sativa and African rice as Oryza glaberrima. Contributed By: Erin Hynes Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.