Oryza, rice, is a genus of 24 species of annual or sometimes perennial grasses in the Poaceae (grass family) endemic in tropical and often swampy parts of Africa, Asia, and Australia, with a few species native to Central and South America. The genus includes the species of rice (Oryza sativa) that is one of the two most important cereal crops world for human consumption (the other is wheat, Triticum species; corn, Zea mays is produced in larger amounts, but a sizable portion of it is used for livestock feed and making ethanol for biofuel), and that is now cultivated in wet areas of tropical, semi-tropical, and temperate regions worldwide. Rice has also been important as a model system in plant biology, and is the first plant species for which the genome has been fully mapped.
Several other species of rice are cultivated to a much lesser extent than O. sativa. African rice, O. glaberrima, is the other species most widely cultivated as a food crop, but it is largely being replaced by O. sativa. Three other species of rice are cultivated locally in parts of Africa: Oryza barthii, a progenitor of O. glaberrima); Oryza longistaminata, a perennial with a high water requirement; and Oryza punctata, which is endemic in eastern Africa and commonly used in central Sudan. The name “wild rice” may refer to any of these species, or of the non-cultivated species of Oryza, but is generally used to refer to North American species in the genus Zizania.
Oryza grasses may grow in a tuft (clump) or spread out from rhizomes (creeping roots). They generally have upright culms (stems) up to 2 m or more tall, with long, flat leaf blades. The flowers grow on broad, open terminal panicles (branched clusters). The oblong spikelets, which each contain a single flower (that develops into a single kernel of grain), are sparse along the stem rather than forming dense clusters. The harvested kernel, known as a rice paddy, is enveloped in a hull or husk that is removed during milling.
Oryza sativa has hundreds of cultivars with different grain color, size, and shape, as well as environmental tolerances and seasonality—the types are generally categorized as valley rice, upland rice, spring rice, and summer rice. It is generally grown in fields that are flooded for part of the growing season—whether from irrigation (the majority of cultivation), rainfed or floodplain systems--which help reduces competition from other plants, among other benefits; some upland varieties can be grown without flooding, but they account for only 4% of rice cultivated worldwide.
Rice is thought to have been domesticated in India and brought to China by 3,000 B.C. It was cultivated in Babylon and the Middle East by 2,000 years ago, and spread to the Europe during medieval times. Rice is now cultivated in countries around the world, and serves as a major calorie source for as much as half the world’s population.
The FAO estimates that the total commercial harvest of rice in 2010 was 672.0 million metric tons, harvested from 153.7 million hectares—around 3% of the planet’s agricultural land. China and India were the leading producers, followed by Indonesia, Bangladesh, and Vietnam; the U.S. is ranked 10th. Within the U.S., Arkansas accounts for the largest share of rice cultivation, followed by California, Louisiana, Mississippi, Missouri, and Texas.
(Bailey et al. 1976, Ecocrop 2012, Encyclopedia Britannica 1993, FAOSTAT 2012, Flora of China 1994, Gillis 2005, Hedrick 1919, NRC 1996, Science 2002, USDA 2012, van Wyk 2005, Wikipedia 2012.)