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Glossary: O

See: Avena sativa.
Obligate parasite
A parasite that is able to extract nutrients only from a living host. It cannot extract nutrients from non-living material. See also: Facultative parasite.
This is the generic name given to the conidial stage of all powdery mildews, the Erysiphales. The conidia are consistently similar throughout this family, being unbranched and producing chains of hyaline, oval conidia.
Oil insecticides
A thin film of mineral oil (e.g., kerosene) on water will kill mosquito larvae by depriving them of oxygen. This is an example of a stable insecticide which is beyond the capacity for micro-evolutionary change of the parasite. However their use is rarely environmentally acceptable.
Oil palm
See: Elaeis guineensis.
Oil seed crops
Any crop that is cultivated specifically for its seed, which has a high vegetable oil content. Temperate oil seeds include canola, sunflower, and linseed. Tropical oil seeds include oil palm, sesame, and coconut. Oil is also extracted, on an industrial basis, from other seeds, such as maize, soybean, peanut, and cotton, which are not cultivated specifically for their oil. Oil is also extracted from the fruit tissues of olives, avocado, and oil palm.
See: Abelmoschus esculentus.
Old encounter parasite
A parasite that has been in continual contact with its crop host since the earliest domestication. Wheat rust in Europe is an old encounter parasite. If the crop host is moved to a new area (e.g., from the Old World to the New), and the parasite is moved with it, as happened with wheat rust in North America, it is still an old encounter parasite. See also: New encounter, re-encounter.
Olea europaea
The olive. This crop is an excellent example of both ancient clones that demonstrate the utility and durability of horizontal resistance, and of an ancient domestication that achieved results that modern plant breeding cannot improve. However, an entirely new, modern requirement is the need for mechanical harvesting, which will necessitate fruits that ripen simultaneously, and that are easily detached. This is a task for professional breeders, and this crop is not recommended for amateur breeders.
Oligocyclic parasite
A parasite that has several, but not many, life cycles in each crop cycle, or season. See also: Polycyclic, Monocyclic.
See: Olea europaea.
A consumer of both animal and plant foods. Humans are omnivores as some two million years of hunter-gathering demonstrate. Our teeth also indicate our fundamental omnivorous nature. See also: Vegetarian, Vegan.
One-pathotype technique
A technique for ensuring that all vertical resistance are matched during the process of screening for horizontal resistance. The technique requires the designation of a single vertical pathotype of the parasite in question. All the original parents of the breeding population must be susceptible to (i.e., matched by) the designated pathotype, which is then used in all screening for resistance to that parasite, during the entire the breeding program. The designated pathotype is usually cultured on the matching designated host. See also: Saturation technique.
See: Allium cepa.
Onobrychis viciifolia
Sainfoin. This is a fodder legume that was often used in place of alfalfa, but which is now in decline from competition with improved strains of clovers and alfalfa. It may be of local limited interest to amateur breeders
On-site selection
Because the epidemiological competence of parasites varies from one agro‑ecosystem to another, the requirement for horizontal resistance, to each of these parasite, also varies. If a cultivar is to be fully adapted to its agro-ecosystem, its selection during breeding must be conducted within that agro-ecosystem.
Although this is called on-site selection, it means three things: that the selection work is conducted in the area of future cultivation, during the time of year of future cultivation, and according to the farming system of future cultivation.
The purpose of on-site selection is to achieve local optimisation of the many quantitative variables that can occur within a cultivar, including the various horizontal resistances to locally important parasites.
The microscopic spores produced by sexual fusion in many parasitic fungi belonging to the Peronosporles (downy mildews).
Most oospores are very hardy, and are formed at the end of a discontinuous epidemic. They are resistant to desiccation and cold, and they enable the fungus to survive an adverse season, such as a tropical dry season, or a temperate winter, when no host tissue is available to the parasite.
Being the result of sexual recombination, they also produce a wide diversity of vertical pathotypes at the beginning of the epidemic, when there is a wide diversity of vertical pathodemes to be matched.
Oospores should not be confused with conidia that are produced asexually.
Open pollination
See: cross-pollination.
Open-pollinated crops
This term is synonymous with cross-pollination. Open-pollinated crops can be divided into those that are obligately cross-pollinated, and those that have an optional selfpollination.
It may be generally assumed that cross-pollinated crops do not tolerate inbreeding, otherwise they would be cultivated as pure lines. However, inbreeding is often employed in order to produce hybrid varieties.
Many open-pollinated crops are cultivated as clones, because this is the only way of preserving their agriculturally valuable characteristics.
Ophiobolus graminis is the old name for Gaumanniensis graminis, the fungus that causes “Take-All” disease of cereals.
See: Papaver somniferum.
Opposite leaves, branches
A pair of leaves or branches that occur on opposite sides of each node on a stem.
Optical microscope
The light microscope, as opposed to an electron microscope.
There are two basic types of light microscope. A compound microscope has two sets of lenses; the first set is called the objective, and it determines the resolution. The second set is called the eye-piece, and it determines to final magnification.
A dissecting microscope consists of two compound microscopes so aligned that they focus on a single point. This provides stereoscopic vision. A dissecting microscope has a low magnification and it is used for delicate operations that require vision in depth.
See: Citrus spp.
A level in the taxonomic hierarchy. An order is a group of closely related families.
Any significant, macroscopic component of an organism.
The internal, microscopic organs of a single cell.
Organic chemicals
Originally, chemical substances that had been produced by living organisms were called ‘organic’ chemicals, as opposed to the ‘inorganic’ chemicals such as rocks and water, which had not been produced by living organisms.
Nowadays, the term ‘organic chemical’ refers to any carbon-based compound, including the synthetic organic chemicals. The original meaning is retained in terms such as organic and inorganic fertilisers, farming, etc.
Organic farming
In simplest terms, organic farming is a form of agriculture that avoids any use of synthetic chemicals or GMOs.
Organic agriculture is an example of a complex system that aims to maximize the biodiversity of insects and microbial life, in order to allow the local ecology to operate as a self-organizing system. This system both minimizes the impact of pests and diseases, and maximizes the nutritional content of organic food.
Organic fertilisers
Any manure that has been produced by a living organism. The term includes farmyard manure, night soil, guano, sewage solids, bone meal, dried blood, and green manure.
The term may also apply to minerals that have been approved for organic agricuture, such as various rock powders.
Organic food
Food that has been produced on an organic farm without any use of synthetic chemicals or GMOs. Recent studies have proved conclusively that organic foods are higher in nutrient content than conventional foods, as well as being free of pesticide residues, additives and preservatives.
Any living individual; the word is derived from organised.
Original parents
In a program of recurrent mass selection, the parents of the first polycross.
Horticultural crops grown for a decorative function. Ornamentals are usually cut flowers but the term also includes decorative foliage, dried flowers, etc.
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.
The flooding of rice paddies 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 beriberi.
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 resistances 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.
The passage of a solvent, such as water, through a semi-permeable 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.
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.).
The progeny of a cross-pollination.
The animal equivalent of an ovule.
The method that an organism uses for surviving a winter. See also: Aestivation.
The female cell of a plant which, when fertilised by a pollen cell, develops into an embryo.