The Oryza genus is thought to have originated about 14 million years ago in what is now SouthEast Asia and the Philippines. Since then, it has evolved, diversified, and dispersed into wild Oryza species which are now distributed throughout the tropics.
Two Oryza species are important cereals for human nutrition: Oryza sativa (Asian rice), grown worldwide, and Oryza glaberrima (African rice), grown in parts of West Africa. Both are cultivated plant species, which means they have developed as a result of humans growing them over many centuries, selecting the traits they consider the most important (e.g. flavor, yield and water requirements).
Asian rice was first domesticated from the wild rice Oryza rufipogon in China between 8,200 and 13,500 years ago, then spread to South and Southeast Asia. Today it is cultivated on every continent save Antarctica. Worldwide there are more than 40,000 different varieties of Oryza sativa, classified into four major categories: indica, japonica, aromatic and glutinous.
Oryza sativa contains two major subspecies: the sticky, short grained japonica or sinica,and the non-sticky, long-grained indica. Japonica varieties are usually cultivated in dry fields, in temperate East Asia, upland areas of Southeast Asia and high elevations in South Asia, while indica varieties are mainly lowland rices, grown mostly submerged, throughout tropical Asia. Recent genetic evidence show that all forms of Asian rice, both indica and japonica, come from a single domestication event that occurred 8,200–13,500 years ago in the Pearl River valley region of China.
Japonica is a group of rice varieties from northern and eastern China grown extensively in some areas of the world. It is found in the cooler zones of the subtropics and in the temperate zones.
It is a relatively short plant with narrow, dark green leaves and medium-height tillers. Japonica grains are short and round, do not shatter easily and have low amylose content, making them moist and sticky when cooked.
1.1 Javanica or tropical Japonica
Once thought to be a third subspecies, javanica is now known as tropical japonica.
Examples of this variety include the medium grain 'Tinawon' and 'Unoy' cultivars, which are grown in the high-elevation rice terraces of the Cordillera Mountains of northern Luzon, Philippines.
Javanica plants are tall with broad, stiff, light green leaves. The grains are long, broad, and thick, do not shatter easily, and have low amylose content.
Indica rice is the major type of rice grown in the tropics and subtropics, including the Philippines, India, Pakistan, Java, Sri Lanka, Indonesia, central and southern China, and in some African countries.
Indica plants are tall with broad to narrow, light green leaves. The grains are long to short, slender, somewhat flat, tend to shatter more easily and have high amylose content, making them drier and flakier when cooked than japonica varieties.
Oryza glaberrima, commonly known as African rice, is a domesticated rice species. African rice is believed to have been domesticated 2,000–3,000 years ago in the inland delta of the Upper Niger river, in what is now Mali. Its ancestor, which still grows wild in Africa, is Oryza barthii.
This species is grown in West Africa. O. glaberrima shows several characteristics that make it less suitable for cultivation compared to O. sativa, such as brittle grains and poor milling quality. African rice also has lower yields than O. sativa, but it often shows more tolerance to fluctuations in water depth, iron toxicity, infertile soils, severe climatic conditions and human neglect. It also exhibits better resistance to various pests and diseases, such as nematodes, midges, viruses and the parasitic plants Striga.
Scientists from the Africa Rice Center managed to cross-breed African rice with Asian rice varieties to produce a rice variety called NERICA, which is an acronym for New Rice for Africa.
Classification of cultivated rice species
Oryza species can be classified into 11 genomic groups labeled AA to LL, and most of the species can be grouped into four complexes of closely related species in two major sections of the genus. Just two species have no close relatives and are placed in their own sections of the genus: O. australiensis and O. brachyantha.
The Oryza genus also includes about 20 other rice species that are sometimes called wild rice. These other Oryza species should not be confused with another plant which is also called wild rice, which is actually from the genus Zizonia.