Orobanche spp

Broomrape. These species are parasitic angiosperms that lack chlorophyll. They attack a wide range of herbaceous crops and can be an agricultural nuisance. Orthotropic branches

In a plant with dimorphic branching, the orthotropic branch is the vertical stem that carries the apical meristem. This is the branch that must be used for cuttings in crops such as coffee, cotton, and black pepper. See also: Plagiotropic. Oryza sativa

Rice. There are three subspecies of Oryza sativa, called japonica, indica, and javanica. As their names imply, they are suited to temperate, subtropical and tropical regions respectively. There are many thousands of cultivars, worldwide. The most recent are the so-called 'miracle' or 'dwarf' rices of the Green Revolution which, having short straw, can take large applications of nitrogenous fertiliser without lodging.

Rice is the second most important food crop after wheat. Rice is a warm season crop cultivated in flooded fields. It is very high yielding and, in tropical areas, two or three crops can be grown each year. Rice countries are usually densely populated for this reason. Fuel is scarce in these regions, and the main objective of the 'stir-fry' method of cooking is to conserve fuel. However, the flooding provides excellent soil conservation, and most rice cultures are ancient and continue to be productive. This is in contrast to many of the ancient wheat cultures, mainly in the Middle East, which have declined or disappeared because of soil erosion.

Rice seed is usually germinated in a seedbed and transplanted when the seedlings are several inches high. The flooded fields are allowed to dry out prior to harvest. The rice is usually reaped by hand and carried to a threshing floor. Both the growing crop, and the unhusked grain, are known as paddy. In the United States, rice cultivation is fully mechanised.

After harvesting, the husks are removed, by milling and winnowing, to produce brown rice. Further milling removes the outer layers of the seed, which contain most of the proteins and vitamins. This milling produces white rice but, unlike the milling of wheat, which grinds the entire grain to flour, rice milling aims to preserve the grain. Rice is usually boiled or steamed to produce the most digestible of all foods, and it is often prescribed for invalids. But undue reliance on a diet of white rice can lead to nutrient deficiencies such as beri-beri. Rice is also fermented to produce beer and saki, and it has many other uses of less commercial importance.

The principle disease is 'Blast' cause by the fungus Piricularia oryzae. There is quantitative vertical resistance to this disease and breeding for horizontal resistance will need careful use of the one-pathotype technique. The chief insect pest in the miracle rices is the brown plant hopper, and there are vertical resistance against this parasite also. Nevertheless, this is a crop suitable for amateur breeders , and there is great need for improvements in horizontal resistance. Rice is normally self-pollinated but the use of male gametocides is feasible. However, the multiplication rate of rice is so great that relatively few hand-pollinations are necessary for recurrent mass selection .

Upland rice is grown on land that is not flooded but it still requires a high rainfall. At the opposite extreme, swamp rice is grown in flood plains and its stems can grow as rapidly as the flood rises. An inferior rice (Oryza glaberrima) originated in Africa but is generally being replaced with O. sativa. The so-called wild rice (Zizania aquatica) of North America is not related.

See also: IRRI. Osmosis

The passage of a solvent, such as water, through a semipermeable membrane, such as a cell membrane, from a less concentrated solution into a more concentrated solution. This process produces osmotic pressure, and is responsible for the turgidity of plant cells. See also: Reverse osmosis. Outbreeder

A species of plant that is allogamous (i.e., cross-pollinating). Outbreeding cereals

The outbreeding cereals are maize, sorghum, millets, and rye. See also: Inbreeding cereals. Outbreeding legumes

Most cultivated legumes are inbreeders. The outbreeding grain legumes are: pigeon pea (Cajanus cajan), broad bean (Vicia faba), and cowpea (Vigna unguiculata). The outbreeding fodder legumes are: alfalfa (Medicago sativa) and various clovers (Trifolium spp.). Outcross

The progeny of a cross-pollination.

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