Wheat

Wheat

Triticum aestivum

Family- Poaceae

The important wheat growing countries are Russia, USA, India, France, Canada, Italy and Argentina. In India there are three species of wheat under cultivation. Nearly 88% of the wheat area is under bread wheat (Triticum aestivum L.) and a little over 11% under macaroni wheat (Triticum durum Desf.). Emmer wheat (Triticum dicoccum)is grown in a limited area in Maharashtra (Khapli), Gujarat (Popatiya), Tamil Nadu (Samba) and Andhra Pradesh (Ravva Godhumalu). The area under emmer wheat is dwindling as it is being replaced by higher-yielding varieties of bread wheat.

Origin and Distribution 

The ancient civilisations of Babylonia, Crete, Egypt, Greece and Rome used wheat and the Chinese also grew this plant as long back as 2700 B.C. One of the oldest archaeological samples of cereals includes the carbonised spikelets of wheat found in the excavations of Jarmo in Iraq. The estimated date of this site is 6700 B.C. The carbonised wheats were identified as T. boeoticum Boiss. em. Schiem (the Wild Einkorn) and T. dicoccoides Korn (the Wild Emmer). It seems that in South-Western Asia, man had begun to domesticate the two wild wheats by 6000 B.C.

 Until 300 B.C. Emmer wheats dominated the world, but later Macaroni wheat (T. durum Desf.) began to appear. Durum and Macaroni wheats soon displaced the Emmer wheats in Europe. From Europe, the wheat cultivation reached Asia. T. sphaerococcum Perc. the Indian Dwarf Wheat, a form closely related to T. aestivum Linn. have been found at Mohenjodaro in Indus Valley. The earliest archaeological sites in Iraq, dated 6000-7000 B.C. yielded grains of the einkorn type (diploid). diploid einkorn type of wheats are the earliest and most primitive while the hexaploid, including the bread wheat T. aestivum, constitutes the most recent and latest step in the evolution of wheat complex.

Diploid wheats

T. boeoticum Boiss. em. Schiem. and T. dicoccoides Korn were domesticated in South-Western Asia by 6000-7000 B.C. The cultivated einkorn, T. monococcum Linn. probably arose from T. boeoticum under field culture through gene mutation and selection. This einkorn is now grown to a small extent in the mountainous regions of Yugoslavia, Asia Minor, Transcaucasia and North Africa.

Tetraploid wheats

T. dicoccoides. the wild emmer, arose by a natural crossing between T. monococcum or the related T. boeoticum (having genomes AA) and a grass related to Aegilops speltoides (with genome similar to BB), followed by chromosome doubling in the hybrid to give rise to the genomic combination AABB. This probably took place in South-West Asia, long before the beginning of agriculture. 

The cultivated emmer, T. dococcum, probably arose from T. dicoccoides by several steps of gene mutations, natural hybridisation; and by automatic and directed selection. The emmer wheat is now cultivated in Transcaucasia, Iran, Pakistan, India and USA.

T. durum, macaroni wheat, perhaps arose in Egypt or some other Mediterranean country within the first millennium B.C. through mutation in T. dicoccum. It is by far the most important tetraploid wheat species today.

Hexaploid wheats

T. spelta the spelt wheat, is a grain wheat which arose in South-Western Asia, from natural crossings of T. dicoccoides and Aegilops squarrosa folowed by chromes doubling in the resulting hybrids, and later for natural crosses of T. dicoccum and Aegilops squarrosa. It is grown in Germany, Spain, Greece, Switzerland, Italy and Iran.

T. aestivum: It is a naked-grain wheat. It probably arose at different times through gene mutation in T. spelta, or through natural crossings of its different forms. It is the most widely cultivated species and forms the bulk of the wheat crop throughout the world.

T. compactum: It is a naked-grain wheat. The plant seems to have arisen as a mutation from T. spelta or T. aestivum or from the natural crossings of different forms of T. spelta  T. compactum is cultivated in USA, Chile, Asia Minor, Transcaucasia, Kazakhstan and Afghanistan.

Cytogenetics

The discovery of Sakamura and Sax that wheats belong to a polyploid series stimulated many investigators to study the cytology of interspecific hybrids. Kihara (1924) and Gains & Aase (1926) and Aase (1930) independently proposed that the cultivated wheats had the following genome formulae: diploid species AA, tetraploid species AABB and hexaploid species AABBDD. In general, hybrids within the groups have complete or nearly complete chromosome pairing and are highly fertile, while hybrids between groups have seven or more univalents and are highly sterile. This hypothesis which may be briefly referred to as ABD hypothesis has been amply confirmed by the works of many investigators. 

The source of A genome was recognised as the wild monococcum. On morphological and cytological evidence it was early recognised that some species of genus Aegilops was the source of D gene. Sarkar and Stebbins (1956) concluded that T. speltoides is the 

probable source of B genome.

Triticale:

Triticale (Triticale hexaploides lart.) is a new cereal created by crossing, wheat and rye (Secale cereale). In 1890 A.D. Rimpau in Germany succeeded in crossing wheat and rye. But the wheat rye hybrids are completely sterile and therefore, it was not until 1937 when the drug colchicine was discovered that experimental work on the synthesis of fertile wheat-rye hybrids was undertaken through polyploidy. The main objective of wheat-rye hybridization seems to be to combine  earliness, poor-soil adaptability and resistance to cold and drought of rye with the yield and grain qualities of wheat. The fertile triticales obtained are of- 

1.  Octoploid types (2n=8x=56 chromosomes) synthesised from crossing common hexaploid wheat species, T. aestivum (2n=6x=42 chromosomes) and diploid rye (2n=2x=14 chromosomes).
2. Hexaploid triticales (2n=6x=42) by crossing the tetraploid wheat T. durum (2n=4x=28) and rye.

In contrast to octoploid types, the hexaploid triticales are more vigorous, having longer spikes, better floret fertility, larger grains and thus increased yield potential.

Botany of cultivated Wheat plant

• An annual grass 60-150 cm in height;
• Stems tufted, nodes 5-7; 
• Leaves long and narrow, with prominent parallel veins, and having ligule and the auricles, glabrous and hairy on one or both surfaces;
• Inflorescence a spike of spikelets (ear) with tough rachis, awned or awnless, glabrous or hairy;
• Spikelets 2-ranked, compressed, parallel to rachis, 5-9 flowered,
• Glumes loose and broad, lemmas thin, pale and rounded on back;
• Fruit or the grain threshes free from the glumes and lemma, highly variable, oblong with blunt ends, 5-10 mm, long, usually plump and swollen, with a shallow groove on the face.

Ecology

 

Wheat adapts itself to a variety of climates. The great wheat regions of the world are found in temperate zones between 30-60 degree N and 24-40 degree S, but wheat has also been grown North of the Arctic circle and close to the Equator.

Wheat is grown on a variety of soils but it prefers a loamy texture, good structure and a moderate water-holding capacity. The wheat growing areas of India can broadly be classified into five main divisions, in terms of soil conditions. Generally, soils with a pH of 8.3 are good for the cultivation of wheat.

Uses

Wheat is consumed mostly in form of flour obtained by milling the grain, a small quantity is converted to breakfast foods such as wheat flakes, puffed wheat and shredded wheat. In India, wheat is ground generally into atta (whole meal), suji (semolina) and maida (flour).

Bulgur- A parboiled wheat product is a traditional item of food in West Asian countries and in North Africa.

Farina- farina or semolina obtained from hard wheat is extensively used in the preparation of a variety of breakfast foods.

Gluten- The protein gluten obtained by processing wheat is used in the preparation of rye and raisin bread and for upgrading weak flours.

The bye-products of wheat milling viz., bran, germ and middlings constitute valuable feed for livestock.

Wheat Grain

A grains wheat is a caryopsis. The seed consists of four parts,

1. The seed-coat or testa (spermoderm)
2. The embryo
3. The nucellar layer
4. The endosperm or floury part- a thin-walled parenchymatous tissue stored with food.

The embryo is found to weigh from 2.8 to 3.5 percent of the grain, the pericarp, seed coat, nucellar layer and aleurone layer from 7.8 to 8.6 percent, the rest, or some 87 to 89 percent, being the endosperm parenchyma, containing starch and gluten. Ordinary wheat flour consists chiefly of the finely ground endosperm.

The seed

Testa:

It is a reddish-brown seed coat which covers the embryo and endosperm, and is folded in the centre of the furrow, where it joins the funicle of the seed.

Nucellar layer:

Next to the seed-coat is a hyaline layer the nucellar layer.

Endosperm:

With the exception of the small space occupied by the embryo the endosperm tissue fills the interior of the grain. It is divided into two parts, viz., 1) Aleurone layer, 2) Starch and gluten parenchyma.

The aleurone layer surrounds the rest of the endosperm, closely following the contour of the seed-coat. It is almost entirely one cell thick measuring about 65-70 mu across. The individual cells are rectangular in t.s. and l.s, varying in diameter from 25-75 mu and having walls about 6 mu thick. In each cell is a round or oval nucleus about 72 mu in diameter and possessing 3-4 nucleoli. The rest of the cell cavity is filled with minute spherical bodies, the aleurone grains, embedded in a small amount of waxy or oily cytoplasm.

The starch- and gluten parenchyma, forming by far the greatest portion of the wheat grain consists of thin-walled polyhedral cells. The cells are filled with starch grains imbedded in a protoplasmic matrix, from which gluten is derived on treatment with water. The starch grain in the cell are small- 8-10 mu in diameter.

Embryo: It is highly differentiated body, possessing a short hypocotyl, at the apex of which is a plumule or primary bud, composed of rudimentary leaves surrounded by the coleoptile or plumule-sheath and at its base, the root system, completely enclosed by the coleorhiza or root-sheath.

Attached to one side of the axis and forming a greater portion of the embryo as it exists in the grain is a fleshy shielf like structure, the scutellum. On the other side, opposite to the scutellum, is a small tongue-like organ themed the epiblast or lobule.

Macaroni industry:

Macaroni, spaghetti and vermicelli are produced from semolina. Vermicelli is produced mainly on a cottage industry scale and used in the preparation of sweet dishes. Macaroni obtained from wheat and tapioca are used as rice substitute or 'mock-rice'.