Wheat
Scientific classification
Kingdom:	Plantae
(unranked):	Angiosperms
(unranked):	Monocots
(unranked):	Commelinids
Order:	Poales
Family:	Poaceae
Subfamily:	Pooideae
Tribe:	Triticeae
Genus:	Triticum L.
Species
T. aestivum T. aethiopicum T. araraticum T. boeoticum T. carthlicum T. compactum T. dicoccoides T. dicoccum T. durum T. ispahanicum T. karamyschevii T. macha T. militinae T. monococcum T. polonicum T. spelta T. sphaerococcum T. timopheevii T. turanicum T. turgidum T. urartu T. vavilovii T. zhukovskyi References:   Serial No. 42236 ITIS 2002-09-22

Wheat (Triticum spp.)^[1] is a cereal grain, originally from the Levant region of the Near East and Ethiopian Highlands, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize (784 million tons) and rice (651 million tons).^[2] In 2009, world production of wheat was 682 million tons, making it the second most-produced cereal after maize (817 million tons), and with rice as close third (679 million tons).^[3]

Wheat is grown on more land area than any other commercial crop and is the most important staple food for humans. World trade in wheat is greater than for all other crops combined.^[4] Globally, wheat is the leading source of vegetable protein in human food, having a higher protein content than either maize (corn) or rice, the other major cereals. In terms of total production tonnages used for food, it is currently second to rice as the main human food crop and ahead of maize, after allowing for maize's more extensive use in animal feeds.

Wheat was a key factor enabling the emergence of city-based societies at the start of civilization because it was one of the first crops that could be easily cultivated on a large scale, and had the additional advantage of yielding a harvest that provides long-term storage of food. Wheat contributed to the emergence of city-states in the Fertile Crescent, including the Babylonian and Assyrian empires. Wheat grain is a staple food used to make flour for leavened, flat and steamed breads, biscuits, cookies, cakes, breakfast cereal, pasta, noodles, couscous^[5] and for fermentation to make beer,^[6] other alcoholic beverages,^[7] or biofuel.^[8]

Wheat is planted to a limited extent as a forage crop for livestock, and its straw can be used as a construction material for roofing thatch.^[9][10] The whole grain can be milled to leave just the endosperm for white flour. The by-products of this are bran and germ. The whole grain is a concentrated source of vitamins, minerals, and protein, while the refined grain is mostly starch.

History[link]

Wild wheat Triticum araraticum, Armenia, Erebuni Reserve

Wheat is one of the first cereals known to have been domesticated, and wheat's ability to self-pollinate greatly facilitated the selection of many distinct domesticated varieties. The archaeological record suggests that this first occurred in the regions known as the Fertile Crescent, and the Nile Delta. These include southeastern parts of Turkey, Lebanon, Syria, the Levant, Israel, Egypt and Ethiopia. Recent findings narrow the first domestication of wheat down to a small region of southeastern Turkey,^[11] and domesticated Einkorn wheat at Nevalı Çori—40 miles (64 km) northwest of Gobekli Tepe in Turkey—has been dated to 9,000 B.C.^[12] However evidence for the exploitation of wild barley has been dated to 23,000 B.C. and some say this is also true of pre-domesticated wheat.^[13]

Origin[link]

Archaeological analysis of wild emmer indicates that it was first cultivated in the southern Levant with finds at Iraq ed-Dubb in northern Jordan dating back as far as 9600 BC.^[14][15] Genetic analysis of wild einkorn wheat suggests that it was first grown in the Karacadag Mountains in southeastern Turkey. Dated archeological remains of einkorn wheat in settlement sites near this region, including those at Abu Hureyra in Syria, suggest the domestication of einkorn near the Karacadag Mountain Range. With the anomalous exception of two grains from Iraq ed-Dubb, the earliest carbon-14 date for einkorn wheat remains at Abu Hureyra is 7800 to 7500 years BC.^[16] Remains of harvested emmer from several sites near the Karacadag Range have been dated to between 8600 (at Cayonu) and 8400 BC (Abu Hureyra), that is, in the Neolithic period. With the exception of Iraq ed-Dubb, the earliest carbon-14 dated remains of domesticated emmer wheat were found in the earliest levels of Tell Aswad, in the Damascus basin, near Mount Hermon in Syria. These remains were dated by Willem van Zeist and his assistant Johanna Bakker-Heeres to 8800 BC. They also concluded that the settlers of Tell Aswad did not develop this form of emmer themselves, but brought the domesticated grains with them from an as yet unidentified location elsewhere.^[17]

Cultivation and repeated harvesting and sowing of the grains of wild grasses led to the creation of domestic strains, as mutant forms ('sports') of wheat were preferentially chosen by farmers. In domesticated wheat, grains are larger, and the seeds (spikelets) remain attached to the ear by a toughened rachis during harvesting. In wild strains, a more fragile rachis allows the ear to easily shatter and disperse the spikelets.^[18] Selection for these traits by farmers might not have been deliberately intended, but simply have occurred because these traits made gathering the seeds easier; nevertheless such 'incidental' selection was an important part of crop domestication. As the traits that improve wheat as a food source also involve the loss of the plant's natural seed dispersal mechanisms, highly domesticated strains of wheat cannot survive in the wild.

Wheat harvest on the Palouse, Idaho, United States

Cultivation of wheat began to spread beyond the Fertile Crescent after about 8000 BC. Jared Diamond traces the spread of cultivated emmer wheat starting in the Fertile Crescent about 8500 BC, reaching Greece, Cyprus and India by 6500 BC, Egypt shortly after 6000 BC, and Germany and Spain by 5000 BC.^[19] "The early Egyptians were developers of bread and the use of the oven and developed baking into one of the first large-scale food production industries." ^[20] By 3000 BC, wheat had reached England and Scandinavia. A millennium later it reached China. The first identifiable bread wheat (Triticum aestivum) with sufficient gluten for yeasted breads has been identified using DNA analysis in samples from a granary dating to c 1350 BC at Assiros in Greek Macedonia.^[21]

Wheat continued to spread throughout Europe. In England, thatch was used for roofing in the Bronze Age, and was in common use until the late 19th century.^[22]

Farming techniques[link]

Technological advances in soil preparation and seed placement at planting time, use of crop rotation and fertilizers to improve plant growth, and advances in harvesting methods have all combined to promote wheat as a viable crop. Agricultural cultivation using horse collar leveraged plows (at about 3000 BC) was one of the first innovations that increased productivity. Much later, when the use of seed drills replaced broadcasting sowing of seed in the 18th century, another great increase in productivity occurred. Yields of wheat per unit area increased as methods of crop rotation were applied to long cultivated land, and the use of fertilizers became widespread. Improved agricultural husbandry has more recently included threshing machines and reaping machines (the 'combine harvester'), tractor-drawn cultivators and planters, and better varieties (see Green Revolution and Norin 10 wheat). Great expansions of wheat production occurred as new arable land was farmed in the Americas and Australia in the 19th and 20th centuries.

Controversial techniques[link]

US EPA Title 40 Section 503 allows for wheat to be grown on sewage sludge.^[23] Some uptake of heavy metals is possible.^[24] Flour grown on sewage sludge is not USDA Certified Organic.^[25] Cereal crops such as wheat might become contaminated with E. coli.^[26]

Genetics[link]

Spikelets of a hulled wheat, einkorn

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).^[27]

• Einkorn wheat (T. monococcum) is diploid (AA, two complements of seven chromosomes, 2n=14).^[1]
• Most tetraploid wheats (e.g. emmer and durum wheat) are derived from wild emmer, T. dicoccoides. Wild emmer is itself the result of a hybridization between two diploid wild grasses, T. urartu and a wild goatgrass such as Aegilops searsii or Ae. speltoides. The unknown grass has never been identified among now surviving wild grasses, but the closest living relative is Aegilops speltoides.^{[citation needed]} The hybridization that formed wild emmer (AABB) occurred in the wild, long before domestication,^[27] and was driven by natural selection.

• Hexaploid wheats evolved in farmers' fields. Either domesticated emmer or durum wheat hybridized with yet another wild diploid grass (Aegilops tauschii) to make the hexaploid wheats, spelt wheat and bread wheat.^[27] These have three sets of paired chromosomes, three times as many as in diploid wheat.

The presence of certain versions of wheat genes has been important for crop yields. Apart from mutant versions of genes selected in antiquity during domestication, there has been more recent deliberate selection of alleles that affect growth characteristics. Genes for the 'dwarfing' trait, first used by Japanese wheat breeders to produce short-stalked wheat, have had a huge effect on wheat yields world-wide, and were major factors in the success of the Green Revolution in Mexico and Asia. Dwarfing genes enable the carbon that is fixed in the plant during photosynthesis to be diverted towards seed production, and they also help prevent the problem of lodging. 'Lodging' occurs when an ear stalk falls over in the wind and rots on the ground, and heavy nitrogenous fertilization of wheat makes the grass grow taller and become more susceptible to this problem. By 1997, 81% of the developing world's wheat area was planted to semi-dwarf wheats, giving both increased yields and better response to nitrogenous fertilizer.

Wild grasses in the genus Triticum and related genera, and grasses such as rye have been a source of many disease-resistance traits for cultivated wheat breeding since the 1930s.^[28]

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 perfect and normally self-pollinate. Commercial hybrid wheat seed has been produced using chemical hybridizing agents; these chemicals 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.^[29] F1 hybrid wheat cultivars should not be confused with the standard method of breeding inbred wheat cultivars by crossing two lines using hand emasculation, then selfing or inbreeding the progeny many (ten or more) generations before release selections are identified to be released as a variety or cultivar.

Synthetic hexaploids made by crossing the wild goatgrass wheat ancestor Aegilops tauschii and various durum wheats are now being deployed, and these increase the genetic diversity of cultivated wheats.^[30]

Stomata (or leaf pores) are involved in both uptake of carbon dioxide gas from the atmosphere and water vapor losses from the leaf due to water transpiration. Basic physiological investigation of these gas exchange processes has yielded valuable carbon isotope based methods that are used for breeding wheat varieties with improved water-use efficiency. These varieties can improve crop productivity in rain-fed dry-land wheat farms.^[31]

In 2010, a team of UK scientists funded by BBSRC announced they had decoded the wheat genome for the first time (95% of the genome of a variety of wheat known as Chinese Spring line 42).^[32] This genome was released in a basic format for scientists and plant breeders to use but was not a fully annotated sequence which was reported in some of the media.^[33] The gene rich regions of several chromosomes have been sequenced^[34][35] and made available at http://www.wheatgenome.info,^[36] and a fully annotated sequence is being assembled by a global consortium (IWGSC http://www.wheatgenome.org/), including members of this team.

Plant breeding[link]

Sheaved and stooked wheat

Wheat

Wheat

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.^[37]

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.^[37] 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 United States and South Africa.^[38]

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 the following, arranged in a rough order of their significance from cooler to warmer climates: eyespot, Stagonospora nodorum blotch (also known as glume blotch), yellow or stripe rust, powdery mildew, Septoria tritici blotch (sometimes known as leaf blotch), brown or leaf rust, Fusarium head blight, tan spot and stem rust. In tropical areas, spot blotch (also known as Helminthosporium leaf blight) is also important.

Hulled versus free-threshing wheat[link]

A mature wheat field in Israel

The four wild species of wheat, along with the domesticated varieties einkorn,^[39] emmer^[40] and spelt,^[41] have hulls. This more primitive morphology (in evolutionary terms) 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.^[39]

Naming[link]

Sack of wheat