Wheat

Wheatis a group of wild anddomesticatedgrassesof the genusTriticum(/ˈtrɪtɪkəm/).They arecultivatedfor theircerealgrains, which arestaple foodsaround the world. Well-knownwheat species and hybridsinclude the most widely growncommon wheat(T. aestivum),spelt,durum,emmer,einkorn, andKhorasan or Kamut. Thearchaeological recordsuggests that wheat was first cultivated in the regions of theFertile Crescentaround 9600 BC.
Wheat is grown on a larger area of land than any other food crop (220.7 million hectares or 545 million acres in 2021). World trade in wheat is greater than that of all other crops combined. In 2021, world wheat production was 771 milliontonnes(850 million short tons), making it the second most-produced cereal aftermaize(known as corn in North America and Australia; wheat is often called corn in countries including Britain).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 ofglutento the food industry.
Wheat is an important source ofcarbohydrates. Globally, it is the leading source ofvegetable proteinsin human food, having a protein content of about 13%, which is relatively high compared to other major cereals but relatively low inprotein quality(supplyingessential amino acids). When eaten as thewhole grain, wheat is a source of multiplenutrientsanddietary fiber. In a small part of the general population, gluten – which comprises most of the protein in wheat – can triggercoeliac disease,noncoeliac gluten sensitivity,gluten ataxia, anddermatitis herpetiformis.
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 flowers arewind-pollinated, with over 99% of pollination events beingself-pollinationsand the restcross-pollinations.The flower is housed in a pair of small leaflikeglumes.  The two (male)stamensand (female)stigmasprotrude outside the glumes. The flowers are grouped intospikelets, each with between two and six flowers. Each fertilisedcarpeldevelops into a wheat grain or berry; botanically acaryopsisfruit, it is often called a seed. The grains ripen to a golden yellow; a head of grain is called an ear.
Leaves emerge from the shoot apicalmeristemin a telescoping fashion until the transition to reproduction i.e. flowering.The last leaf produced by a wheat plant is known as the flag leaf. It is denser and has a higherphotosyntheticrate than other leaves, to supplycarbohydrateto 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.Wheat is unusual among plants in having morestomataon the upper (adaxial) side of the leaf, than on the under (abaxial) side.It has been theorised that this might be an effect of it having beendomesticatedand cultivated longer than any other plant.Winter wheatgenerally produces up to 15 leaves per shoot and spring wheat up to 9and winter crops may have up to 35tillers(shoots) per plant (depending on cultivar).
Wheatrootsare among the deepest of arable crops, extending as far down as 2 metres (6 ft 7 in).While the roots of a wheat plant are growing, the plant also accumulates an energy store in its stem, in the form offructans,which helps the plant to yield under drought and disease pressure,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.
Depending on variety, wheat may beawnedor not awned. Producing awns incurs a cost in grain number,but wheat awns photosynthesise more efficiently than their leaves with regards to water usage,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 toclimate change. In Europe, however, a decline inclimate resilienceof wheat has been observed.
Hunter-gatherersin West Asia harvested wild wheats for thousands of years before they weredomesticated,perhaps as early as 21,000 BC,but they formed a minor component of their diets.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.
Repeated harvesting and sowing of the grains ofwild grassesled 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 thespikelets) remain attached to the ear by a toughenedrachisduring harvesting.In wild strains, a more fragile rachis allows the ear toshattereasily, dispersing the spikelets.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 cropdomestication. As the traits that improve wheat as a food source involve the loss of the plant's naturalseed dispersalmechanisms, highly domesticated strains of wheat cannot survive in the wild.
Wildeinkorn wheat(T. monococcumsubsp.boeoticum) grows across Southwest Asia in openparklandandsteppeenvironments.It comprises three distinctraces, only one of which, native toSoutheast Anatolia, was domesticated.The main feature that distinguishes domestic einkorn from wild is that its ears do notshatterwithout pressure, making it dependent on humans for dispersal and reproduction.It also tends to have wider grains.Wild einkorn was collected at sites such asTell Abu Hureyra(c.10,700–9000 BC) andMureybet(c.9800–9300 BC), but the earliest archaeological evidence for the domestic form comes afterc.8800 BCin southern Turkey, atÇayönü,Cafer Höyük, and possiblyNevalı Çori.Genetic evidence indicates that it was domesticated in multiple places independently.
Wildemmer wheat(T. turgidumsubsp.dicoccoides) is less widespread than einkorn, favouring the rockybasalticandlimestonesoils found in thehilly flanksof the Fertile Crescent.It is more diverse, with domesticated varieties falling into two major groups: hulled or non-shattering, in which threshing separates the wholespikelet; and free-threshing, where the individual grains are separated. Both varieties probably existed in prehistory, but over time free-threshing cultivars became more common.Wild emmer was first cultivated in the southernLevant, as early as 9600 BC.Genetic studies have found that, like einkorn, it was domesticated in southeastern Anatolia, but only once.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.Slightly earlier finds have been reported fromTell Aswadin Syria,c.8500–8200 BC, but these were identified using a less reliable method based on grain size.
Einkorn and emmer are considered two of thefounder cropscultivated by the first farming societies inNeolithicWest Asia.These communities also cultivated naked wheats (T. aestivumandT. durum) and a now-extinct domesticated form ofZanduri wheat(T. timopheevii),as well as a wide variety of other cereal and non-cereal crops.Wheat was relatively uncommon for the first thousand years of the Neolithic (whenbarleypredominated), but became a staple after around 8500 BC.Early wheat cultivation did not demand much labour. Initially, farmers took advantage of wheat's ability to establish itself inannual grasslandsby 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.They may also have exploited natural wetlands and floodplains to practicedécrue farming, sowing seeds in the soil left behind by receding floodwater.It was harvested withstone-bladedsickles.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.
Wheat grain was stored afterthreshing, with thechaffremoved.It was then processed into flour usingground stonemortars.Breadmade from ground einkorn and the tubers of a form ofclub rush(Bolboschoenus glaucus) was made as early as 12,400 BC.AtÇatalhöyük(c.7100–6000 BC), both wholegrain wheat and flour was used to prepare bread,porridgeandgruel.Apart from food, wheat may also have been important to Neolithic societies as a source ofstraw, which could be used for fuel,wicker-making, orwattle and daubconstruction.
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 einkornc.7500 BC;emmer reachedGreeceby 6500 BC,Egyptshortly after 6000 BC, andGermanyandSpainby 5000 BC."The early Egyptians were developers ofbreadand the use of the oven and developed baking into one of the first large-scale food production industries."By 4000 BC, wheat had reached theBritish IslesandScandinavia.Wheat likely appeared inChina's lowerYellow Riveraround 2600 BC.
The oldest evidence forhexaploidwheat has been confirmed throughDNA analysisof wheat seeds, dating to around 6400–6200 BC, recovered fromÇatalhöyük.As of 2023,the earliest known wheat with sufficient gluten for yeasted breads was found in a granary atAssirosinMacedoniadated to 1350 BC.From theMiddle East, wheat continued to spread across Europe and to theAmericasin theColumbian exchange. In the British Isles, wheat straw (thatch) was used for roofing in theBronze Age, and remained in common use until the late 19th century.White wheat bread was historically a high status food, but during the nineteenth century it became in Britain an item of mass consumption, displacingoats,barleyandryefrom diets in the North of the country. It became "a sign of a high degree of culture".After 1860, the enormous expansion ofwheat production in the United Statesflooded the world market, lowering prices by 40%, and (along with the expansion ofpotatogrowing) made a major contribution to the nutritional welfare of the poor.
Some wheat species arediploid, with two sets ofchromosomes, but many are stablepolyploids, with four sets of chromosomes (tetraploid) or six (hexaploid).Einkornwheat (Triticum monococcum) is diploid (AA, two complements of seven chromosomes, 2n=14).Most tetraploid wheats (e.g.emmeranddurumwheat) are derived fromwild emmer,T. dicoccoides. Wild emmer is itself the result of a hybridization between two diploid wild grasses,T. urartuand a wild goatgrass such asAe. speltoides.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 bynatural selection. Hexaploid wheats evolved in farmers' fields as wild emmer hybridized with another goatgrass,Ae. squarrosaorAe. tauschii, to make thehexaploidwheats includingbread wheat.
A 2007molecular phylogenyof the wheats gives the following not fully-resolvedcladogramof major cultivated species; the large amount of hybridisation makes resolution difficult. Markings like "6N" indicate the degree ofpolyploidyof each species:
During 10,000 years of cultivation, numerous forms of wheat, many of themhybrids, have developed under a combination ofartificialandnatural selection. This complexity and diversity of status has led to much confusion in the naming of wheats.
Hexaploid species (6N)
Tetraploid species (4N)
Diploid species (2N)
The wild species of wheat, along with the domesticated varietieseinkorn,emmerandspelt,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. Hulled wheats are often stored as spikelets because the toughened glumes give good protection against pests of stored grain.In free-threshing (or naked) forms, such as durum wheat and common wheat, the glumes are fragile and the rachis tough. On threshing, thechaffbreaks up, releasing the grains.
Classification of wheat greatly varies by the producing country.
Argentina's grain classes were formerly related to the production region or port of shipment:Rosafe(grown inSanta Fe province, shipped throughRosario),Bahia Blanca(grown inBuenos AiresandLa Pampaprovinces and shipped throughBahia Blanca),Buenos Aires(shipped through theport of Buenos Aires). While mostly similar to the US Hard Red Spring wheat, the classification caused inconsistencies, so Argentina introduced three new classes of wheat, with all names using a prefixTrigo Dura Argentina(TDA) and a number.
The grain classification inAustraliais within the purview of its National Pool Classification Panel. Australia chose to measure the protein content at 11% moisture basis.
The decisions on the wheat classification inCanadaare coordinated by the Variety Registration Office of theCanadian Food Inspection Agency. Like in the US system, the eight classes in theWestern Canadaand six classes in theEastern Canadaare based on colour, season, and hardness. Canada has a unique requirement that the varieties of wheat grains should allow for purely visual identification.
The wheat grain classes used in theUnited Statesare named by colour, season, and hardness:
Wheat is a staple cereal worldwide.Rawwheat berriescan be ground intoflouror, using harddurum wheatonly, can be ground intosemolina; germinated and dried creatingmalt; crushed or cut into cracked wheat; parboiled (or steamed), dried, crushed and de-branned intobulguralso known asgroats.If the raw wheat is broken into parts at the mill, as is usually done, the outer husk orbrancan be used in several ways. Wheat is a major ingredient in such foods asbread,porridge,crackers,biscuits,muesli,pancakes,pasta,pies,pastries,pizza,semolina,cakes,cookies,muffins,rolls,doughnuts,gravy,beer,vodka,boza(afermented beverage), andbreakfast cereals.In manufacturing wheat products, gluten is valuable to impartviscoelasticfunctional qualities indough,enabling the preparation of diverse processed foods such as breads, noodles, and pasta that facilitate wheat consumption.
Raw red winter wheat is 13% water, 71%carbohydratesincluding 12%dietary fiber, 13%protein, and 2%fat(table). Some 75–80% of the protein content is asgluten.In a reference amount of 100 grams (3.5 oz), wheat provides 1,368 kilojoules (327 kilocalories) offood energyand is a rich source (20% or more of theDaily Value, DV) of multipledietary minerals, such asmanganese,phosphorus,magnesium,zinc, andiron(table). TheB vitamins,niacin(36% DV),thiamine(33% DV), andvitamin B6(23% DV), are present in significant amounts (table).
Wheat is a significant source ofvegetable proteinsin human food, having a relatively high protein content compared to other major cereals.However, wheat proteins have a low quality for human nutrition, according to theDIAASprotein quality evaluation method.Though they contain adequate amounts of the other essential amino acids, at least for adults, wheat proteins are deficient in theessential amino acidlysine.Because the proteins present in the wheatendosperm(gluten proteins) are particularly poor in lysine,white floursare more deficient in lysine compared with whole grains.Significant efforts in plant breeding are made to develop lysine-rich wheat varieties, without success, as of 2017.Supplementation with proteins from other food sources (mainlylegumes) is commonly used to compensate for this deficiency,since the limitation of a single essential amino acid causes the others to break down and become excreted, which is especially important during growth.
Consumed worldwide by billions of people, wheat is a significant food for human nutrition, particularly in theleast developed countrieswhere wheat products are primary foods.When eaten as thewhole grain, wheat supplies multiple nutrients anddietary fiberrecommended for children and adults.In genetically susceptible people, wheat gluten can triggercoeliac disease.Coeliac disease affects about 1% of the general population indeveloped countries.The only known effective treatment is a strict lifelonggluten-free diet.While coeliac disease is caused by a reaction to wheat proteins, it is not the same as awheat allergy.Other diseasestriggered by eating wheatarenon-coeliac gluten sensitivity(estimated to affect 0.5% to 13% of the general population),gluten ataxia, anddermatitis herpetiformis.Certain short-chain carbohydrates present in wheat, known asFODMAPs(mainlyfructose polymers), may be the cause of non-coeliac gluten sensitivity. As of 2019, reviews have concluded that FODMAPs only explain certain gastrointestinal symptoms, such asbloating, but not theextra-digestive symptomsthat people with non-coeliac gluten sensitivity may develop.Other wheat proteins, amylase-trypsin inhibitors, have been identified as the possible activator of theinnate immune systemin coeliac disease and non-coeliac gluten sensitivity.These proteins are part of the plant's natural defense against insects and may cause intestinalinflammationin humans.
In 2022, world wheat production was 808.4 million tonnes, led by China, India, and Russia which collectively provided 43.22% of the world total.As of 2019,the largest exporters wereRussia (32 million tonnes), United States (27), Canada (23) and France (20), while the largest importers were Indonesia (11 million tonnes), Egypt (10.4) and Turkey (10.0).In 2021, wheat was grown on 220.7 million hectares or 545 million acres worldwide, more than any other food crop.World trade in wheat is greater than for all other crops combined.Global demand for wheat is increasing due to the uniqueviscoelasticand adhesive properties ofglutenproteins, which facilitate the production of processed foods, whose consumption is increasing as a result of the worldwide industrialization process andwesternization of diets.
Wheat became a central agriculture endeavor in the worldwideBritish Empirein the 19th century, and remains of great importance in Australia, Canada and India.In Australia, with vast lands and a limited work force, expanded production depended on technological advances, especially regarding irrigation and machinery. By the 1840s there were 900 growers inSouth Australia. They used "Ridley's Stripper", a reaper-harvester perfected byJohn Ridleyin 1843,to remove the heads of grain.  In Canada, modern farm implements made large scale wheat farming possible from the late 1840s. By 1879,Saskatchewanwas the center, followed byAlberta,ManitobaandOntario, as the spread of railway lines allowed easy exports to Britain. By 1910, wheat made up 22% of Canada's exports, rising to 25% in 1930 despite the sharp decline in prices during the worldwideGreat Depression.Efforts to expand wheat production in South Africa, Kenya and India were stymied by low yields and disease. However, by 2000 India had become the second largest producer of wheat in the world.In the 19th century the American wheat frontier moved rapidly westward. By the 1880s 70% of American exports went to British ports. The first successfulgrain elevatorwas built in Buffalo in 1842.The cost of transport fell rapidly. In 1869 it cost 37 cents to transport a bushel of wheat fromChicagotoLiverpool. In 1905 it was 10 cents.
In the 20th century, global wheat output expanded by about 5-fold, but until about 1955 most of this reflected increases in wheat crop area, with lesser (about 20%) increases in crop yields per unit area. After 1955 however, there was a ten-fold increase in the rate of wheat yield improvement per year, and this became the major factor allowing global wheat production to increase. Thus technological innovation and scientific crop management withsynthetic nitrogen fertilizer, irrigation and wheat breeding were the main drivers of wheat output growth in the second half of the century. There were some significant decreases in wheat crop area, for instance in North America.Better seed storage and germination ability (and hence a smaller requirement to retain harvested crop for next year's seed) is another 20th-century technological innovation. In Medieval England, farmers saved one-quarter of their wheat harvest as seed for the next crop, leaving only three-quarters for food and feed consumption. By 1999, the global average seed use of wheat was about 6% of output.In the 21st century, rising temperatures associated withglobal warmingare reducing wheat yield in several locations.
Wheat is anannualcrop. It can be planted in autumn and harvested in early summer aswinter wheatin climates that are not too severe, or planted in spring and harvested in autumn as spring wheat. It is normally planted aftertillingthe soil byploughingand thenharrowingto kill weeds and create an even surface. The seeds are then scattered on the surface, ordrilledinto the soil in rows. Winter wheat lies dormant during a winter freeze. It needs to develop to a height of 10 to 15 cm before the cold intervenes, so as to be able to survive the winter; it requires a period with the temperature at or near freezing, itsdormancythen being broken by the thaw or rise in temperature. Spring wheat does not undergo dormancy. Wheat requires a deepsoil, preferably aloamwith organic matter, and available minerals including soil nitrogen, phosphorus, and potassium. An acid andpeatysoil is not suitable. Wheat needs some 30 to 38 cm of rain in the growing season to form a good crop of grain.
The farmer may intervene while the crop is growing to addfertilizer, water byirrigation, or pesticides such asherbicidesto kill broad-leaved weeds orinsecticidesto kill insect pests. The farmer may assess soil minerals, soil water, weed growth, or the arrival of pests to decide timely and cost-effective corrective actions, and crop ripeness and water content to select the right moment to harvest. Harvesting involvesreaping, cutting the stems to gather the crop; andthreshing, breaking the ears to release the grain; both steps are carried out by acombine harvester. The grain is then dried so that it can be stored safe frommouldfungi.
Wheat normally needs between 110 and 130 days between sowing and harvest, depending upon climate, seed type, and soil conditions. Optimal crop management requires that the farmer have a detailed understanding of each stage of development in the growing plants. In particular, springfertilizers,herbicides,fungicides, andgrowth regulatorsare typically applied only at specific stages of plant development. For example, it is currently recommended that the second application of nitrogen is best done when the ear (not visible at this stage) is about 1 cm in size (Z31 onZadoks scale). Knowledge of stages is also important to identify periods of higher risk from the climate. Farmers 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 so should be preserved from disease or insect attacks to ensure a good yield. Several systems exist to identify crop stages, with theFeekesand 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.For example, the stage of pollen formation from the mother cell, and the stages betweenanthesisand maturity, are susceptible to high temperatures, and this adverse effect is made worse by water stress.
Technological advances in soil preparation and seed placement at planting time, use ofcrop rotationandfertilizersto improve plant growth, and advances in harvesting methods have all combined to promote wheat as a viable crop. When the use ofseed drillsreplaced broadcasting sowing of seed in the 18th century, another great increase in productivity occurred. Yields of pure wheat per unit area increased as methods of crop rotation were applied to land that had long been in cultivation, and the use of fertilizers became widespread.
Improved agricultural husbandry has more recently included pervasiveautomation, starting with the use ofthreshing machines,and progressing to large and costly machines like thecombine harvesterwhich greatly increased productivity.At the same time, better varieties such asNorin 10 wheat, developed in Japan in the 1930s,or the dwarf wheat developed byNorman Borlaugin theGreen Revolution, greatly increased yields.
In addition to gaps in farming system technology and knowledge, some large wheat grain-producing countries have significant losses after harvest at the farm and because of poor roads, inadequate storage technologies, inefficient supply chains and farmers' inability to bring the produce into retail markets dominated by small shopkeepers. Some 10% of total wheat production is lost at farm level, another 10% is lost because of poor storage and road networks, and additional amounts are lost at the retail level.
In thePunjab regionof the Indian subcontinent, as well as North China, irrigation has been a major contributor to increased grain output. More widely over the last 40 years, a massive increase in fertilizer use together with the increased availability of semi-dwarf varieties in developing countries, has greatly increased yields per hectare.In developing countries, use of (mainly nitrogenous) fertilizer increased 25-fold in this period. However, farming systems rely on much more than fertilizer and breeding to improve productivity. A good illustration of this is Australian wheat growing in the southern winter cropping zone, where, despite low rainfall (300 mm), wheat cropping is successful even with relatively little use of nitrogenous fertilizer. This is achieved by crop rotation with leguminous pastures. The inclusion  of acanolacrop in the rotations has boosted wheat yields by a further 25%.In these low rainfall areas, better use of available soil-water (and better control of soil erosion) is achieved by retaining the stubble after harvesting and by minimizing tillage.
Pests and diseases consume 21.47% of the world's wheat crop annually.
There are many wheat diseases, mainly caused by fungi, bacteria, andviruses.Plant breedingto develop new disease-resistant varieties, and sound crop management practices are important for preventing disease. Fungicides, used to prevent the significant crop losses from fungal disease, can be a significant variable cost in wheat production. Estimates of the amount of wheat production lost owing to plant diseases vary between 10 and 25% in Missouri.A wide range of organisms infect wheat, of which the most important are viruses and fungi.
The main wheat-disease categories are:
A historically significant disease of cereals including wheat, though commoner inryeisergot; it is unusual among plant diseases in also causing sickness in humans who ate grain contaminated with the fungus involved,Claviceps purpurea.
Among insect pests of wheat is thewheat stem sawfly,
a chronic pest in the Northern Great Plains of the United States and in theCanadian Prairies.Wheat is the food plant of thelarvaeof someLepidoptera(butterflyandmoth) species includingthe flame,rustic shoulder-knot,setaceous Hebrew characterandturnip moth. Early in the season, many species of birds and rodents feed upon wheat crops. These animals can cause significant damage to a crop by digging up and eating newly planted seeds or young plants. They can also damage the crop late in the season by eating the grain from the mature spike. Recent post-harvest losses in cereals amount to billions of dollars per year in the United States alone, and damage to wheat by various borers, beetles and weevils is no exception.Rodents can also cause major losses during storage, and in major grain growing regions, field mice numbers can sometimes build up explosively to plague proportions because of the ready availability of food.To reduce the amount of wheat lost to post-harvest pests,Agricultural Research Servicescientists have developed an "insect-o-graph", which can detect insects in wheat that are not visible to the naked eye. The device uses electrical signals to detect the insects as the wheat is being milled. The new technology is so precise that it can detect 5–10 infested seeds out of 30,000 good ones.
In traditional agricultural systems, wheat populations consist oflandraces, informal farmer-maintained populations that often maintain high levels of morphological diversity. Although landraces of wheat are no longer extensively grown in Europe and North America, they continue to be important elsewhere. The origins offormal wheat breedinglie 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 ofMendelian genetics. The standard method of breeding inbred wheat cultivars is by crossing two lines using hand emasculation, then selfing or inbreeding the progeny. Selections areidentified(shown to have the genes responsible for the varietal differences) ten or more generations before release as a variety or cultivar.
Major breeding objectives include highgrain yield, good quality,disease-and insect resistance and tolerance to abiotic stresses, including mineral, moisture and heat tolerance.Wheat has been the subject ofmutation breeding, with the use ofgamma-,x-rays,ultraviolet light(collectively,radiation breeding), and sometimes harsh chemicals. The varieties of wheat created through these methods are in the hundreds (going as far back as 1960), more of them being created in higher populated countries such as China.Bread wheat with high grain iron and zinc content has been developed through gamma radiation breeding,and through conventional selection breeding.International wheat breeding is led by the International Maize and Wheat Improvement Center in Mexico.ICARDAis another major public sector international wheat breeder, but it was forced to relocate from Syria to Lebanon in theSyrian Civil War.
Pathogens and wheat are in a constant process ofcoevolution.Spore-producing wheat rusts are substantiallyadaptedtowards successful spore propagation, which is essentially to say itsR0.These pathogens tend towards high-R0evolutionary attractors.
The presence of certain versions of wheat genes has been important for crop yields. Genes for the 'dwarfing' trait, first used by Japanese wheat breeders to produceNorin 10short-stalked wheat, have had a huge effect on wheat yields worldwide, and were major factors in the success of theGreen Revolutionin Mexico and Asia, an initiative led byNorman Borlaug.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.
T. turgidumsubsp.polonicum, known for its longerglumesand grains, has been bred into main wheat lines for its grain size effect, and likely has contributed these traits toTriticum petropavlovskyiand the PortugueselandracegroupArrancada.As with many plants,MADS-boxinfluences flower development, and more specifically, as with other agricultural Poaceae, influences yield. Despite that importance, as of 2021little research has been done into MADS-box and other such spikelet and flower genetics in wheat specifically.
The world record wheat yield is about 17tonnesperhectare(15,000 pounds per acre), reached in New Zealand in 2017.A project in the UK, led byRothamsted Researchhas aimed to raise wheat yields in the country to 20 t/ha (18,000 lb/acre) by 2020, but in 2018 the UK record stood at 16 t/ha (14,000 lb/acre), and the average yield was just 8 t/ha (7,100 lb/acre).
Wild grasses in the genusTriticumand related genera, and grasses such asryehave been a source of many disease-resistance traits for cultivated wheatbreedingsince the 1930s.Someresistance geneshave been identified againstPyrenophora tritici-repentis, especially races 1 and 5, those most problematic inKazakhstan.Wild relative,Aegilops tauschiiis the source of several genes effective againstTTKSK/Ug99 -Sr33,Sr45,Sr46, andSrTA1662- of whichSr33andSrTA1662are the work of Olsonet al., 2013, andSr45andSr46are also briefly reviewed therein.
Resistance to Fusarium head blight(FHB, Fusarium ear blight) is also an important breeding target.Marker-assisted breedingpanels involvingkompetitive allele specific PCRcan be used. Singh et al. 2019 identify a KASPgenetic markerfor apore-forming toxin-like gene providing FHB resistance.
In 2003 the first resistance genes against fungal diseases in wheat were isolated.In 2021, novel resistance genes were identified in wheat againstpowdery mildewandwheat leaf rust.Modified resistance genes have been tested in transgenic wheat and barley plants.
Because wheat self-pollinates, creatinghybrid seedto provide the possible benefits ofheterosis, hybrid vigor (as in the familiar F1 hybrids of maize), is extremely labor-intensive; the high cost of hybrid wheat seed relative to its moderate benefits have kept farmers from adopting them widelydespite nearly 90 years of effort.Commercial hybrid wheat seed has been produced using chemical hybridizing agents,plant growth regulatorsthat selectively interfere with pollen development, or naturally occurringcytoplasmic male sterilitysystems. Hybrid wheat has been a limited commercial success in Europe (particularly France), the United States and South Africa.
Synthetic hexaploids made by crossing the wild goatgrass wheat ancestorAegilops tauschii,and otherAegilops,and various durum wheats are now being deployed, and these increase the genetic diversity of cultivated wheats.
Modern bread wheat varieties have beencross-bredto contain greater amounts of gluten,which affords significant advantages for improving the quality of breads and pastas from a functional point of view.However, a 2020 study that grew and analyzed 60 wheat cultivars from between 1891 and 2010 found no changes in albumin/globulin and gluten contents over time. "Overall, the harvest year had a more significant effect on protein composition than the cultivar. At the protein level, we found no evidence to support an increasedimmunostimulatorypotential of modern winter wheat."
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 watertranspiration. Basic physiological investigation of these gas exchange processes has yielded carbonisotopebased method used for breeding wheat varieties with improved water-use efficiency. These varieties can improve crop productivity in rain-fed dry-land wheat farms.
The complex genome of wheat has made its improvement difficult. Comparison of hexaploid wheat genomes using a range of chromosome pseudomolecule and molecular scaffold assemblies in 2020 has enabled the resistance potential of its genes to be assessed. Findings include the identification of "a detailed multi-genome-derived nucleotide-binding leucine-rich repeat protein repertoire" which contributes to disease resistance, while the geneSm1provides a degree of insect resistance,for instance against theorange wheat blossom midge.
In 2010, 95% of the genome of Chinese Spring line 42 wheat was decoded.This genome was released in a basic format for scientists and plant breeders to use but was not fully annotated.In 2012, an essentially complete gene set of bread wheat was published.Random shotgun librariesof total DNA and cDNA from theT. aestivumcv. Chinese Spring (CS42) were sequenced to generate 85 Gb of sequence (220 million reads) and identified between 94,000 and 96,000 genes.In 2018, a more complete Chinese Spring genome was released by a different team.In 2020, 15 genome sequences from various locations and varieties around the world were reported, with examples of their own use of the sequences to localize particular insect and disease resistance factors.Wheat Blast Resistanceis controlled byR geneswhich are highly race-specific.
For decades, the primarygenetic modificationtechnique has beennon-homologous end joining(NHEJ). However, since its introduction, theCRISPR/Cas9tool has been extensively adopted, for example:
As of 2021these examples illustrate the rapid deployment and results that CRISPR/Cas9 has shown in wheat disease resistance improvement.
The Dutch artistVincent van Goghcreated the seriesWheat Fieldsbetween 1885 and 1890, consisting of dozens of paintings made mostly in different parts of rural France. They depict wheat crops, sometimes with farm workers, in varied seasons and styles, sometimes green, sometimes at harvest.Wheatfield with Crowswas one of his last paintings, and is considered to be among his greatest works.
In 1967, the American artistThomas Hart Bentonmade his oil on wood paintingWheat, showing a row of uncut wheat plants, occupying almost the whole height of the painting, between rows of freshly-cut stubble. The painting is held by theSmithsonian American Art Museum.
In 1982, the American conceptual artistAgnes Denesgrew a two-acre field of wheat atBattery Park, Manhattan. Theephemeral artworkhas been described as an act of protest. The harvested wheat was divided and sent to 28 world cities for an exhibition entitled "The International Art Show for the End of World Hunger".