Wheat
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Monocots
Clade:	Commelinids
Order:	Poales
Family:	Poaceae
Subfamily:	Pooideae
Tribe:	Triticeae
Genus:	Triticum L.[1]
Type species
Triticum aestivum
Species[2]
List of Triticum species: T. aestivum T. aethiopicum T. araraticum T. boeoticum T. carthlicum T. compactum T. dicoccoides T. dicoccon T. durum T. ispahanicum T. karamyschevii T. monococcum T. polonicum T. spelta T. thaoudar T. timopheevii T. turanicum T. turgidum T. urartu T. vavilovii T. zhukovskyi

Wheat is a grass widely cultivated for its seed, a cereal grain that is a worldwide staple food. The many species of wheat together make up the genus Triticum (/ˈtrɪtɪkəm/);^[3] the most widely grown is common wheat (T. aestivum). The archaeological record suggests that wheat was first cultivated in the regions of the Fertile Crescent around 9600 BC. Botanically, the wheat kernel is a caryopsis, a type of fruit.

Wheat is grown on more land area than any other food crop (220.7 million hectares or 545 million acres in 2021). World trade in wheat is greater than for all other crops combined. In 2021, world wheat production was 771 million tonnes (850 million short tons), making it the second most-produced cereal after maize (known as corn in the US and Australia; wheat is often called corn in other countries). Since 1960, world production of wheat and other grain crops has tripled and is expected to grow further through the middle of the 21st century. Global demand for wheat is increasing because of the usefulness of gluten to the food industry.

Wheat is an important source of carbohydrates. Globally, it is the leading source of vegetable proteins in human food, having a protein content of about 13%, which is relatively high compared to other major cereals but relatively low in protein quality (supplying essential amino acids). When eaten as the whole grain, wheat is a source of multiple nutrients and dietary fiber. In a small part of the general population, gluten – which comprises most of the protein in wheat – can trigger coeliac disease, noncoeliac gluten sensitivity, gluten ataxia, and dermatitis herpetiformis.

Description

Wheat is a stout grass of medium to tall height. Its stem is jointed and usually hollow, forming a straw. There can be many stems on one plant. It has long narrow leaves, their bases sheathing the stem, one above each joint. At the top of the stem is the flower head, containing some 20 to 100 flowers. Each flower contains both male and female parts. The flower, which is wind-pollinated, is housed in a pair of small leaflike glumes. The two (male) stamens and (female) stigmas protrude outside the glumes. The flowers are grouped into spikelets, each with between two and six flowers. Each fertilised carpel develops into a wheat grain or berry; botanically a fruit, it is often called a seed. The grains ripen to a golden yellow; a head of grain is called an ear.^[4]

Leaves emerge from the shoot apical meristem in a telescoping fashion until the transition to reproduction i.e. flowering.^[5] The last leaf produced by a wheat plant is known as the flag leaf. It is denser and has a higher photosynthetic rate than other leaves, to supply carbohydrate to the developing ear. In temperate countries the flag leaf, along with the second and third highest leaf on the plant, supply the majority of carbohydrate in the grain and their condition is paramount to yield formation.^[6][7] Wheat is unusual among plants in having more stomata on the upper (adaxial) side of the leaf, than on the under (abaxial) side.^[8] It has been theorised that this might be an effect of it having been domesticated and cultivated longer than any other plant.^[9] Winter wheat generally produces up to 15 leaves per shoot and spring wheat up to 9^[10] and winter crops may have up to 35 tillers (shoots) per plant (depending on cultivar).^[10]

Wheat roots are among the deepest of arable crops, extending as far down as 2 metres (6 ft 7 in).^[11] While the roots of a wheat plant are growing, the plant also accumulates an energy store in its stem, in the form of fructans,^[12] which helps the plant to yield under drought and disease pressure,^[13] but it has been observed that there is a trade-off between root growth and stem non-structural carbohydrate reserves. Root growth is likely to be prioritised in drought-adapted crops, while stem non-structural carbohydrate is prioritised in varieties developed for countries where disease is a bigger issue.^[14]

Depending on variety, wheat may be awned or not awned. Producing awns incurs a cost in grain number,^[15] but wheat awns photosynthesise more efficiently than their leaves with regards to water usage,^[16] so awns are much more frequent in varieties of wheat grown in hot drought-prone countries than those generally seen in temperate countries. For this reason, awned varieties could become more widely grown due to climate change. In Europe, however, a decline in climate resilience of wheat has been observed.^[17]

History

Domestication

Hunter-gatherers in West Asia harvested wild wheats for thousands of years before they were domesticated,^[18] perhaps as early as 21,000 BC,^[19] but they formed a minor component of their diets.^[20] In this phase of pre-domestication cultivation, early cultivars were spread around the region and slowly developed the traits that came to characterise their domesticated forms.^[21]

Repeated harvesting and sowing of the grains of wild grasses led to the creation of domestic strains, as mutant forms ('sports') of wheat were more amenable to cultivation. In domesticated wheat, grains are larger, and the seeds (inside the spikelets) remain attached to the ear by a toughened rachis during harvesting.^[22] In wild strains, a more fragile rachis allows the ear to shatter easily, dispersing the spikelets.^[23] Selection for larger grains and non-shattering heads 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 involve the loss of the plant's natural seed dispersal mechanisms, highly domesticated strains of wheat cannot survive in the wild.^[24]

Wild einkorn wheat (T. monococcum subsp. boeoticum) grows across Southwest Asia in open parkland and steppe environments.^[25] It comprises three distinct races, only one of which, native to Southeast Anatolia, was domesticated.^[26] The main feature that distinguishes domestic einkorn from wild is that its ears do not shatter without pressure, making it dependent on humans for dispersal and reproduction.^[25] It also tends to have wider grains.^[25] Wild einkorn was collected at sites such as Tell Abu Hureyra (c. 10,700–9000 BC) and Mureybet (c. 9800–9300 BC), but the earliest archaeological evidence for the domestic form comes after c.  8800 BC in southern Turkey, at Çayönü, Cafer Höyük, and possibly Nevalı Çori.^[25] Genetic evidence indicates that it was domesticated in multiple places independently.^[26]

Wild emmer wheat (T. turgidum subsp. dicoccoides) is less widespread than einkorn, favouring the rocky basaltic and limestone soils found in the hilly flanks of the Fertile Crescent.^[25] It is more diverse, with domesticated varieties falling into two major groups: hulled or non-shattering, in which threshing separates the whole spikelet; and free-threshing, where the individual grains are separated. Both varieties probably existed in prehistory, but over time free-threshing cultivars became more common.^[25] Wild emmer was first cultivated in the southern Levant, as early as 9600 BC.^[27][28] Genetic studies have found that, like einkorn, it was domesticated in southeastern Anatolia, but only once.^[26][29] The earliest secure archaeological evidence for domestic emmer comes from Çayönü, c. 8300–7600 BC, where distinctive scars on the spikelets indicated that they came from a hulled domestic variety.^[25] Slightly earlier finds have been reported from Tell Aswad in Syria, c. 8500–8200 BC, but these were identified using a less reliable method based on grain size.^[25]

Early farming

Einkorn and emmer are considered two of the founder crops cultivated by the first farming societies in Neolithic West Asia.^[25] These communities also cultivated naked wheats (T. aestivum and T. durum) and a now-extinct domesticated form of Zanduri wheat (T. timopheevii),^[30] as well as a wide variety of other cereal and non-cereal crops.^[31] Wheat was relatively uncommon for the first thousand years of the Neolithic (when barley predominated), but became a staple after around 8500 BC.^[31] Early wheat cultivation did not demand much labour. Initially, farmers took advantage of wheat's ability to establish itself in annual grasslands by enclosing fields against grazing animals and re-sowing stands after they had been harvested, without the need to systematically remove vegetation or till the soil.^[32] They may also have exploited natural wetlands and floodplains to practice décrue farming, sowing seeds in the soil left behind by receding floodwater.^[33][34][35] It was harvested with stone-bladed sickles.^[36] The ease of storing wheat and other cereals led farming households to become gradually more reliant on it over time, especially after they developed individual storage facilities that were large enough to hold more than a year's supply.^[37]

Wheat grain was stored after threshing, with the chaff removed.^[37] It was then processed into flour using ground stone mortars.^[38] Bread made from ground einkorn and the tubers of a form of club rush (Bolboschoenus glaucus) was made as early as 12,400 BC.^[39] At Çatalhöyük (c. 7100–6000 BC), both wholegrain wheat and flour was used to prepare bread, porridge and gruel.^[40][41] Apart from food, wheat may also have been important to Neolithic societies as a source of straw, which could be used for fuel, wicker-making, or wattle and daub construction.^[42]

Spread

Domestic wheat was quickly spread to regions where its wild ancestors did not grow naturally. Emmer was introduced to Cyprus as early as 8600 BC and einkorn c. 7500 BC;^[43][44] emmer reached Greece by 6500 BC, Egypt shortly after 6000 BC, and Germany and Spain by 5000 BC.^[45] "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."^[46] By 4000 BC, wheat had reached the British Isles and Scandinavia.^[47][48][49] Wheat likely appeared in China's lower Yellow River around 2600 BC.^[50]

The oldest evidence for hexaploid wheat has been confirmed through DNA analysis of wheat seeds, dating to around 6400–6200 BC, recovered from Çatalhöyük.^[51] As of 2023,^[update] the earliest known wheat with sufficient gluten for yeasted breads was found in a granary at Assiros in Macedonia dated to 1350 BC.^[52] From the Middle East, wheat continued to spread across Europe and to the Americas in the Columbian exchange. In the British Isles, wheat straw (thatch) was used for roofing in the Bronze Age, and remained in common use until the late 19th century.^[53][54] White wheat bread was historically a high status food, but during the nineteenth century it became in Britain an item of mass consumption, displacing oats, barley and rye from diets in the North of the country. It became "a sign of a high degree of culture".^[55] After 1860, the enormous expansion of wheat production in the United States flooded the world market, lowering prices by 40%, and (along with the expansion of potato growing) made a major contribution to the nutritional welfare of the poor.^[56]

• 
  Sumerian cylinder seal impression dating to c. 3200 BC showing an ensi and his acolyte feeding a sacred herd wheat stalks; Ninurta was an agricultural deity and, in a poem known as the "Sumerian Georgica", he offers detailed advice on farming
• 
  Threshing of wheat in ancient Egypt
• 
  Traditional wheat harvesting in Madhya Pradesh, 2012

Evolution

Phylogeny

Some wheat species are diploid, with two sets of chromosomes, but many are stable polyploids, with four sets of chromosomes (tetraploid) or six (hexaploid).^[57] Einkorn wheat (Triticum monococcum) is diploid (AA, two complements of seven chromosomes, 2n=14).^[58] 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 Ae. speltoides.^[59] The hybridization that formed wild emmer (AABB, four complements of seven chromosomes in two groups, 4n=28) occurred in the wild, long before domestication, and was driven by natural selection. Hexaploid wheats evolved in farmers' fields as wild emmer hybridized with another goatgrass, Ae. squarrosa or Ae. tauschii, to make the hexaploid wheats including bread wheat.^[57][60]

A 2007 molecular phylogeny of the wheats gives the following not fully-resolved cladogram of major cultivated species; the large amount of hybridisation makes resolution difficult. Markings like "6N" indicate the degree of polyploidy of each species:^[57]

Taxonomy

Zdroj:https://en.wikipedia.org?pojem=Wheat
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