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|DESCRIPTION: Wheat is an hexaploid annual grass; culms simple, erect, hollow or pithy, glabrous, up to 1.2 m tall; leaves flat, narrow, 20-38 cm long, about 1.3 cm broad; spikes long, slender, dorsally compressed, somewhat flattened; rachis tough, not separating from spikelet at maturity; spikelets 2-5 flowered, relatively far apart on stem, slightly overlapping, nearly erect, pressed close to rachis; glumes keeled in upper half, firm, glabrous, shorter than the lemmas; lemmas awned or awnless, less than 1.3 cm long; palea as long as the lemma, remaining entire at maturity; caryopsis free-threshing, soft or hard, red or white.
The roots of the wheat plant may be divided into two groups, namely, the seminal roots, or those belonging initially to the embryo, or which develop later from the hypocotyl or near it, and the adventitious roots, which spring from the nodes of the plant, within the soil or just above it.
The terminal four to seven internodes of the shoot elongate to form the flowering stem or culm and internode elongation is complete by the time of anthesis. Internodes increase in final length from the base of the culm to the uppermost internode, which carries the ear, the peduncle. The basal internodes are shorter than the enclosing sheath of the subtending leaf, while the peduncle and the penultimate internode are longer than enclosing sheaths, revealing a length of bare stem and carrying the emerged ear clear of the sheath. Sometimes in environmentally stressful conditions internode elongation is restricted and the ear remains partially enclosed in the flag leaf sheath.
The strong, thickened sheaths of the culm leaves are structurally important for stem strength and stiffness and the knot or pulvinus is instrumental in carrying the ear aloft if the plant is lodged.
The leaf is divided at the ligule into a cylindrical sheath and the flat blade or lamina. The sheath is tubular at the base but nearer to the blade it is split with the margins overlapping. The lamina has a fairly well marked midrib, along which runs the major vascular bundle of the leaf. It divides the blade into two sub-equal parts, each of which has a number of parallel lateral ribs or veins. Each vein marks the position of a vascular bundle and the tissue over the bundle is raised producing a ridge, so that the adaxial surface of the blade is corrugated. The abaxial surface is more or less flat. The midrib extends down into the sheath for a short distance as a pronounced ridge. The leaf blade naturally assumes a twist and just below the tip, usually about two-thirds along the leaf there is frequently a constriction. This constriction is produced by the constraint upon growth produced by the closely investing ligule of the subtending leaf during development. The ligule is a thin colourless flap of tissue about 1-2 mm in length, which encircles the leaf or the culm above it beyond where the blade diverges. Associated with the ligule are the auricles, two small ear like projections fringed with unicellular hairs.
At the base of the leaf sheath of the culm leaves there is a thicker zone, called variously the joint knot or pulvinus. It is considerably thicker and generally lighter green than the sheath above or the stem below. The node of the stem is below the joint and its position is marked by a slight constriction of the stem. The joint has an important function lifting the ear of a lodged stem off the ground and restoring it to a more or less vertical position.
Leaf shape and size change with leaf position. The lowermost leaf on the main shoot has parallel sides to within 1 cm or so of the tip so that the tip itself is characteristically blunt. The leaves above the first have more or less parallel sides for about two-thirds their length above which they taper to a sharp point. The last leaf produced upon the culm, the flag leaf, tapers from about the lower third giving the leaf an elongated ovate shape.
In spring wheat lamina length increases with increasing leaf number from the base, reaching a maximum one or two leaves before the flag leaf after which the length declines so the flag leaf is somewhat shorter than the longest leaf. Lamina width increases with leaf position so that the flag leaf is the widest leaf. Sheath length also increases with leaf position, markedly so for the culm leaves. Winter wheat shows comparable size changes, but associated with the long vegetative period the first-produced leaves show little change. Heteroblastic development is also seen in some anatomical features of the leaf.
In the inflorescence each spikelet comprises an axis, the rachilla, which bears two glumes and a number of florets. Within each spikelet there are usually from two to four potentially fertile florets.
The caryopsis is a dry indehiscent fruit. The dorsal side (with respect to the spikelet axis) is smoothly rounded while the ventral side has the deep crease. The embryo or germ is situated at the point of attachment of the spikelet axis and the distal end has a brush of fine hairs. The embryo is made up of the scutellum, the plumule and the radicle. The scutellum is the region that secretes some of the enzymes involved in germination and absorbs the soluble sugars from the breakdown of starch in the endosperm. The pollen grain, which has a life span of about 5 hours, when settled on a stigma germinates in about 1.5 hours to produce a pollen tube. This grows down the style, between the cells, and eventually reaches and enters the embryo sac via the micropyle. Wheat is a self-pollinating crop with a very low percentage of cross-pollination - from 1-4%.
USES: Wheat is one of the most important food plants of man. It enters into international trade more than any other food. The economic stability of many nations is affected by the exchange in wheat. Wheat is utilized mainly as flour (whole grain or refined) for the production of a large variety of leavened and flat breads, and for the manufacture of a wide variety of other baking products such as biscuits, and confectionary. Fermented grains are made into various alcoholic drinks and industrial alcohol. Starch is used as cloth-stiffeners. Straws are fed to livestock, used for animal bedding and used in basketry and woven products. According to the phytomass files, annual productivity ranges from 4 to 18 MT/ha. Chaff is estimated to constitute 25% of the grain. Wheat straw is calculated at 1/2-2 times grain yield, more frequently, 1-1/2 times. However, in some countries, wheat biomass averages more than 6 MT/ha, double this if double cropped. The highest phytomass figure is 18 MT/ha/yr. The seeds are used in folk remedies for cancers, corns, tumors, warts, and whitlow. Reported to be antivinous, bilious, demulcent, discutient, diuretic, emollient, excipient, intoxicant, laxative, useful as a poultice, restorative, sedative, used as a shampoo and vulnerary, common wheat is a folk remedy for burns, cancer, diarrhea, dysentery, ecchymosis, epistaxis, fertility, fever, flux, gravel, hematuria, hemoptysis, hemorrhage, incontinence, leprosy, leucorrhea, menorrhagia, neurasthenia, night sweat, perspiration, scald, tumor, warts, whitlow, and wounds.
KILLING T.: Winter wheat may withstand -20°C in the early and dormant stages, later it is like spring wheat and is sensitive to frost.
GROWING PERIOD: Some cultivars of wheat are winter annual (winter wheat) and are sown in the autumn others are annual and sown in the spring (spring wheat). Normally the severity of the winter determines whether winter or spring types are grown. If winters are severe spring types are used, if winters are less cold, winter cultivars are grown. Sown in the autumn it may be harvested after 180-250 days, sown in the spring it may require 90-130 days.
COMMON NAMES: Common wheat, wheat, bread wheat, blé, trigo, weizen.
FURTHER INF.: Scientific synonyms: T. cereale Schrank, Baier., T. hybernum L., T. sativum Lam., T. vulgare. Wheat is grown from the tropics to 60oN and 40oS. In temperate regions and in the subtropics it can be grown at altitudes from sea level to 3000 m, in the tropics between 1500-3700 m or where suitable conditions exist in the lowlands. In the tropics and subtropics, it is grown only during the winter season. Principal wheat-growing areas of the world have similar growing conditions: the Russian prairies, the fertile pampas of Argentina, the Wheat belt of United States, all have fertile dark soils rich in nitrogen; rather hot, cloudless summers; rainfall which, although low, is well-distributed. A good wheat soil has physical structure, which holds together, making good water retention and favorable conditions for nitrate formation. Hot, humid conditions are unfavorable for wheat growing. Wheat prefers a low humidity of about 10%; high humidity encourages disease infestation, especially in combination with high temperatures. The photosynthesis pathway is C 3:1. The production of one ton of wheat removes 20-30 kg N, 6-8 kg P, and 20-28 kg K from the field; this includes the mineral contents in the grain plus the straw. The optimum yield of grain in temperate regions is 6.3 t/ha; the optimum yield in the tropics is 2.5 t/ha, while the average yield in Africa is 1.5 t/ha.
|Sims D (pers. comm.)
Doorenbos J 1979 pp 164
Rehm S 1991 pp 21-23 [TEMP, RAIN, PH, FER]
Dube P 1982 pp 34
Sys C 1984 pp 70
Kassam A 1976 pp 31
Maas E 1990 pp 272
Landon J 1984 pp 280 286 288 291 [TEXT, DRA, DEP, PH, FER, SAL]
Hartmann T 1981 pp 502-504 [RAIN, DEP, DRA, FER]
Roecklein J 1987 pp 45 [USE]
Eswaran H 1986
Kung P 1970 pp 205
Edwards S up pp 151
Purseglove J 1972 pp 291-297 [RAIN, TEXT, FER, DRA]
Van Waveren E 1993 pp 46
Jansen P 1991 pp 179
Onwueme I 1991 pp 215-224 [RAIN, TEMP, TEXT, FER, DRA]