Rice is a type of grass (genus Oryza) that belongs to a family of plants that includes other cereals such as wheat and corn. Rice grain is rich in nutrients, vitamins, and minerals and is the staple food for more than 3 billion people.
Rice is grown across the world, and there are many varieties of rice and different ways of cultivating them. However, all rice plants share common features and go through three main stages of growth – vegetative, reproductive, and ripening – before the seed can be harvested.
Cultivated rice is generally considered a semiaquatic annual grass, although in the tropics it can survive as a perennial, producing new tillers from nodes after harvest (ratooning).
At maturity, the rice plant has a main stem and several tillers. Each productive tiller bears a terminal flowering head or panicle. Plant height varies by variety and environmental conditions, ranging from approximately 0.4 meter (m) to more than 5 m in some floating rice. The morphology of rice is divided into the vegetative phase (including germination, seedling, and tillering stages) and the reproductive phase (including panicle initiation and heading stages).
The growth duration of the rice plant is 3–6 months, depending on the variety and the environment under which it is grown. During this time, rice completes two distinct growth phases: vegetative and reproductive. The vegetative phase is subdivided into germination, early seedling growth, and tillering; the reproductive phase is subdivided into the time before and after heading, that is, panicle exsertion. The time after heading is better known as the ripening period.
Two rice species are important cereals for human nutrition: Oryza sativa, grown worldwide, and O. glaberrima, grown in parts of West Africa. These two cultigens—species known only by cultivated plants—belong to a genus that includes about 25 other species, although the taxonomy is still a matter of research and debate.
Oryza is thought to have originated about 14 million years ago in Malesia. Since then, it has evolved, diversified, and dispersed, and wild Oryza species are now distributed throughout the tropics.