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ITIS 42236 2002-09-22
Wheat (Triticum spp.) is a domesticated grass from the Levant that is cultivated worldwide. Globally, wheat is an important human food, its production being second only to maize 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, 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.
Wheat originated in Southwest Asia in the area known as the Fertile Crescent. The genetic relationships between einkorn and emmer indicate that the most likely site of domestication is near Diyarbakır in Turkey . These 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 which 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 cultivation of wheat began to spread beyond the Fertile Crescent during the Neolithic period. By 5,000 years ago, wheat had reached Ethiopia, India, Ireland and Spain. A millennium later it reached China. 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 (the ‘combine harvester’), tractor-drawn cultivators and planters, and better varieties (see green revolution and Norin 10 wheat). With population growth rates falling, while yields continue to rise, the area devoted to wheat may now begin to decline for the first time in modern human history. But now in 2007 wheat stocks have reached their lowest since 1981, and 2006 was the first year in which the world consumed more wheat than the world produced - a gap that is continuously widening as the requirement for wheat increases beyond production.
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.