1. Wheat originated in southwest Asia and is descended from crosses between wild grasses and wheat species. It was first domesticated before 7000 BC and spread throughout Europe and later to North and South America. 2. Wheat is the most important cereal crop worldwide and a staple food in many countries. Major producers include China, India, USA, Russia, and Canada. 3. There are two main types - winter wheat planted in fall and spring wheat planted in spring. Winter wheat can survive cold temperatures while spring wheat is damaged by frost.

1. Taxonomy
Family Poaceae
Genus Triticum L.
Species Triticum aestivum L
Origin and history
The origin of wheat is believed to be southwestern Asia. A cross between wild Emmer
wheat (Triticum dicoccoides) Aegilops squarrosa, a grass, produced a spelt-like plant.
This suggests that the common or bread wheat (T. aestivum) is descended from a cross
between spelt and the progenitor of Persian wheat (T. persicum). Persian wheat occurs in
the wild in the Russian Caucasus. The Persian wheat probably is descended from the
wheat of the Neolithic Swiss lake dwellers, which in turn might have originated from
across between einkorn and a grass, Agropyron triticum. Archeological findings indicate
that Emmer wheat was cultivated before 7000 BC. Similarly, wheat was cultivated in
Europe in prehistoric times. In the United States, wheat was first cultivated along the
Atlantic Coast in early seventeenth century, moving westwards as the country was settled
Economic importance
Wheat is the most important cereal grain crop in the world. It is the principal cereal grain
crop used for food consumption in the United States and most parts of the world. In the
United States, it usually ranks fourth after corn, hay, and soybeans in that order of
importance. Wheat is grown commercially in nearly every state in the USA, with a
concentration of production in the Great Plains, an area spanning the states from Texas to
Montana. USDA production trends indicate that in 1866, wheat was harvested from an
area of 15.4 million acres, yielding an average of 11bushels/acre. By 1950, production
occurred on 61.6 million acres, with an average yield of 16.5 bushels/acre. In 1990, the
acreage was 69.2 million acres, yield average being 39.5bushels/acre. The Central and
Southern Plains (Texas, Oklahoma, Kansas, among others) produce more than the
Northern Plains (e.g., Montana, North Dakota) the two regions accounting for two-thirds
of the US wheat production and about 80% of wheat acreage. Kansas leads all states in
wheat production. Hard white wheat is also grown in the Central Plains and Northern
Plains.
World production of wheat in 2001 was 583.9 million metric tons, occurring on 219.5
million acres. World wheat consumption in that period was 590.6 million tons.
Developing countries (excluding those in Eastern Europe or the former Soviet Union)
account for nearly 50% of the world’s wheat production, the leading producers being
China, India, Turkey, Pakistan, and Argentina. The success of wheat production in these
countries is credited to the impact of the Green Revolution that occurred in the 1960s and
1970s. In 2000, China produced 111.9 million metric tons while India produced 26.5
metric tons. Latin America and Asia (excluding China and India) each produce about 20
million metric tons a year. Wheat is produced in Europe, including the United Kingdom,

2. Denmark, The Netherlands, Belgium, Switzerland, and West Germany.
Adaptation
Wheat is best adapted to cool temperate climates where rainfall is not excessive (15–
24inches/annum). Based on season of production, there are two types of wheat – winter
wheat and spring wheat.
 Winter wheat
Winter wheat is sown in the fall (autumn) so that it can have some growth before the
onset of cold weather in winter. Growth ceases and the plants remain dormant through
winter, resuming growth in spring for harvesting in summer. About two-thirds of US
wheat is winter wheat. Winter wheat can survive cold temperatures as low as –40F if
protected by snow.
 Spring wheat
Spring wheat is less tolerant of low temperatures and is damaged by even a light frost of
28–30F. Spring wheat is planted in early spring and harvested in July–August. Wheat is a
long day plant. Short days of high temperatures stimulate tillering and leaf formation but
delay flowering of wheat plants. Early maturing cultivars are available for production
under any photoperiod conditions. However, the quality (nutritional uses such as baking)
of wheat is influenced by the production environment. For example, growing hard wheats
in soft wheat regions results in grains that are starchy or “yellow berry” (soft and
starchy).
History of breeding in the United States
Wheat is one of a few food crops (the others being corn and rice) that have been
associated with the Nobel Peace Prize. The 1970 Nobel Peace Prize awarded to Norman
Borlaug, the Father of the Green Revolution, recognized his contribution to agricultural
productivity through the introduction of superior genotypes of wheat. These superior
varieties were high yielding, shorter (semidwarf wheat) more lodging resistant, and
responsive to high levels of fertilizer. A significant contributor to this effort was Orville
Vogel, a USDA wheat breeder stationed at Washington State University. Under his
leadership, the first successful commercial semidwarf wheat variety in the Western
Hemisphere was released to farmers in 1961. This variety, Gaines, was a soft white wheat
and yielded in excess of 100bushels/acre under both dry-land and irrigated production. In
spite of its agronomic qualities, Gaines had milling quality problems. In response to the
demands of the milling industry, a new selection with more desirable milling qualities,
called Nugaines, was released in 1965.
Commercial wheat classes
Wheat breeders specialize in one of the special market classes of wheat. There is a

3. genetic basis for this classification. Wheat may be classified into seven groups based on
the time of year of planting and kernel characteristics (hardness, color, shape). However,
for commercial production, the varieties may be narrowed down to six basic classes: hard
red winter, soft red winter, hard red spring, hard white, soft white, and durum wheat. The
hard red wheat accounts for about 40% of total US wheat production and is the dominant
class in US wheat export.
1. Hard red winter wheat
This is grown mainly in the Great Plains (Kansas, Oklahoma, Nebraska, Texas,
Colorado). It is also grown in the former Soviet Union, Argentina, and Danube Valley of
Europe. It is used for bread flour.
2. Hard red spring wheat
This class of wheat is grown in regions with severe winters in the North Central states
(North Dakota, Montana, South Dakota, Minnesota). It is also produced in Canada,
Russia, and Poland. It is the standard wheat for bread flour.
3.Soft red winter wheat
This class of wheat is grown predominantly in the eastern United States (Ohio, Missouri,
Indiana, Illinois, Pennsylvania). It is also grown in Western Europe. Soft red winter
wheats are used mainly for pastry, cake, biscuit, and household flour. For bread making, it
needs to be blended with hard red wheat flour.
4. White wheat
White wheat (hard or soft) is produced in the four western states (North Dakota, South
Dakota, Nebraska, Minnesota) and in the northeastern states (Washington, Oregon,
Michigan, California, New York). Some of this is club wheat. It is produced also in
northern, eastern, and southern Europe, Australia, South Africa, South America, and Asia.
5 .Durum wheat
Durum wheat is grown mainly in North Dakota, South Dakota, and Minnesota. Other
smaller production states are California, Arizona, Oregon, and Texas. Elsewhere, it is
grown in North Africa, Southern Europe, and the former Soviet Union. Durum wheat is
used in making semolina, which is used for producing products such as macaroni and
spaghetti.
Germplasm resources
Plant breeders have access to over 400 000 accessions in natural and international
germplasm banks. These banks include the USDA National Seed Storage Lab at Fort
Collins, Colorado, the CIMMYT, in Mexico, and the N.I. Vavilov All-Union Institute of
Plant Industry, St Petersburg, Russia. Over 40 000 accessions are held at Aberdeen,
Idaho, as a working collection and parts of the United States National Small Grains
Collection.
Cytogenetics
The species of Triticum are grouped into three ploidy classes: Diploid (2n¼2x¼14),

4. tetraploid (2n¼ 2x¼28), and hexaploid (2n¼6x¼42). The cytoplasmic male sterility
(cms) gene used in modern wheat breeding is derived from T. timopheevii, a wild
tetraploid variety. Three genomes (A, B, D) comprise the polyploid series of wheat. The
A genome comes from T. monococcum, while the D comes from Aegilops squarrosa (or
T. tauschii). The origin of the B genome is debatable. The genomic formula of the ploidy
classes are AA or BB for diploids and AABB for the tetraploid or Emmer wheat.
Common wheat (T. aestivum) is an allohexaploid of genomic formula AABBDD. In
hexaploid wheat, the 21 chromosomes are divided into seven homeologous groups
(partially homologous chromosomes) identified group numbers from 1 to 7. The three
chromosomes within the ABD homeologous group usually share some loci in common
for a specific trait. An example of this is the two genes for rust resistance that occur on
chromosome 2A, three genes on 2B, and three genes on 2D. Tetraploid and hexaploid
wheat reproduce naturally as diploids (2n¼28 or 2n¼42). This reproductive mechanism
is made possible by the presence of a gene on chromosome 5B, Ph1, which enables
diploid pairing to occur. The Ph1 gene causes truly homologous paring within the same
genome. When absent, paring between one chromosome and a homeologous chromosome
from another genome is possible. The homeology that exists in its three component
genomes allows the species to tolerate a range of aneuploidy. T. aestivum exhibits vigor
and morphology similar to disomic wheat. Among other applications, aneuploidy has
been used to locate genes that confer agronomically important traits (e.g., the mlo locus
for resistance to powdery mildew). Classical wheat genetics was advanced through the
work of E. R. Sears of the University of Missouri. He developed a compatible set of the
possible 21 monosomics (2n1) of wheat and sets of related aneuploid forms in the
hexaploid wheat cultivar, “Chinese Spring”. Introgression of alien genes is problematic
because of the lack of ability for crossing between hexaploid and diploid species, as well
as the numerous problems that manifest at various stages in the ontogeny of the hybrid.
Genes for crossing (kr1kr1, kr2kr2) located on chromosomes 5B,5 A, and 5 D,
respectively) have been identified in Chinese Spring wheat, which facilitates wheat rye
cross. Some breeders also use genetic bridges and chromosome number doubling to
overcome problems with ploidy differences. Alien autotetraploids of Agropyron
cristatum, Psathyrostachys juncea, especially, have been used to overcomehexaploid
diploid alien species crossability barriers. Generally, in practice, the parent with the
higher ploidy is used as the female in crosses. However, successes with the reverse have
also been recorded. Widening the genetic base of T. aestivum through intergeneric crosses
often involves complex wheat and alien chromosome combinations. Research has shown
that alien genes must be epistatic to those of wheat or interact with them to produce the
desired effect. Modifications of the expression of disease and pest resistance genes
usually occur when they are introduced into a new genetic background. Nonetheless,
successes with spontaneous translocations have been reported in triticale/wheat crosses.
One of the notable induced translocations was conducted by Sears and involved
chromosome 6B and an Ae. umbelluta chromosome, resulting in leaf rust resistance in the
release cultivar, “Transfer”. Fertile. wheat alien amphiploids can result from
chromosome doubling, the most successful so far being triticale (wheat rye). Other wheat
x alien amphiploids are less successful, being of poor fertility and often exhibiting
undesirable alien traits. The technique of alien chromosome additions has been used in an
attempt to reduce the undesirable effects introduced by the wild species.

5. Genetics
Dwarfing genes occur in wheat and have been used in breeding to develop cultivars with
short stature (semidwarf wheat) (see Green Revolution in Chapter 1). Early work in Japan
produced dwarfing genes. Designated Rht, over 20 dwarfing genes have been identified,
the most commonly used in wheat breeding including Rht1, Rht2, andRht8. The first two,
called the Norin 10 dwarfing genes , also belong to a group of dwarfing genes called
gibberellic acid-insensitive dwarfing genes. Cultivars with these genes fail to respond to
the application of gibberllic acid. Rht3 and Rht10 genes confer extreme dwarfism on
plants, the latter more so in its effect. Practical application to commercial breeding is yet
to materialize. Rht4 and Rht8 plus others are called the gibberellic acid sensitive dwarfing
genes. Monosomic analysis was used to locate theRht1 gene and Rht2 gene on
chromosomes 4A and 4D, respectively. Chromosome substitution can be used to transfer
these genes in a breeding program. The dwarfing genes increase grain yield by increasing
tillering and number of seeds per plant. Other genes of interest in wheat breeding include
awnedness, pubescence, grain color, and glume color. The awnedness trait is inhibited by
three dominant alleles at three independent loci. Hd conditions hooded awn, while B1 and
B2 condition awnless or tipped awned phenotype. A genotype of hdb1b2 produces a
bearded or fully awned phenotype. Pubescence in the glume and other parts of the plant is
conditioned by a variety of dominant alleles, for example, Hg producing hairy glume,
while Hp conditions hairy peduncle. Red grain color is conditioned by three independent
dominant alleles acting in additive fashion (R1R2R3), while white grain occurs when the
genotype is r1r2r3. Consequently, when all three alleles occur in one genotype, the seed
color is very dark red. Anthocyanin pigmentation occurs in various parts of the plant. For
example, red auricles are conditioned by a single dominant allele, Ra. The red color of
glumes is controlled by two dominant alleles, Rg1 and Rg2, while photoinsensitivity is
controlled by alleles at three independent loci, designated ppd1, ppd2, and ppd3.
General botany
Wheat (Tricticum spp) is an annual plant. It has a spikelet inflorescence. A floret is
composed of a lemma, palea, and a caryopsis or grain that has a deep furrow and a hairy
tip or brush. The floret may be awned or awnless. Awned varieties are common in regions
of low rainfall and warm temperatures. The presence of awns also tends to influence
transpiration rate, accelerating the drying of ripe grain. Consequently, the tips of awnless
spikes tend to be blasted in hot dry weather. The grain may also be amber, red, purple, or
creamy white in color. Under normal high density production conditions, a wheat plant
may produce 2–3 tillers. However, when amply spaced on fertile soils, a plant may
produce 30–100 tillers. The spike (head) of a plant may contain 14–17 spikelets, each
spike containing about 25–30 grains. Large spikes may contain between 50 and 75 grains.
The grain size varies within the spikelet, the largest being the second grain from the
bottom and decreasing in size progressively towards the tip of the spike.
Wheat is predominantly self-pollinated. Anthers assume a pendant position soon after the
flower opens. Blooming occurs at temperatures between 13 and 25F starting with the
spikelet around the middle of spike and proceeding upwards and downwards. The wheat

6. kernel or berry is a caryopsis varying between 3 and 10 mm in length and 3 and 5 mm in
width. It has a multilayered pericarp that is removed along with the testa, nucellus and
aleurone layers during milling. The endosperm makes up about 85% of a well-developed
kernel. Below the aleurone layer occurs a complex protein called gluten that has cohesive
properties. It is responsible for the ability of wheat flour to hold together, stretch, and
retain gas as fermented dough rises. This property is available to the flour of only one
other species, the rye flour. Wheat is classified based on three primary characteristics –
agronomic, kernel color, and endosperm quality. There are two seed coat colors – red or
white. Red is conditioned by three dominant genes, the true whites comprising of
recessive alleles of all three genes. Most wheat varieties in the United States are red.
Kernel hardness is classified into two – hard or soft. Upon milling, hard wheat yields
coarse flour. White wheats, lacking in this starch–protein complex, produce a higher yield
of fine flour upon milling. Hard wheat is used for bread making because its gluten protein
is cohesive and elastic.

7. kernel or berry is a caryopsis varying between 3 and 10 mm in length and 3 and 5 mm in
width. It has a multilayered pericarp that is removed along with the testa, nucellus and
aleurone layers during milling. The endosperm makes up about 85% of a well-developed
kernel. Below the aleurone layer occurs a complex protein called gluten that has cohesive
properties. It is responsible for the ability of wheat flour to hold together, stretch, and
retain gas as fermented dough rises. This property is available to the flour of only one
other species, the rye flour. Wheat is classified based on three primary characteristics –
agronomic, kernel color, and endosperm quality. There are two seed coat colors – red or
white. Red is conditioned by three dominant genes, the true whites comprising of
recessive alleles of all three genes. Most wheat varieties in the United States are red.
Kernel hardness is classified into two – hard or soft. Upon milling, hard wheat yields
coarse flour. White wheats, lacking in this starch–protein complex, produce a higher yield
of fine flour upon milling. Hard wheat is used for bread making because its gluten protein
is cohesive and elastic.