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Glossary: Q-R

Qualitative variation
Genetic variation in which a character shows differences in kind. The character is either present or absent, with no intermediates. This variation is typical of Mendelian genetics. The term ‘discontinuous variation’ is synonymous. See also: Quantitative variation, Continuous variation.
Quality of crop product
This is one of the four main objectives in plant breeding, the others being yield, agronomic suitability, and resistance to parasites.
Quantitative variation
Genetic variation in which a character shows differences in degree. The character can be present at any level between a minimum and a maximum. This variation is typical of biometrical genetics. The term ‘continuous variation’ is synonymous. See also: Qualitative variation.
Quantitative vertical resistance
Quantitative vertical resistance is confusing because its inheritance is qualitative while its effects are quantitative. It can easily be confused with horizontal resistance, and the best way to avoid it in a breeding program is by choosing only parents that exhibit the normal, qualitative, vertical resistance.
Fortunately, quantitative vertical resistance is rare, and occurs mainly in the small grain cereals, such as wheat and barley. Vertical resistance to wheat Hessian fly (Mayetiola destructor), and rice blast, are examples.
Quarantine
See: Plant quarantine.
Queensland arrowroot
See: Canna edulis.
Quinine
See: Cinchona spp.
Quinoa
See: Chenopodium quinoa.
Race
Vertical pathotypes are often called physiologic races, or pathologic races, and some plant pathologists still use these antiquated terms.
Race-non-specific resistance
This clumsy term is often used as a synonym for horizontal resistance. Note that ‘non-race-specific resistance’ is meaningless.
Race-specific resistance
This term is closely synonymous with vertical resistance.
Radish
See: Raphanus sativus.
Ragi
See: Eleusine coracana.
Random polycross
A polycross in which the pollination is random. This is possible with allogamous species, and with an autogamous species, which responds to a male gametocide, or which has an easily controlled genetic male sterility. The advantage of a random polycross is that it produces very large numbers of crosses with very little labour. The disadvantage is that there is no control over the pollination and some parents may be more widely represented than others.
Ramie
See: Boehmeria nivea.
Rape seed
See: Brassica campestris.
Raphanus sativus
The radish. There are four basic types, and all of them belong to this one species. The small radish is the temperate zone garden vegetable, grown commercially on quite a large scale. The large radish is popular in the Far East. Mougri-radish is grown in Southeast Asia, solely for its leaves and young seed pods, as it has no fleshy root. Fodder-radish is similar to Mougri. Suitable for amateur breeders.
Rapid multiplication
With some crop species, it is possible to use a rapid multiplication technique in order to accelerate production of a new cultivar. For example, green cuttings of a potato clone can be rooted in a mist propagator and, when planted out, the cuttings will themselves produce more cuttings.
Raspberry
See: Rubus spp.
Ratio
The relation between two similar numbers determined by the number of times one contains the other.
Ratoon
When a sugarcane crop is harvested, the root systems can be left to sprout new canes, which are known as ratoons. Several successive ratoon crops may be taken from one field, and they are known as first ratoon, second ratoon, etc. However, the yields of the successive ratoons gradually decline, and there is a limit to the number of ratoons that can be commercially viable. The first crop to be harvested, before any ratoons are taken, is known as the ‘plant crop’.
Recessive character
A genetic character, or an allele, is described as recessive when it is eclipsed by the dominant allele.
Reciprocal cross
A second cross, which is similar to the first except that the sexes of the parents are interchanged. For example (A♀ x B♂) is the reciprocal cross of (A♂ x B♀).
Recombination
Sexual recombination occurs at the time of fertilisation and the mixing of the alleles of the male and female gametes,
Recurrent mass selection
The breeding method of the Biometricians, designed to increase the levels of desirable qualities -- which are quantitative variables -- by changing the frequency of polygenes.
In each screening generation, the best individuals are selected, and they become the parents of the next screening generation. This process is repeated for as many generations as necessary, but the rate of progress declines dramatically after a few generations.
See also: early selection, Family selection, Late selection, Pedigree breeding, population breeding, recurrent mass selection.
Red gram
See: Cajanus cajan.
Reducers
Evolution has produced three basic types of organism called producers, reducers, and consumers. Reducers break down the organic chemicals of dead organisms, and they make these nutrients available for re-use by other organisms.
Reduction division
See: Meiosis.
Reductionism
In science, reductionism has two quite distinct meanings. The first meaning concerns the search for basic fundamentals, and this is good science. The second means working at the lower systems levels, and this is the opposite of the holistic approach. It is called the merological approach, which can be very dangerous because it leads so easily to suboptimisation.
Redwood
Redwood trees are evergreens that live for two millennia or more and they are a good example of the durability of horizontal resistance. Having continuous pathosystems, they may be assumed to have no vertical resistances.
Re-encounter parasite
When a crop host is taken to another part of the world, some of its parasites may be left behind in the area of origin, as happened with tropical rust, when maize was taken from the New World to Africa.
If the parasite arrives in the new area at a later date, it is described as a re‑encounter parasite. A re‑encounter parasite is usually very damaging because the crop host tends to lose horizontal resistance during the absence of that parasite.
See also: Old encounter, New encounter.
Relative measurements
Horizontal resistance can be measured only in terms of the level of parasitism. Because this level is affected by so many other factors, it is impossible to devise an absolute scale of measurement of horizontal resistance. Consequently, we can measure horizontal resistance only in terms of its relation to the level in other cultivar of known field performance. That is, we can say that cultivar ‘A’ has more resistance to a given parasite than cultivar ‘B’. But we are unable to develop a scale of resistance similar to the Celsius scale of temperature.
Renfrew hypothesis
This is the hypothesis that the proto-Indo-European language (PIE) spread across Europe, and across the Middle East to India, with the cultivation of wheat. It postulates that wheat farmers had population densities about fifty times greater than those of hunter-gatherers, and they gradually spread into the hunter-gatherer territories and swamped both their languages and their genes with their superior numbers. See also: Austronesian family of languages, Coconut.
Reproduction
There are several kinds of biological reproduction. First perhaps is the distinction between r-strategists (quantity breeders) and K-strategist (quality breeders). Second, there is the distinction between sexual and asexual reproduction.
The most rapid reproduction occurs with asexual r‑strategists that may also be crop parasites. It is the population explosions of such parasites that can be so damaging and so difficult to control.
The vertical subsystem (i.e., gene-for-gene relationship) evolved to stabilise such population explosions by operating as a system of biochemical locks and keys.
Reproductive advantage and disadvantage
In ecological terms, individuals within a population which have a reproductive advantage (e.g., more resistance to parasites) tend to proliferate, while those with a reproductive disadvantage (e.g., less resistance to parasites) tend to disappear. See also: Micro-evolution.
Research
Study designed to discover or confirm new facts or concepts.
Experimental research involves the use of experiments, which can occasionally be highly original and innovative.
Conceptual research involves original thinking that attempts to discover a new idea, concept, hypothesis, or theory.
Modelling, particularly computer modelling, is a form of research designed to test ideas that would be difficult or expensive to test in reality.
Finally, library research is designed to discover the extent of existing knowledge in order to save time, and to avoid unnecessary repetition.
Some amateurs believe that research can only be done by highly trained scientists. This is not true, and amateur breeders are encouraged to conduct their own research if they think it is within their capacity, and will solve one or more their problems.
Resistance
The ability of a host to impede or prevent parasitism, in spite of the parasitic ability of the parasite. There are two kinds of resistance called vertical resistance and horizontal resistance respectively.
Resistant rootstocks
Many vegetatively propagated tree crops have superb agricultural or horticultural characteristics but are susceptible to various soil-borne parasites. They are then grafted on to resistant rootstocks. The classic example of this was the grafting of classic, European, wine grapes on to American rootstocks to control Phylloxera.
Many fruit trees (e.g., stone and pome fruits, citrus) and other high-yielding clones (e.g., rubber) are grafted on to resistant rootstocks for this reason.
Respiration
The inhaling or absorbing of air for the purpose of obtaining oxygen. Plants normally respire at night, absorbing oxygen through the stomata. This is in direct contrast to their behaviour during daylight when they absorb carbon dioxide and expel oxygen in order to photosynthesise and produce carbohydrates.
Resting spore
The spore of a microscopic organism that remains dormant during an adverse season such as a temperate winter or a tropical dry season.
Resting spores are usually produced following sexual recombination and this provides a wide variety of phenotypes when the dormancy ends.
This is important, for example, with vertical pathotypes, which have to match a wide variety of vertical pathodemes in the system of locks and keys of a vertical subsystem.
Reverse osmosis
Osmosis is the passage of a solvent (e.g., water) through a semi‑permeable membrane from a less concentrated solution to a more concentrated solution. This is the phenomenon that keeps plant cells turgid. Without water, turgidity is lost and the plant wilts.
Reverse osmosis involves forcing a solvent in the opposite direction with physical pressure. It is an artificial process used for purifying water, and for concentrating fruit juices.
Rheum rhaponticum
Rhubarb. An ancient clone that does not breed true. Not recommended for amateur breeders.
Rhizobium
This is the bacterium that forms a symbiotic relationship in the root nodules of plants belonging to the family Leguminosae. The plant provides the bacterium with carbohydrates while the bacterium provides the plant with nitrates obtained by nitrogen fixation.
The chemistry of this process remains a scientific mystery which industrial chemists would dearly like to imitate. A further scientific mystery is the fact that the nitrogen-fixing nodules are pink inside, and this colour is due to a form of haemoglobin that is closely related to mammalian haemoglobin.
Rhizoctonia
One of the soil-borne fungi that can cause damping-off of seedlings, and other root and stem rots.
Rhizome
An underground stem with buds that produce new roots and shoots, and which often acts as a food storage organ, and as a survival mechanism from one season to the next.
Ginger, irises, and asparagus are examples of plants that create rhizomes.
Rhizomorph
See: Armillaria.
Rhizosphere
The micro-environment that surrounds a root and is influenced by that root.
Rhodes grass
See: Chloris gayana.
Rhubarb
See: Rheum rhaponticum.
Ribes grossularia
The gooseberry. An ancient crop known to the classical Greeks and Romans. A powdery mildew (Sphaerotheca mors-uvae) native to North America has prevented cultivation on that continent, and its introduction to Europe in 1905 caused severe damage. There is scope for breeding for horizontal resistance by amateurs.
Ribes spp.
The garden currants. There are a number of cultivated species of black and red currants, and white currants are a variety of Ribes sativum, the main red currant. Ribes spp., are the alternate host of white pine blister rust, which largely prevents their cultivation in North America.
There is much scope for amateur breeders willing to work with horizontal resistance to a number of pests and diseases in these crops of somewhat limited commercial importance.
Rice
See: Oryza sativa.
Ricinus communis
The castor oil plant. Castor oil has extraordinary lubrication properties because its viscosity changes only slightly with temperature, and it has a wide range of industrial uses.
The plant cannot stand frost and its cultivation is limited to the tropics and sub-tropics. This is an easy crop to breed but its commercial importance is limited.
Rimpau
Rimpau was a European farmer, who lived in Schlanstedt, and worked with rye, which is open-pollinated.
At each harvest, he would collect the best looking heads and keep them for seed and, after twenty years, in the mid-nineteenth century, his rye was famous as the ‘Schlanstedt Rye’, with long heads and kernels that were nearly double the size of the unimproved, local, rye landraces.
RNA
Ribonucleic acid, which exists in a variety of forms. These substances are present in all living cells and their function is to act as messengers carrying instructions from DNA for controlling the production of proteins.
Rogue
In the course of seed certification for purity of cultivar, the parent crop is inspected, and any plants that belong to another cultivar are called rogues. They must be removed, in a process called ‘roguing’, before the seed from that crop can be certified.
Root
Many of the larger plants have two kinds of root called feeder roots and anchor roots.
The feeder roots are generally near the soil surface and their function is to absorb water and nutrients that are sent up the root and stem systems to the leaves and flowers.
The anchor roots may have these functions also, but their primary function is to anchor the plant in the ground.
Many other plants (e.g., grasses) have only one type of root, which fulfils both functions.
There are complex symbiotic interactions, still barely understood, between feeder roots, soil microbes and soil within the rhizosphere. Organic agriculture aims to support these interactions, whereas conventional agriculture overpowers them.
Root hairs
Elongated cells that emerge from the roots like hairs. Their function is to absorb water and nutrients from the soil.
Root nodules
See: Rhizobium.
Rooting hormones
Synthetic plant hormones that stimulate the production of roots on cuttings. With the development of mist propagators, the need for these hormones has largely disappeared.
Rorippa nasturtium-aquaticum
Watercress. Diploid and tetraploid forms of this ancient crop occur. There is scope for amateur breeders working with horizontal resistance, except that the crop is of limited commercial importance.
Rosaceae
The rose family which includes the stone and pome fruits, and berry fruits such as strawberry, raspberry, and blackberry.
Roses
Probably the most popular of all ornamentals, roses are divided into long-stem and short-stem varieties. Not recommended for amateur breeders because of the intense competition from professional breeders.
Rot
Any process of decay, usually induced by rotting organisms. Some plant diseases are called rots, particularly the root rots and fruit rots.
Rotation
A sequential changing of crop species on an annual basis, usually in a regular pattern, primarily in order to control soil-borne pests and diseases. Rotation can also facilitate weed control, and it can optimise fertiliser use.
Rotenone
A natural insecticide extracted from the roots of Derris elliptica in S.E. Asia, where it is used to control body lice, and from Lonchocarpus in South America, where it is used for paralysing fish.
These plants are cultivated in a number of tropical countries and improved cultivars are available. No resistance to rotenone has ever been known to develop in any species of insect.
Royalties
Plant breeders can earn royalties on the sale of their cultivars in the same way that authors earns royalties on their books. This is a relatively recent legal development and it was designed specifically to encourage private endeavour in plant breeding.
r-strategist
A species in which the population size is governed by the rate of reproduction, which is normally abbreviated to r. In its turn, the rate of reproduction is governed by the season.
An r-strategist reproduces very cheaply, and very rapidly, with large numbers of very small offspring, whenever the weather and food supply permit. This behaviour produces a population explosion that is inevitably followed by a population extinction.
Many plant parasites are r‑strategists, and it is their population explosions that can be so alarming, so damaging, and so difficult to control.
The gene-for-gene relationship, and the system of locking of the vertical subsystem, apparently evolved for the sole function of dampening the population explosions of r‑strategist parasites.
See also: K-strategist.
Rubber
See: Hevea brasiliensis.
Rubus spp.
The raspberries and blackberries. Several species are involved but this is not easy breeding for amateur breeders.
Runner
Many plants have runners, which are a form of vegetative propagation. Runners are probably best known in strawberries, which can send out a stem that eventually produces a plantlet at its end.
Russet Burbank
Possibly the most famous potato cultivar of them all.
Rusts
See: Uredinales.
Ryania
A natural insecticide extracted from the roots of a shrub of this name in Trinidad. It is so safe that it can be used on food crops without a waiting period. However, it is difficult to obtain.
Rye
See: Secale cereale.