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Glossary: T

Tamarind
See: Tamarindus indica.
Tamarindus indica
A tropical, leguminous tree native to Africa but cultivated in India since antiquity. The seeds are coated with a highly prized sweet/sour pulp, used as a flavouring reminiscent of lemon.
Take-all disease
See: Gaümannomyces graminis.
Tannia
See: Xanthosoma sagittifolium.
Tannier
See: Xanthosoma sagittifolium.
Taraxacum
A genus of the family Compositae that includes T. officinale, the dandelion, a widespread "weed" that is now resistant to various herbicides.
Taro
See: Colocasia esculenta.
Taxon
A taxonomic rank (e.g., family, genus, species).
Taxonomy, taxonomist
The classification and naming of living organisms, and a person who studies this classification. See also: Linnaeus.
Tea
See: Thea assamensis and T. sinensis.
Teak
See: Tectona grandis.
Tectona grandis
Teak, a tropical hardwood used in plantation forests. Not recommended for amateur breeders.
Teff
See: Eragrostis tef.
Temperate grasses
The most important sown temperate grasses are Lolium, Festuca, Dactylis, Phleum, and Bromus.
Temperate regions
The regions between the Arctic Circle and the Tropic of Cancer in the northern hemisphere, and between the Antarctic Circle and the Tropic of Capricorn in the southern hemisphere.
In terms of agriculture, there are just the two regions of temperate and tropical, although many people also recognise the subtropics.
Tropical regions are characterised by short days and freedom from frost. Temperate regions have freezing, short-day winters, as well as warm, long-day summers.
The crops of each region are profoundly different, although a few tropical annuals can be grown in a temperate summer (e.g., tomatoes, tobacco).
See also: Day-length.
Temporary resistance
Vertical resistances are temporary in that they stop functioning on the appearance of a matching vertical pathotype. They are within the capacity for micro-evolutionary change of the parasite. However, this is not an exclusive trait, and other kinds of resistance, such as the single-gene resistances of genetic engineering, are also expected to be within the capacity for micro-evolutionary change of the parasite.
Tendril
A thread-like appendage used by vines as an aid to climbing. Tendrils usually twist around suitably sized objects.
Terminal
This word means ‘at the end of’ as in terminal bud, etc.
Terracing
Converting hillsides into terraces that follow the contour is a method of soil conservation. It is an expensive process but, once completed, it is both effective and easily maintained.
Terracotta pots
Flower pots made of terracotta (baked clay) are more expensive than plastic pots, but they provide a superior aeration to plant roots.
Tetraploid
A cell or plant with four sets of chromosomes. A tetraploid usually develops from a more normal diploid, by an accidental doubling of its two sets of chromosomes.
See also: Doubled monoploid, Haploid, Auto‑polyploid, Allopolyploid.
Thea assamensis
Often called Camellia assamensis, this is the tea of India and Sri Lanka (Ceylon). It contains considerable germplasm from Thea sinensis, and it represents a vast hybrid swarm between these two species.
Most commercial tea plantations are grown from true seed and the tea bushes show enormous variation, often with some 60% of the yield coming from about 30% of the bushes. Furthermore, the fermentation time of each bush is also variable, and this causes difficulties in the tea factory.
The use of high-yielding, high quality clones eliminates these difficulties, and produces greatly increased yield and quality. However, a tea plantation is usually good for a hundred years, and replanting is an expensive business.
The selection of tea clones is an appropriate task for amateur breeders. They should select the best yielding bushes out of a large area of seedling tea (up to one million bushes), and then gradually narrow down these selections with a series of increasingly stringent tests.
Thea sinensis
Often called Camellia sinensis, this is the tea of China. See also: Thea assamensis.
Theobroma cacao
The tree is often called cacao, while the product is called cocoa, from which chocolate is manufactured. But it is entirely correct to call them both cocoa. Note that cocoa was originally spelled coco, as in coconut and coco-yam, and the ‘a’ was added as a printing error in Johnson’s dictionary. The old-fashioned spelling ‘cocoanut’ is incorrect.
The centre of origin of cocoa is on the eastern equatorial slopes of the Andes, and cocoa occurs throughout the Amazon Valley where it provides an interesting example of a cline. All the wild trees in the centre of origin are self‑incompatible. As one moves down the Amazon, self-compatible types become increasingly common and, at the river mouth, they are all self-compatible.
All the cocoa in West Africa is self-compatible and very uniform, with a very narrow genetic base. When a very destructive African virus, called ‘swollen shoot’, appeared, the Government introduced a very unpopular eradication program which was ineffective. In those days, horizontal resistance was not recognised.
There is now considerable scope for a university breeding club to test buds of carefully quarantined foreign material grafted on to virus-infected trees. It should not be difficult to accumulate adequate horizontal resistance once the genetic base has been widened.
In Latin America, most cocoa populations are heterogeneous and are suitable targets for negative screening, with a view to eliminating the parasite interference coming from a few susceptible trees, in order to establish population immunity against witch’s broom disease.
Theoretical science
The agricultural sciences were largely dominated by empiricism during the twentieth century, and the theoretical aspects of agriculture are behind the times.
Good science should have a nice balance of both facts and ideas. One of the many advantages of theoretical science is its ability to predict novelty. For example, when Dimitri Mendeleev developed the periodic table, he was able to predict the existence of chemical elements that had not yet been discovered.
Similarly, the Person-Habgood differential interaction can predict new vertical resistances and vertical parasitic abilities that have not yet been discovered.
The n/2 model is also a prediction of novelty based on theoretical science. This model has yet to be shown to occur in nature but, if this demonstration is made, it will provide an elegant example of the powers of prediction of theoretical science.
Thorn
A sharp-pointed projection on a plant that provides a defence against grazing animals. Thorns occur typically on roses but also on cacti, when they are generally called spines.
Thousand seed weight
See: Hundred seed weight.
Threshing
The separation of grain from its husk. Ancient threshing consisted of pounding the grain and then throwing it into the wind, which would carry off the light husks and allow the clean grain to fall to a mat. Modern methods use a wide range of machines that vary from small hand-driven machines to combine harvesters.
Thrips
Small (0.5-2.0 mm) insects of the Order Thysanoptera, which are mostly plant feeders. Many are crop parasites, and some are responsible for spreading plant virus diseases.
Ticks
Blood-sucking arachnids.
Tiller
(1) A side shoot of a cereal. (2) One who tills (i.e., cultivates) the soil.
Tilth
The condition of soil: “In good tilth”.
Timber
Timber refers to the growing tree. Lumber is the sawn planks that come out of a saw mill. Wood is the same material when it is a finished product in buildings or furniture.
Timber trees
In general, trees have a long breeding cycle and are not suitable for amateur breeders. But selection within existing populations is possible.
For example, most of the five-needle pines in North America have been killed by white pine blister rust. Those that survive are likely to be resistant and they merit study.
Similarly, selection of the fastest growing gum trees is possible in areas that depend on firewood for cooking.
The main species used in plantation forests are divided into softwoods and hardwoods. The principle softwoods are various species of pine, spruce, fir, and larch,. The principle hardwoods are species of gum trees, beech, birch, poplar, and teak.
Toadstool
Similar to a mushroom except that toadstools are usually inedible, even poisonous.
Tobacco
See: Nicotiana tabacum.
Tolerance
This is a poorly defined and often misused term that is sometimes taken to mean horizontal resistance. Strictly speaking, tolerance means that, if two different plants are equally diseased, the tolerant one will suffer less of a yield loss. However, to demonstrate tolerance, it must first be shown that those two plants have equal yields when disease-free. A term to be avoided whenever possible.
See also: Field resistance.
Tollocan
The name of a Mexican, short-day potato which has an exceptionally high level of horizontal resistance to blight.
Tomato
See: Lycopersicon esculentum.
Toxicity
The degree of poisonousness of a substance such as a pesticide. Toxicity is usually measured in terms of the LD50, which stands for the lethal dose required to kill 50% of a population, usually of insects or laboratory rats. The LD50 is normally expressed as milligrams of poison per kilogram of body weight, but other ratios are possible.
For the purposes of labelling, the USA recognises four categories of toxicity:
Category One involve substances with an acute oral LD50 of 0‑50 mg/kg and these are very dangerous.
Category Two are moderately toxic substances with LD50 of 50-500 mg/kg.
Category Three substances have LD50 of 500-5000 mg/kg and are only mildly toxic.
Category Four have an LD50 greater than 5000 mg/kg.
However, these categories involve oral ingestion only, and they take no account of inhalation and skin penetration, or of hormone mimics that can damage the development of young children and foetuses.
Toxicology
The scientific study of toxins. See also: Toxicity.
Toxin
A poisonous substance. The term is sometimes restricted to toxins produced by a living organism but, it the context of crop science, it can also be applied to pesticides and other synthetic chemicals.
Tramlines
Parallel tracks in crop fields, visible from the air, and produced by tractor wheels when spraying the crop with pesticides.
Transgressive segregation
The phenomenon in which some of the progeny have a higher level of a quantitative character, such as horizontal resistance, than either of their parents.
Suppose that two parents, which are highly susceptible, each have only 10% of all of the alleles contributing to horizontal resistance to a parasite. If each parent has a different 10% of alleles, some of their progeny will have more than 10% of the total available alleles. These individuals in the progeny will then be more resistant than either of their parents.
Transgressive segregation can continue in each generation of recurrent mass selection until no further progress is possible, because the maximum number of alleles has been accumulated.
Transmission
Many virus diseases are transmitted by plant parasitic insects that migrate from one host to another. This transmission clearly involves allo-infections.
Transpiration
The loss of water from a plant. The rate of transpiration is controlled by the stomata. See also: Guttation.
Tree
Technically, any plant with woody tissues, as opposed to a herb that has no woody tissues. In practice, many of the smaller trees are called shrubs.
Tree tomato
See: Cyphomandra betacea.
Trifoliate
This term means ‘three leaves’ and refers to the leaves of plants such as Trifolium spp., (clovers), or Oxalis, that are divided into three leaflets.
Trifolium spp.
The clovers are important as fodder crops, usually sown in mixtures with grasses. They also occur commonly in natural grasslands in humid temperate areas, and in tropical highlands. There are ten species of clover that are considered agriculturally important:
T. alexandrinum Egyptian or Berseem clover (annual)
T. ambiguum Caucasian or Kura clover (perennial)
T. dubium Yellow suckling clover (annual)
T. fragiferum Strawberry clover (perennial)
T. hybridum Alsike clover (perennial)
T. incarnatum Crimson clover (annual)
T. pratense Red clover (perennial)
T. repens White clover (perennial)
T. resupinatum Persian clover (annual)
T. subterraneum Subterranean clover (annual)
The clovers are important because of their nitrogen-fixation with Rhizobium root nodules. Because the deliberate cultivation of pasture crops is fairly recent, most clover cultivars are fairly close to their wild progenitors.
With only minor exceptions, the annual species are self-compatible while the perennial species are self-incompatible.
Pollination is by insects and the clovers are suitable crops for amateur breeders.
Trigonella foenum-graecum
Fenugreek is a member of the family Leguminosae and its seeds are used as a component of curry powder in India.
Trillion
A trillion is 1012 or 1,000,000,000,000. See also: Billion.
Triploid
A plant that has three sets of chromosomes in place of the usual two. Triploids are usually sterile, and they are difficult to breed (e.g., banana).
Tristeza
‘Tristeza’ means sadness in Spanish, and this is the name of a virus disease of citrus, also known as ‘stem pitting’.
The diagnostic symptom is a flattening of the branches and, when the bark is peeled off, there are pits in the wood, with corresponding projections in the bark. Diseased trees usually die, following severe dieback.
Tristeza is a graft incompatibility disease, and it is serious mainly on trees grafted on to sour orange rootstocks. Resistant scion-stock combinations have rootstocks of sweet orange, rough lemon, and ‘Cleopatra mandarin’.
Triticale
A modern inter-generic hybrid between wheat and rye. It has not proved particularly successful as the uses for its grain are limited.
Triticum spp.
Triticum aestivum (also known as T. vulgare) is a hexaploid, and is bread wheat, which is the most important crop in the world. Triticum durum is a tetraploid and is pasta wheat, which has a very high gluten content, and is used for making pasta (e.g., macaroni, spaghetti, etc.) and couscous, or semolina. Diploid wheats also occur but none is economically important.
Wheat is a major staple (c.f., rice and maize) and it permitted the growth of civilisations in the Fertile Crescent, ancient Egypt, Greece, Rome, northern India, and modern Europe. The Indo-European languages are associated with the spread of wheat cultivation, according to the Renfrew hypothesis.
Wheat breeding for pest and disease resistance during the twentieth century has concentrated almost totally on both vertical resistance, and early maturity, which was aimed at disease escape.
There is enormous scope for breeding for horizontal resistance, and wheat is an excellent challenge for the more adventurous plant breeders. The use of a male gametocide is recommended in order to obtain large heterogeneous populations for recurrent mass selection.
Tropical grasses
The most important cultivated tropical grasses are Digitaria, Eragrostis, Chloris, Cenchrus, Melinis, Panicum, Pennisetum, Cynodon, and Paspalum.
Tropics
The Tropic of Cancer (23°27´ N), and the Tropic of Capricorn (23°27´ S). In common usage, "the tropics" refers to any latitude between the two Tropics, which lie to the north and south of the equator, and which mark the limits at which the sun is vertically overhead for at least one day of the year.
The tropical regions have no winter or summer, and their seasons are often defined by rainy and dry periods, as the inter-tropical convergence zone moves from the northern to the southern hemisphere, and back again, in the course of one year.
Tuber
The swollen part of a rhizome that is used as a storage organ, as in a potato.
Tumour
An abnormal swelling of tissue. In plants, tumours are often called galls.
Tung oil
See: Aleurites spp.
Turmeric
See: Curcuma domestica.
Turnips
See: Brassica campestris.