There are three cereals which nourish the world's population: rice, wheat and maize Rice constitutes 49% of the calories consumed worldwide and is the primary food source for over a third of the world's population. Over 90% of rice is grown and consumed in Asia: home to 60% of the world's population. Rice is grown on 150 million hectares of land per year, which constitutes 11% of farmland worldwide. Cultivated rice (Oryza sativa L.) belongs to the Poaceae (Gramineae) family; subfamilies Bambusoideae and tribe Oryzeae.
This tribe is made up of 11 genera, of which, the Oryza genus is the only one with cultivated species. Oryza has two cultivated and 22 wild species. O. sativa, Asian rice, is grown worldwide, but O. glaberrima, African rice, is cultivated on a limited scale in West Africa. Both genera are thought to be an example of parallel evolution in crop plants.

The wild progenitor of O. sativa is O. rufipogon, which shows a wide range of variation between perennial and annual types. In a parallel evolutionary path, O. glaberrima was domesticated from annual O. breviligulata, which in turn evolved from perennial O. longistaminata. O. rufipogon is distributed from Pakistan to China and Indonesia and its populations vary between perennial and annual types, which differ markedly in life history traits. In short, the perennial types have higher outcrossing rates and lower seed productivity than annual types. In monsoonal Asia, the perennial types grow in deep swamps, which retain moisture throughout the year while annual types occur in temporary marshes, which are parched during dry season.

All these wild rices cross with cultivated rice under natural conditions producing hybrid swarms in the field. Domestication of wild rices probably started around 9000 years ago. This may have occurred concurrently at various sites bordering a broad belt that extends from the plains below the eastern foothills of the Himalayas in India, through Upper Myanmar, northern Thailand, Laos and Vietnam to southwest or south China. Archaeological remains dating back to between 4000 and 2500 BC have been found in each of these regions. O. glaberrima, the African cultivar, originated in the Niger River delta. The primary centre of diversity [for O.glaberrima] is the swampy basin of the Niger River and two secondary centres near to the Ivory Coast. The primary centre was formed around 1500 BC while the secondary centres were formed 500 years later.

Wild and cultivated rice differ in respect to several characteristics. Wild species are easily degrained and show high phenotypic plasticity in the development of various organs. Domestication is a gradual process guided by the forces of natural selection and entails numerous genetic changes. The morphological and physiological properties accompanying this domestication include, amongst others: synchronised tillering, reduction in seed dormancy and uniform maturation. Oryza sativa is a species that is distributed worldwide in which several varietal groups can be observed. Further to allelic frequency research carried out on varieties from distant geographical regions, 6 varietal groups have been classified: Group I corresponds to Indian varieties, group VI to Japanese varieties, Group II to short-cycle varieties and drought-tolerant varieties cultivated in Bangladesh and Western India.

The floating rice species of Bangladesh and India belong to groups III and IV. Group V includes aromatic rices cultivated on the Indian subcontinent. The dispersion of rice began in the Himalayan foothills, spreading to the north and west of India, Afghanistan, Iran and southern Sri Lanka. The cultivation of rice may have been introduced in Greece and its neighbouring countries by members of Alexander the Great's army around 324 BC. In Spain it is thought to have been introduced by Arab settlers in the Andalusian provinces during the 8th century, spreading from here to Sicily and the north of Italy, but was not grown in the region of Ribera del JĂșcar in Valencia until the year 1200.

 

Calendario de Cultivo

El arroz