Wheat (
Triticum spp.)
[1][2] is a
cereal grain, originally from the
Levant region of the
Near East but now cultivated worldwide. In 2013, world production of wheat was 713 million tons, making it the third most-produced
cereal after
maize (1,016 million tons) and
rice (745 million tons).
[3]
Wheat was the second most-produced cereal in 2009; world production in
that year was 682 million tons, after maize (817 million tons), and with
rice as a close third (679 million tons).
[4]
This grain is grown on more land area than any other commercial food.
[citation needed] World trade in wheat is greater than for all other crops combined.
[5]
Globally, wheat is the leading source of vegetable protein in human
food, having a higher protein content than other major cereals, maize
(corn) or rice.
[6]
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 Asian
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[7] and for
fermentation to make
beer,
[8] other
alcoholic beverages,
[9] and
biofuel.
[10]
There are six wheat classifications: 1) hard red winter, 2) hard red
spring, 3) soft red winter, 4) durum (hard), 5) hard white, and 6) soft
white wheat.
[citation needed]
The hard wheats have the most amount of gluten and are used for making
bread, rolls and all-purpose flour. The soft wheats are used for making
flat bread, cakes, pastries, crackers, muffins, and biscuits. A high
percentage of wheat production in the EU is used as animal feed, often
surplus to human requirements or low-quality wheat.
[11]
Wheat is planted to a limited extent as a
forage crop for livestock, although the straw cannot be used as feed.
[12] Its straw can be used as a construction material for roofing
thatch.
[13][14] 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.
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. Recent findings estimate the first domestication of wheat down to a small region of southeastern Turkey,
[15] and domesticated
Einkorn wheat at
Wadi el Jilat in
Jordan—has been dated to 7,500-7,300
BCE.
[16]
Origin
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 (inside the
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.
[17]
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.
Cultivation of wheat began to spread beyond the Fertile Crescent after about 8000 BCE.
Jared Diamond traces the spread of cultivated
emmer wheat starting in the Fertile Crescent sometime before 8800 BCE. Archaeological analysis of wild
emmer indicates that it was first cultivated in the southern
Levant with finds dating back as far as 9600 BCE.
[18][19] 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.
[20] 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 BCE.
[21]
Remains of harvested emmer from several sites near the Karacadag Range have been dated to between 8600 (at
Cayonu) and 8400 BCE (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 BCE. 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.
[22]
The cultivation of emmer reached Greece, Cyprus and India by
6500 BCE, Egypt shortly after 6000 BCE, and Germany and Spain by
5000 BCE.
[23]
"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."
[24] By 3000 BCE, 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 approximately 1350 BCE at
Assiros in Greek Macedonia.
[25]
From Asia, wheat continued to spread throughout Europe. In England,
wheat straw (thatch) was used for roofing in the Bronze Age, and was in
common use until the late 19th century.
[26]
Farming techniques
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 BCE) 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 pure 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 expansion of wheat production occurred as new arable land was
farmed in the Americas and Australia in the 19th and 20th centuries.
Genetics
 |
| Green wheat a month before harvest |
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, an initiative led by
Norman 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.
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][31][32]
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.
[33]
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).
[34]
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.
[35]
On 29 November 2012, an essentially complete gene set of bread wheat has been published.
[36] Random shotgun libraries of total DNA and cDNA from the
T. aestivum
cv. Chinese Spring (CS42) were sequenced in Roche 454 pyrosequencer
using GS FLX Titanium and GS FLX+ platforms to generate 85 Gb of
sequence (220 million reads), equivalent to 5X genome coverage and
identified between 94,000 and 96,000 genes.
[36]
This sequence data provides direct access to about 96,000 genes,
relying on orthologous gene sets from other cereals. and represents an
essential step towards a systematic understanding of biology and
engineering the cereal crop for valuable traits. Its implications in
cereal genetics and breeding includes the examination of genome
variation, association mapping using natural populations, performing
wide crosses and alien introgression, studying the expression and
nucleotide polymorphism in transcriptomes, analyzing population genetics
and evolutionary biology, and studying the epigenetic modifications.
Moreover, the availability of large-scale genetic markers generated
through NGS technology will facilitate trait mapping and make
marker-assisted breeding much feasible.
[37]
Moreover, the data not only facilitate in deciphering the complex
phenomena such as heterosis and epigenetics, it may also enable breeders
to predict which fragment of a chromosome is derived from which parent
in the progeny line, thereby recognizing crossover events occurring in
every progeny line and inserting markers on genetic and physical maps
without ambiguity. In due course, this will assist in introducing
specific chromosomal segments from one cultivar to another. Besides, the
researchers had identified diverse classes of genes participating in
energy production, metabolism and growth that were probably linked with
crop yield, which can now be utilized for the development of transgenic
wheat. Thus whole genome sequence of wheat and the availability of
thousands of SNPs will inevitably permit the breeders to stride towards
identifying novel traits, providing biological knowledge and empowering
biodiversity-based breeding.
[37]
Plant breeding
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.
[38]
The major breeding objectives include high grain yield, good quality,
disease and insect resistance and tolerance to abiotic stresses,
including 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.
Wheat has also been the subject of
mutation breeding,
with the use of gamma, x-rays, ultraviolet light, and sometimes harsh
chemicals. The varieties of wheat created through this methods are in
the hundreds (varieties being as far back as 1960), more of them being
created in higher populated countries such as China.
[39]
Hybrid wheat
Because wheat self-pollinates, creating
hybrid varieties
is extremely labor-intensive; the high cost of hybrid wheat seed
relative to its moderate benefits have kept farmers from adopting them
widely
[40][41] despite nearly 90 years of effort.
[42] 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 perfect in the botanical sense, meaning they have both male and female parts, and normally
self-pollinate.
[38] 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.
[43]
Hulled versus free-threshing wheat
The four wild species of wheat, along with the domesticated varieties
einkorn,
[44] emmer[45] and
spelt,
[46]
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.
[44]
Naming
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.[14]
- Protein content. Bread wheat protein content ranges from 10% in some soft wheats with high starch contents, to 15% in hard wheats.
- The quality of the wheat protein gluten.
This protein can determine the suitability of a wheat to a particular
dish. A strong and elastic gluten present in bread wheats enables dough to trap carbon dioxide during leavening, but elastic gluten interferes with the rolling of pasta into thin sheets. The gluten protein in durum wheats used for pasta is strong but not elastic.
- Grain color (red, white or amber). Many wheat varieties are
reddish-brown due to phenolic compounds present in the bran layer which
are transformed to pigments by browning enzymes. White wheats have a
lower content of phenolics and browning enzymes, and are generally less
astringent in taste than red wheats. The yellowish color of durum wheat
and semolina flour made from it is due to a carotenoid pigment called lutein, which can be oxidized to a colorless form by enzymes present in the grain.
Major cultivated species of wheat[citation needed]
Hexaploid Species
- Common wheat or Bread wheat (T. aestivum) – A hexaploid species that is the most widely cultivated in the world.
- Spelt (T. spelta) –
Another hexaploid species cultivated in limited quantities. Spelt is
sometimes considered a subspecies of the closely related species common wheat (T. aestivum), in which case its botanical name is considered to be Triticum aestivum subsp. spelta.
Tetraploid Species
- Durum (T. durum) – The only tetraploid form of wheat widely used today, and the second most widely cultivated wheat.
- Emmer (T. dicoccon) – A tetraploid species, cultivated in ancient times but no longer in widespread use.
- Khorasan
(Triticum turgidum ssp. turanicum also called Triticum turanicum) is a
tetraploid wheat species.[2] It is an ancient grain type; Khorasan
refers to a historical region in modern-day Afghanistan and the
northeast of Iran. This grain is twice the size of modern-day wheat and
is known for its rich nutty flavor.[3]
Diploid Species
- Einkorn (T. monococcum) – A diploid species with wild and cultivated variants. Domesticated at the same time as emmer wheat, but never reached the same importance.
Classes used in the
United States:
- Durum – Very hard, translucent, light-colored grain used to make semolina flour for pasta & bulghur; high in protein, specifically, gluten protein.
- Hard Red Spring – Hard, brownish, high-protein
wheat used for bread and hard baked goods. Bread Flour and high-gluten
flours are commonly made from hard red spring wheat. It is primarily
traded at the Minneapolis Grain Exchange.
- Hard Red Winter – Hard, brownish, mellow high-protein wheat
used for bread, hard baked goods and as an adjunct in other flours to
increase protein in pastry flour for pie crusts. Some brands of
unbleached all-purpose flours are commonly made from hard red winter
wheat alone. It is primarily traded on the Kansas City Board of Trade. One variety is known as "turkey red wheat", and was brought to Kansas by Mennonite immigrants from Russia.[47]
- Soft Red Winter – Soft, low-protein wheat used for cakes, pie crusts, biscuits, and muffins. Cake flour, pastry flour, and some self-rising flours with baking powder and salt added, for example, are made from soft red winter wheat. It is primarily traded on the Chicago Board of Trade.
- Hard White – Hard, light-colored, opaque, chalky, medium-protein wheat planted in dry, temperate areas. Used for bread and brewing.
- Soft White – Soft, light-colored, very low protein wheat
grown in temperate moist areas. Used for pie crusts and pastry. Pastry
flour, for example, is sometimes made from soft white winter wheat.
Red wheats may need bleaching; therefore, white wheats usually command higher prices than red wheats on the commodities market.
As a food
Raw wheat can be ground into flour or, using hard durum wheat only, can be ground into semolina; germinated and dried creating malt; crushed or cut into cracked wheat; parboiled (or steamed), dried, crushed and de-branned into bulgur also known as groats. If the raw wheat is broken into parts at the mill, as is usually done, the outer husk or bran can be used several ways. Wheat is a major ingredient in such foods as bread, porridge, crackers, biscuits, Muesli, pancakes, pies, pastries, cakes, cookies, muffins, rolls, doughnuts, gravy, boza (a fermented beverage), and breakfast cereals (e.g., Wheatena, Cream of Wheat, Shredded Wheat, and Wheaties).
Commercial use
Harvested wheat grain that enters trade is classified according to grain properties for the purposes of the
commodity markets.
Wheat buyers use these to decide which wheat to buy, as each class has
special uses, and producers use them to decide which classes of wheat
will be most profitable to cultivate.
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, for
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