Wheat (Triticum spp.) is a worldwide cultivated grass from the Levant area of the Middle East. Globally, after maize, wheat is the second most produced food among the cereal crops; rice ranks third. Wheat grain is a staple food used to make flour for leavened, flat and steamed breads; cookies, cakes, pasta, juice, noodles and couscous; and for fermentation to make beer, alcohol, vodka or biofuel. Wheat is planted to a limited extent as a forage crop for livestock, and the straw can be used as fodder for livestock or as a construction material for roofing thatch.
Although wheat supplies much of the world's dietary protein and food supply, as many as one in every 100 to 200 people has Coeliac disease, a condition which results from an immune system response to a protein found in wheat: gluten (based on figures for the United States).
Wild wheats were domesticated as part of the origins of agriculture in the Fertile Crescent. Cultivation and repeated harvesting and sowing of the grains of wild grasses led to the domestication of wheat through selection of mutant forms with tough ears that remained intact during harvesting, larger grains, and a tendency for the spikelets to stay on the stalk until harvested. Because of the loss of seed dispersal mechanisms, domesticated wheats have limited capacity to propagate in the wild.
The exact timing of the first appearance of domesticated wheats is currently uncertain, but is either in the PPNA period (9800-8800 cal BC) or the early-mid PPNB (8800-7500 cal BC). Domesticated einkorn and emmer wheat has been identified at three PPNA sites in the northern Levant , Iraq ed-Dubb, Jericho and Tell Aswad, but both the dating and the domesticated status of these cereals is disputed. Domesticated wheats (and other Neolithic founder crops) are unambiguously present at early-mid PPNB sites in the northern Levant, such as Ain Ghazal, Abu Hureyra and Tell Aswad, and in southeast Turkey at Cafer Höyük and Çayönü. As a round figure, it is correct to say that wheats have been domesticated for about 10,000 years.
The cultivation of wheat began to spread beyond the Fertile Crescent during the Neolithic period, reaching the Aegean by 8500 cal BC and the Indian subcontinent by 6000 cal BC. By 5,000 years ago, wheat had reached Ethiopia, Great Britain, Ireland and Spain. A millennium later it reached China. Claims have been made for independent domestication of wheat outside the fertile crescent, but these lack evidence of the presence of wild wheats or of early domesticated wheat.
Three thousand years ago agricultural cultivation with horse-drawn plows increased cereal grain production, as did the use of seed drills to replace broadcast sowing in the 18th century. Yields of wheat continued to increase, as new land came under cultivation and with improved agricultural husbandry involving the use of fertilizers, threshing machines and reaping machines, tractor-drawn cultivators and planters, and varieties adapted to intensive cultivation (see green revolution and Norin 10 wheat).
Wheat genetics is more complicated than that of most other domesticated species. Some wheat species are diploid, with two sets of chromosomes, but many are stable polyploids, with four sets of chromosomes (tetraploid) or six (hexaploid).
In traditional agricultural systems wheat populations often consist of landraces, informal farmer-maintained populations that often maintain high levels of morphological diversity. Although landraces of wheat are no longer grown in Europe and North America, they continue to be important elsewhere. The origins of formal wheat breeding lie in the nineteenth century, when single line varieties were created through selection of seed from a single plant noted to have desired properties. Modern wheat breeding developed in the first years of the twentieth century and was closely linked to the development of Mendelian genetics. The standard method of breeding inbred wheat cultivars is by crossing two lines using hand emasculation, then selfing or inbreeding the progeny. Selections are identified (shown to have the genes responsible for the varietal differences) ten or more generations before release as a variety or cultivar.
F1 hybrid wheat cultivars should not be confused with wheat cultivars deriving from standard plant breeding. Heterosis or hybrid vigor (as in the familiar F1 hybrids of maize) occurs in common (hexaploid) wheat, but it is difficult to produce seed of hybrid cultivars on a commercial scale as is done with maize because wheat flowers are complete and normally self-pollinate. Commercial hybrid wheat seed has been produced using chemical hybridizing agents, plant growth regulators that selectively interfere with pollen development, or naturally occurring cytoplasmic male sterility systems. Hybrid wheat has been a limited commercial success in Europe (particularly France), the USA and South Africa.
The major breeding objectives include high grain yield, good quality, disease and insect resistance and tolerance to abiotic stresses include mineral, moisture and heat tolerance. The major diseases in temperate environments include Fusarium head blight, leaf rust and stem rust, whereas in tropical areas spot blotch (wheat) (also known as Helminthosporium leaf blight). See physiological and molecular wheat breeding
The four wild species of wheat, along with the domesticated varieties einkorn, emmer and spelt, have hulls (in German, Spelzweizen). This more primitive morphology consists of toughened glumes that tightly enclose the grains, and (in domesticated wheats) a semi-brittle rachis that breaks easily on threshing. The result is that when threshed, the wheat ear breaks up into spikelets. To obtain the grain, further processing, such as milling or pounding, is needed to remove the hulls or husks. In contrast, in free-threshing (or naked) forms such as durum wheat and common wheat, the glumes are fragile and the rachis tough. On threshing, the chaff breaks up, releasing the grains. Hulled wheats are often stored as spikelets because the toughened glumes give good protection against pests of stored grain.
There are many botanical classification systems used for wheat species, discussed in a separate article on Wheat taxonomy. The name of a wheat species from one information source may not be the name of a wheat species in another. Within a species, wheat cultivars are further classified by wheat breeders and farmers in terms of growing season, such as winter wheat vs. spring wheat, by gluten content, such as hard wheat (high protein content) vs. soft wheat (high starch content), or by grain color (red, white or amber).
In British English wheat may be referred to as corn.
Classes used in the United States are
Hard wheats are harder to process and red wheats may need bleaching. Therefore, soft and white wheats usually command higher prices than hard and red wheats on the commodities market.
Raw wheat can be powdered into flour, germinated and dried creating malt, crushed and de-branned into cracked wheat, parboiled (or steamed), dried, crushed and de-branned into bulgur, or processed into semolina, pasta, or roux. They are a major ingredient in such foods as bread, breakfast cereals (e.g. Wheatena, Cream of Wheat, Shredded Wheat), porridge, crackers, biscuits, Muesli, pancakes, cakes, gravy and boza (a fermented beverage).
Much of the carbohydrate fraction of wheat is starch. Wheat starch is an important commercial product of wheat, but second in economic value to wheat gluten. The principal parts of wheat flour are gluten and starch. These can be separated in a kind of home experiment, by mixing flour and water to form a small ball of dough, and kneading it gently while rinsing it in a bowl of water. The starch falls out of the dough and sinks to the bottom of the bowl, leaving behind a ball of gluten.
|Top Ten Wheat Producers — 2005 (million metric ton)|
|Source: UN Food & Agriculture Organisation (FAO)|
Wheat is widely cultivated as a cash crop because it produces a good yield per unit area, grows well in a temperate climate even with a moderately short growing season, and yields a versatile, high-quality flour that is widely used in baking. Most breads are made with wheat flour, including many breads named for the other grains they contain like most rye and oat breads. The popularity of foods made from wheat flour creates a large demand for the grain, even in economies with significant food surpluses.
In 2007 there was a dramatic rise in the price of wheat due to freezes and flooding in the northern hemisphere and a drought in Australia. Wheat futures in September, 2007 for December and March delivery had risen above $9.00 a bushel, prices never seen before. There were complaints in Italy about the high price of pasta. This followed a wider trend of escalating food prices around the globe, driven in part by climatic conditions such as drought in Australia, the diversion of arable land to other uses (such as producing government-subsidised bio-oil crops), and later by some food-producing nations placing bans or restrictions on exports in order to satisfy their own consumers.
Other drivers affecting wheat prices include the movement to bio fuels (in 2008, a third of corn crops in the US are expected to be devoted to ethanol production) and rising incomes in developing countries, which is causing a shift in eating patterns from predominantly rice to more meat based diets (a rise in meat production equals a rise in grain consumption - seven kilograms of grain is required to produce one kilogram of beef.
Unlike rice, wheat production is more widespread globally though China's share is almost one-sixth of the world.
While winter wheat lies dormant during a winter freeze, wheat normally requires between 110 and 130 days between planting and harvest, depending upon climate, seed type, and soil conditions. Crop management decisions require the knowledge of stage of development of the crop. In particular, spring fertilizer applications, herbicides, fungicides, growth regulators are typically applied at specific stages of plant development.
For example, current recommendations often indicate the second application of nitrogen be done when the ear (not visible at this stage) is about 1 cm in size (Z31 on Zadoks scale). Knowledge of stages is also interesting to identify periods of higher risk, in terms of climate. For example, the meiosis stage is extremely susceptible to low temperatures (under 4 °C) or high temperatures (over 25 °C). Farmers also benefit from knowing when the flag leaf (last leaf) appears as this leaf represents about 75% of photosynthesis reactions during the grain-filling period and as such should be preserved from disease or insect attacks to ensure a good yield.
Several systems exist to identify crop stages, with the Feekes and Zadoks scales being the most widely used. Each scale is a standard system which describes successive stages reached by the crop during the agricultural season.
Sodium transport in the seedlings of two bread wheat (Triticum aestivum L.) genotypes showing contrasting salt stress tolerance
Mar 01, 2011; Abstract In many plant species, salt sensitivity is associated with the accumulation of sodium (Na+) in photosynthetic tissues....