Glossary: A

Abaca
See: Musa textilis.
Abelmoschus esculentus
Okra, previously called Hibiscus esculentus. This is an annual crop grown for its fruits that are cooked and eaten as a green vegetable. There has been considerable hybridisation with wild species and there is much genetic variation. Scope for amateur breeders working with horizontal resistance.
Abscission
The discarding of plant organs, such as leaves of deciduous trees in the autumn.
Acaricide
See: Miticide.
Acidity
See: pH.
Acre
A measure of land area. One acre is 4840 square yards, or 0.405 hectare.
Acropetal
Growing upwards so that the oldest parts are at the base and the youngest at the tip.
Adlay
See: Coix lachryma-jobi.
Adult plant resistance
Horizontal resistance in many crops, particularly the cereals, is often expressed more in mature plants, and less in young seedlings. This is to be expected because the epidemic intensifies as the growing season progresses. For this reason, horizontal resistance is often called adult plant resistance and, by implication, it is more difficult to observe it, measure it, or screen for it, in young plants.
Aerobic
Living conditions in which there is a plentiful supply of oxygen. Organisms which require oxygen are labelled as aerobic organisms, or aerobes. The converse, meaning without oxygen, is anaerobic.
Aestivation
An organism’s survival of a hot dry summer.
Aflatoxin
Toxins produced by Aspergillus flavus and related fungi. Mouldy feedstuffs contaminated with aflatoxins have caused severe disease and mortalities in livestock, particularly poultry.
African millet
See: Eleusine coracana.
Agaric
Any member of the Agaricaceae, a fungus family in which the fruiting bodies are mushroom shaped.
Agave sisalana
Sisal. Once an important bast fibre crop in its centre of origin in Mexico, and also in East Africa (Kenya and Tanzania), sisal has been largely supplanted by synthetic fibres. Seed set in sisal is extremely rare and breeding this crop is far too difficult for amateur breeders.
Agriculture
Agriculture was independently discovered and developed by many different groups of people in various parts of the world, the main centres being based on the crops wheat (Europe), maize (Central and South America), and rice (Southeast Asia).
Agriculture consists of the propagation and nurturing of domesticated animals and plants. The cultivation of crop plants is now divided into agricultural and horticultural crops. The latter involve a wide array of fruit and vegetables and offer great scope for amateur breeders.
Commercial agriculture is undertaken for financial gain, while subsistence agriculture, mainly in the tropics, is undertaken to feed the farmer’ family, possibly with the sale of some subsistence surpluses. Most subsistence crops also offer great scope for amateur breeders.
Forestry involves the cultivation of trees for timber and it too offers some scope for amateur breeders.
Agrobacterium
Agrobacteriium tumefaciens is the bacterium that causes a disease called crown gall on many different species of host, most particularly on temperate fruit trees. The galls can grow to the size of a soccer ball if left untreated.
Amateur breeders working with rootstocks of fruit trees may care to take resistance to this bacterium into consideration in their breeding. Genetic engineers use this bacterium as a means of introducing foreign DNA into a plant, but this is not a technique for amateurs.
Agro-ecosystem
The ecosystem of a cultivated crop. It differs from the surrounding, natural ecosystem because of the various artificial components of agriculture.
Agro-ecotype
The local landrace of an outbreeding crop is often called an agro-ecotype because, like a wild ecotype, it has responded to selection pressures within its own locality in the agro-ecosystem, and it is well adapted to that locality. In systems terminology, this adaptation is called local optimisation.
In a wide sense, any domesticated variety of plant or animal is an agro-ecotype. Amateur plant breeders may regard their work as improving the domestication of existing agro-ecotypes.
Agronomic suitability
The agronomic suitability of a cultivar is one of the four objectives of plant breeding (the others being yield, quality of crop product, and resistance to pests and diseases).
It is governed by a variety of traits such as plant shape and size (often called crop architecture), time of maturity, suitability for mechanical cultivation and harvesting, frost and/or drought resistance, yield potential, suitability to market requirements, and so on.
This is a factor that amateur breeders must always take into account.
Agronomy
That component of agriculture which is concerned with the theory and practice of growing crops, and with the management of soils.
Aguacate
See: Persea americana.
Air-borne parasites
Plant parasites can be air-borne, soil-borne, water-borne (mainly in irrigation water), and seed-borne. The air-borne parasites include fungi and flying insects, which can sometimes travel for hundreds of miles on prevailing winds.
Akee
See: Blighia sapida.
Aldrin
One of the dirty dozen chemicals called POPS. Aldrin is an insecticide, now banned by international treaty.
Aleurites spp.
Tung, an ancient crop in China, it is now grown in several warm countries. The seeds of A. fordii and A. montana yield a paint oil of exceptional quality. The market has declined from competition with cheaper paints, particularly plastics. Considerable scope for local amateur breeders who are not ambitious about their new cultivars.
Alfalfa
See Medicago sativa.
Alga
(Plural: algae). Primitive plants that have chlorophyll and can photosynthesise. They range in size from single-celled and microscopic, or many-celled and many feet long. They occur mainly in water, which may be either fresh or marine.
Alkaloid
An organic compound containing nitrogen, and with conspicuous physiological properties. Well-known alkaloids include nicotine, caffeine, quinine, morphine, cocaine, and strychnine.
Allele
The alternate copies of a single gene. Each gene normally consists of two alleles. Each allele occurs on one of the two matching chromosomes, one of which comes from the male parent, and the other from the female parent.
In one individual, the two alleles may be both dominant (AA), both recessive (aa), or one of each (Aa). The first two of these combinations are described as homozygous; the third is heterozygous.
Allelopathy
A mechanism that reduces or eliminates competition from other species by the production of toxins. The best known example is that of antibiotics produced by fungi to suppress the growth of bacteria. Equally familiar is the effect of a carpet of pine needles in suppressing the germination of other plants.
Alliaceae
The botanical family that includes the onions and their relatives. However, some taxonomists prefer to classify Allium spp. within either the Liliaceae or the Amaryllidaceae.
Allium ampeloprasum
Leeks and ‘elephant’ garlic. Leeks are tetraploids (4x) and set seed freely, while ‘elephant’ garlic is a hexaploid (6x) and is sterile. We can certainly consider the possibility of breeding leeks for horizontal resistance, but we should steer clear of ‘elephant’ garlic. The breeding procedures are those of open-pollinated crops.
Allium cepa
The common onion, including the shallot. This vegetable is an excellent subject for breeding by amateurs.
There are many different types of onion, ranging from sweet to pungent, and from deep red, and green, to white. And there are many parasite problems of onions, all of which can be either solved or greatly ameliorated by breeding for horizontal resistance.
Onions are open-pollinated but flower only in their second season. The parasite screening should be undertaken in the first season and it should be based on both yield and appearance after exposure to major infestations of parasites.
The best selections are stored, and this constitutes a second screening for resistance to storage rots and pests. The storage survivors are planted out and allowed to flower, but a negative screening decapitates the worst plants, and only the best individuals can form pollen and seed.
New varieties can consist of either improved populations (synthetic varieties) or hybrid varieties. The latter procedure requires more work but has the advantages of higher yields and complete protection of seed production.
The wild progenitors of onion are extinct.
Allium sativum
Garlic. This crop cannot be recommended for amateur breeders as it never sets seed, and it can be propagated vegetatively only. The flowers sometimes produce small bulbils, which can be used for propagation, but these are also vegetative and are not the result of pollination.
The formation of flowers and seeds is a major physiological sink that severely reduces the yield of vegetative parts of the plant. Ancient cultivators probably had a gut-feeling about this, and preferred clones that did not produce flowers or seed.
Garlic provides an excellent example of the durability of horizontal resistance because all the varieties are ancient clones that have been cultivated for centuries without crop protection chemicals, and without serious loss from parasites. Any modern problems with parasites are the result of an environmental erosion of horizontal resistance.
Allium schoenoprasum
Chives. The leaves are used as a garnish. This species is an outbreeder and is easy to breed. Chives can be propagated either vegetatively or from true seed. Chives do not have well-formed bulbs but they do form tillers to produce dense clumps of plant. Easy to breed.
Allogamy
Greek; allo = other, or different; gamy = marriage. The term means cross pollination. An allogamous plant or species is one in which cross-pollination is normal or even obligatory. Cultivated allogamous species include maize, sorghum, millets, and rye; members of the onion family, members of the cucumber family; and various pulses and vegetables.
The converse term, meaning self-pollination, is autogamy.
Allo-infection
Infection is the contact made by one parasite individual with one host individual for the purposes of parasitism. Allo-infection (Greek: allo = other or different) means that the parasite has arrived from somewhere else; it had to travel to its host. The first infection of any host individual must be an allo-infection.
The gene-for-gene relationship provides a system of locking which ensures that most allo-infections are non-matching infections. This is the sole function of vertical resistance in a wild pathosystem.
See also: Auto-infection, Allogamy.
Allopatric
Species, ecotypes, or pathotypes that come from another part of the world.
Allopolyploid
A polyploid has more than two sets of chromosomes (e.g., triploid, tetraploid). In an allopolyploid, the chromosomes are derived from two or more different species. In an autopolyploid, all the chromosomes are derived from the same species.
Allotetraploid
An allotetraploid has four sets of chromosomes derived from two different diploid species. For example, Coffea arabica is believed to be an allotetraploid derived from a cross of the two diploid species Coffea canephora and Coffea eugenioides.
An interspecific cross is usually sterile, but the cross can be made fertile by doubling its chromosome number, and making it an allotetraploid. However specialists should be consulted before such a breeding approach is attempted by amateur breeders in other crops.
Allspice
See: Pimenta dioica.
Alocasia macrorrhiza
One of the aroids, of minor significance, cultivated in S.E. Asia.
Almond
See: Prunus amygdalus.
Alternaria
This genus is an imperfect fungus (i.e., it has no sexual stage) with an extremely wide host range.
Various species of Alternaria cause leaf and fruit spots on citrus, brassicas, flax, potatoes, tomatoes, leeks, onions, and other crops. The spots form concentric rings of colonisation and the disease is often called ‘ring-spot’ or ‘target spot’.
It is easy to accumulate horizontal resistance to this fungus and plant breeders should take it into account when breeding many species of vegetables.
Amaranth
See: Amaranthus.
Amaranthus
Amaranth is an ancient crop of the Americas cultivated either as a grain crop or as a pot herb. It is now a popular ornamental. The Spanish tended to prohibit its cultivation as they believed it was associated with cannibalism, but its full potential is now being recognised.
The grain amaranths consist of three species, A. hypochondriacus and A. cruentas that originated in Mexico and Guatemala, and A. caudatus, which is native to Andean countries such as Peru. Vegetable amaranths are boiled as greens and include A. tricolor, A. dubius, and A. cruentus.
Most amaranths have high levels of horizontal resistance to all their pests and diseases but there is considerable scope for improvements in yield, quality, and agronomic suitability, including possible day-length changes.
The amaranths are wind-pollinated and should be subjected to open-pollinated breeding techniques. An attractive crop for plant breeders.
Amateur plant breeding
Plant breeding that is undertaken by people who are not professional plant breeders, and who may not have any formal training in plant breeding. Using the techniques of horizontal resistance breeding, amateur breeders can easily achieve outstanding results.
The Open Plant Breeding Foundation is here to support this type of breeding -- both with information and practical assistance -- as well as to encourage other plant breeders associations.
Ammonium nitrate
An artificial fertiliser that is exceptionally rich in nitrogen. Ammonium nitrate must be handled with care, as it is powerfully explosive when mixed with a combustible such as oil.
Amphidiploid
An alternative term for allotetraploid.
Amphimictic
The adjectival form of amphimixis.
Amphimixis
The converse of apomixis, and meaning reproduction by seed which has been produced by a normal sexual fusion.
Anacardiaceae
Family of tropical trees that includes mango and cashew.
Anacardium occidentale
Cashew nut. Although it is frost-susceptible, cashew is one of the hardiest of trees and, in warm countries, will grow on poor soils that are unsuitable for other crops. The nuts fetch a high price and the crop is about as valuable as arabica coffee. However, a factory is necessary for the specialised task of shelling the nuts.
Each nut is borne externally on the end of a fairly large fruit. The fruit is edible, but very astringent, and it can be utilised for the manufacture of alcohol. There is a correlation between total yield and quality, the highest yielding trees producing small nuts of low commercial quality.
But there is great variation among trees, and there is scope for selection within existing orchards, by amateur breeders, with a view to vegetative propagation of selected clones.
Anaerobic
Living conditions in which there is an absence of oxygen. Organisms which do not require oxygen are labelled as anaerobic organisms. The converse, meaning with oxygen, is aerobic.
Analogous evolution
Evolution in which similar features have different origins (e.g., the wings of birds, insects, and bats represent analogous evolution). This is the converse of homologous evolution, in which similar features have a common origin (e.g., all the plants in one family have a common ancestor).
Ananas comosus
Pineapple. This is a very difficult crop to breed and it is definitely not recommended for amateur plant breeders.
Anastomsis
Natural grafting that can occur in either stems or roots. For example, mango seeds contain both a nucellar embryo and a normal embryo that is the result of open-pollination. Trees growing from casually discarded seeds often consist of two trunks joined at the base by anastomosis. One trunk is the nucellar seedling and is identical to the maternal parent, while the other is an open-pollinated variant and is visibly different in many characteristics, including fruit quality and resistance to parasites.
Ancient clones
The importance of ancient clones is that they provide proof of the durability of horizontal resistance. Such clones may date from centuries, even millennia, ago.
They are common in figs (Ficus), olives (Olea), date palms (Phoenix), citrus (Citrus), horseradish (Armoracea), garlic (Allium), ginger (Zingiber), turmeric (Curcuma), saffron (Crocus), rhubarb (Rheum), etc.
Andromonoecious
Having both male and hermaphrodite flowers on the same plant.
See: Cucumis melo.
Anethum graveolens
Dill. See also: curry powders.
Angiosperm
Seed-forming plants whose seeds are protected by a seed-coat. This group includes the flowering plants, both monocotyledons and dicotyledons, and it provides virtually all human food, either directly as vegetable matter, or indirectly, as meat.
A few Angiosperms are parasitic on other plants. They lack chlorophyll and they include dodder (Cuscuta spp.), broomrape (Orobanche spp.) and witchweed (Striga spp.). It is possible to breed crops for horizontal resistance to these parasitic Angiosperms. See also: Gymnosperm.
Annual plant
A plant which flowers, fruits, and dies in one season.
Anther
The male reproductive part of a flower that produces pollen.
Anthesis
The time of pollen production.
Anthracnose
A plant disease caused by a species of the fungus called Colletotrichum (pronounced coll-ee-TOT-tree-coom). The symptoms are sunken lesions, several millimetres in diameter, with small, black, sporulating, fungal bodies on the sunken surface.
Antibiotic
A substance that inhibits the growth of micro-organisms, e.g., penicillin. It seems that all antibiotics provide an unstable protection when used singly, and that a cocktail of different antibiotics is much more stable.
Aphids
Plant parasitic insects of the Order Homoptera which are among the most common, and serious, of insect pests of crops.
Also known as greenfly or green bugs, aphids have several different forms, including winged females for alloinfection; wingless, asexual, viviparous females for auto-infection; and winged males and females for sexual reproduction.
Many species of aphid are heteroecious. Many are vectors of virus diseases.
Apical dominance
The suppression of lateral branches by the apical shoot, or apex, of the plant.
Apical meristem
The meristem at the main growing point, or apex, of a plant.
Apis
The genus to which honey bees belong. These are stinging, social, hymenopterous insects, useful in the production of honey, and in the pollinating of many species of crop. Amateur breeders can often make use of them to produce a random polycross.
Apium graveolens
Celery and celeriac. An ancient domestication known to the classical Greeks. Celery is used for its green stems, mainly as a flavouring in soups and salads. Celeriac (var. rapaceum) is grown for its swollen, edible roots.
Apomictic
The adjectival form of apomixis.
Apomixis
Greek: apo = without; mixis = mixing. Asexual reproduction by seeds produced from the maternal tissue of a flower.
Apomictic seeds occur mainly in grasses, and they have the advantage of being the equivalent of vegetative propagation, being free of most vegetatively transmitted diseases (particularly viruses).
The so-called ‘apomictic gene’ is a topic of interest among molecular biologists because it could very easily preserve agricultural characteristics, including hybrid vigour, in heterozygous seeds of open-pollinated crops.
Apothecium
An open fruiting body shaped like a ‘dry martini’ glass, produced by some Ascomycetes, with asci on the open, upper surface. Sometimes called ‘cup fungi’.
Apple
See Malus.
Apple scab
See Venturia inaequalis.
Apricot
See Prunus armeniaca.
Araceae
The family to which the aroids belong; see Alocasia, Colocasia, Cyrtosperma, and Xanthosoma
Arachis hypogea
The peanut, also known as ‘monkey nut’, and groundnut, because the plant thrusts its pods underground as a method of self-sowing.
Originating in South America, ancient domestication produced non-fragile pods and shorter pod-bearing stems. Like the non-shattering character in cereals, these changes made harvesting much easier.
Most groundnut varieties are inbreeders and cross pollination is rather difficult. They are also allotetraploids and crossing with wild diploids is not easy. However, many interspecific crosses have been made and these offer considerable scope for development. A serious challenge for amateur breeders but one with great potential for the courageous.
Arachnid
A member of the Arachnida, the class of arthropods that includes spiders, mites, scorpions, and ticks.
Archetype
The wild ancestor of a modern cultivar.
Areca catechu
This palm is the source of the betel nut, which is chewed as a narcotic by more people than use chewing gum. It is chewed as a ‘quid’ of betel pepper leaves with a dash of slaked lime. This ‘quid’ turns the saliva red and this colours walls and sidewalks from spitting.
The young palm is also a popular houseplant. There is some scope for amateur breeders to select superior palms within existing populations in areca-producing countries.
Areca palm
See: Areca catechu above.
Armillaria
Armillaria mellea is known as the honey fungus, and it can cause a serous disease of many species of tree.
It produces long black rhizomorphs that look like boot-laces, and that can grow through the soil and spread the disease from tree to tree. Armillaria often produces toadstools on dead tree stumps.
In the tropics, it occurs only at high altitudes. It has even been postulated that a large network of rhizomorphs constitutes the largest living organism. Foresters often ring-bark trees about a year before felling them, and this denudes the roots of nutrients. The fungus is then unable to invade them.
Another defence is to dig trenches that the rhizomorphs cannot cross. However, many pathologists think that Armillaria will only attack trees that are weakened from some other cause such as waterlogging or shallow soil.
It is not feasible for amateurs to breed for horizontal resistance to this disease.
Armoracia rusticana
Horse radish. The roots are used to make a peppery condiment, but this species does not flower or set seed. It is definitely not recommended for amateur plant breeders.
There are many clones with widely varying degrees of pungency. These ancient clones have few pests or diseases and they are a good example of both the effectiveness and the durability of horizontal resistance.
Aroids
Aroids are a group of tropical root crops belonging to the family Araceae. See: Alocasia, Colocasia, Cyrtiosperma and Xanthosoma.
Arrowroot
See: Maranta arundunacea.
Arrowroot, Queensland
See: Canna edulis.
Arsenic
Compounds of this well-known poison were frequently used as an insecticide before the days of the much less hazardous modern synthetic insecticides.
Arthropod
An invertebrate animal belonging to the Phylum Arthropoda, which includes insects, spiders, crustaceans, centipedes, and millipedes. This is the largest phylum and it contains more than one million known species.
Arthropods are characterised by an exoskeleton with a segmented body and jointed limbs.
Artichoke, globe
See: Cynara scolymus.
Artichoke, Jerusalem
See: Helianthus tuberosus.
Artificial fertilisers
The term ‘fertilisation’ has two meanings in agriculture. It can mean sexual fertilisation of either plants or animals, or it can mean manuring of crops.
Fertilisers used for manure are divided into the two categories of organic and artificial. Organic manures are either the excrement of farm animals, usually known as farmyard manure (F.Y.M.) or stable dung, bone meal, or quarried deposits of fish-eating bird excrement, known as guano.
Artificial fertilisers are produced in factories, usually by a modification of natural products, such as atmospheric nitrogen, rock phosphate, or potash. Their constituents are known as N, P, and K, the symbols standing for nitrogen, phosphorus, and potassium. Other constituents may include calcium and magnesium, as well as various minor nutrients and trace elements.
Artificial fertilisers are not used in organic farming.
Artificial selection
Genetic selection which is controlled by people, within a genetically diverse population. Artificial selection is the basis of both domestication, and modern plant and animal breeding. See also: natural selection, agro-ecotype.
Artocarpus altilis
Breadfruit, which is an ancient domestication and is the staple food in a number of Pacific Islands.
Ascomycete
Fungi whose sexual reproduction is by means of an ascus. Many plant pathogens are Ascomycetes, such as the powdery mildews, and apple scab (Venturia inaequalis).
Ascospore
A spore produced within an ascus. Ascospores are haploid, being the result of the reduction division (meiosis) of a newly fertilised diploid cell, which is the only diploid component in the life cycle of an Ascomycete.
Being the result of meiosis, an ascus usually contains eight ascospores but, in some species, the ascus contains only four, or two ascospores.
Ascus
The microscopic reproductive organ of an Ascomycete fungus. The ascus consists of a tube containing eight, four, or two haploid ascospores that are the result of meiosis. When the ascospores are mature, the tube bursts at its tip, from internal pressure, and the ascospores are projected into the atmosphere like microscopic bullets.
Asexual reproduction
Reproduction without sex. Asexual reproduction prevents variation and it produces clones. Many microscopic organisms, such as viruses, bacteria, and imperfect fungi, have asexual reproduction only.
Many r-strategists plant parasites, such as fungi and aphids have both sexual and asexual reproduction. This has the advantage of speed and economy for the parasite, and it permits a population explosion.
If continued for too long, asexual reproduction in the higher organisms is a survival disadvantage in a wild population, but it can be very useful in agriculture. The asexual propagation of plants by cuttings, grafts, etc., is called vegetative propagation.
Some Angiosperms have asexual reproduction by apomictic or nucellar seeds.
See also: r‑strategists.
Asparagus officinalis
A dioecious vegetable that is perennial cultivated for its young succulent shoots. Difficult to breed and not recommended for amateurs.
Asparagus pea
See: Psophocarpus tetragonobolus.
Asynchronous flowering
The production of flowers at different times within one season. Asynchronous flowering assists cross-pollination. It also assists survival, if there is bad weather that hinders pollination.
Attention deficit hyperactivity disorder (ADHD)
The syndrome in children which, as its name implies, exhibits hyperactivity and a very short attention span. It has been reported that about two million children suffer from this syndrome in the United States.
It is thought that the cause of the syndrome may be exposure to hormone mimics during foetal development and/or childhood. There have been numerous documented cases in which a switch to an organic diet has eliminated ADHD and other mental disorders.
See also: Dirty dozen, POPS.
Aubergine
See: Solanum melongena.
Austronesian family of languages
Also known as the Malayo-Polynesian family of languages, these are the languages of remote islands extending from Madagascar, in the West, to Easter Island, in the East, and from Hawaii, in the North, to New Zealand, in the South.
The Austronesian people spread these languages by their ability to make long ocean voyages long before either the Chinese or the Europeans developed ocean-going ships.
Autocratic plant breeding
The converse of the democratic plant breeding produced by self-organising crop improvement. Autocratic plant breeding is justified by the expense of breeding for vertical resistance, and by the relatively few cultivars produced by such breeding.
These cultivars have a very wide ecological adaptation and their widespread use justifies their cost. But the farmer has few choices of cultivar, and the breakdown of a vertical resistance can lead to widespread damage.
Autoecious
The converse of heteroecious, which means that a rust or an aphid is obliged to change its species of host in order to complete its life cycle. An autoecious rust is one that completes its entire life cycle on one species of host.
Entomologists use the term ‘monoecious’ in place of autoecious when describing aphids. Unfortunately, in botany, monoecious means that separate male or female flowers occur on a single plant (See also dioecious, hermaphodite).
Autogamy
(Greek: auto = self; gamy = marriage). Self-fertilisation, or self-pollination. An autogamous species is one in which individual flowers, or plants, are fertilised with their own pollen. However, some cross pollination always occurs in an autogamous species and variability is always maintained. (See also: allogamy).
Auto-infection
Infection is the contact made by one parasite individual with one host individual for the purposes of parasitism. Auto-infection (Greek: auto = self) means that the parasite was born on (or in) the host that it infects; it had no need to travel to its host.
Auto-infection is possible only after a matching allo-infection has occurred. The parasite then reproduces asexually to produce a clone in which all individuals are identical. It follows that, in terms of the gene-for-gene relationship, all auto-infection is matching infection. Consequently, vertical resistance cannot control auto-infection, which can be controlled only by horizontal resistance.
Because all parasitism involves auto-infection, it must be concluded that horizontal resistance occurs in every host, against every parasite of that host.
(See also: alloinfection, autogamy).
Autopolyploid
A polyploid has more than two sets of chromosomes (e.g., triploid, tetraploid). In an autopolyploid, all the chromosomes are derived from the same species. In an allopolyploid, the chromosomes are derived from two or more different species.
Auxin
Auxins are plant hormones.
Avena fatua
Wild oats. This species can be a serious weed as it is difficult to control in cereal crops.
Avena sativa
Cultivated oats. This species is a hexaploid and the first controlled crosses were made by a Scottish farmer, Patrick Sheriff, in 1860.
Subsequently, most professional work has used pedigree breeding and back-crossing with a view to introducing vertical resistances. However amateur breeding for horizontal resistance is entirely feasible and a male gametocide, as used with wheat, will probably be effective.
Average
The mean. A figure obtained by dividing the total of given amounts by the number of amounts in the set.
Avocado
See: Persea americana.
Axil
The upper angle between a leaf and the stem.
Axillary bud
A bud that is located in an axil. Many axillary buds are suppressed by auxins emanating from the apical meristem, and they develop only if the apical meristem is damaged or removed.