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(wheat), a genus of herbaceous plants of the family Gramineae.
The genus includes more than 20 wild and cultivated species belonging to three groups: diploids (einkorns), tetraploids (em-mers), and hexaploids (common wheats). The groups are distinguished from each other by the number of chromosomes in their somatic cells. The diploids include three wild species—T. boeoticum, T. thaoudar, and T. urartu —and two cultivated species— T. monococcum and T. sinskajae. The tetraploids include the wild species T. dicoccoides and T. araraticum; the cultivated tunicate-seeded species T. timopheevi, T. karamyschevii (T. palaeo-colchicum, T. georgicum), T. dicoccum, and T. ispahanicum; and the cultivated free-threshing species T. durum, T. turgidum, T. persicum (T. carthlicum), T. turanicum, T. aethiopicum, and T. polonicum. The hexaploids include the cultivated tunicate species T. macha, T. spelta, T. vavilovii, and T. zhukovskyi and the cultivated free-threshing species T. aestivum (T. vulgare), T. compactum, T. sphaerococcum, and T. petropavlovskyi. A number of synthetic octoploid Triticum have been created under laboratory conditions; they include T. fun-gicidum, T. soveticum, and T. cziczinii (T. agropyrotriticum).
On the basis of the type of awn, the color of the spike, the pubescence of the glumes, and the color of the awns and grains, Triticum species are divided into a great number of different types.
Wheat is grown on all the continents of the globe. However, only common, or bread, wheat (T. vulgare) and durum, or macaroni, wheat (T. durum) are widely distributed. The northern boundary of wheat cultivation is 66° N lat. (Sweden); experimental plantings are raised in the USSR to 76°44’ N lat. (Murmansk Oblast). The southern limits of cultivation are the southern borders of Australia, South America, and Africa. Wheat is primarily a steppe crop. It mainly occupies steppe and forest-steppe zones in Europe, prairies in North America, the pampas in South America (Argentina), and on plains and semidesert areas in Australia. Wheat is also grown in piedmont and mountain regions; there are plantings at 4,000 m above sea level.
Botanical description. The root system of the wheat plant is fibrous and develops in the upper (arable) stratum of the soil; some roots extend to a depth of 180 cm. The stem is a straw measuring 40 to 130 cm tall. Its height is determined by the plant’s resistance to lodging and is connected with yield. New high-yield varieties developed in Mexico, the USA, the USSR, and India, which are distinguished by a short (50–85 cm), rigid straw, excel long-stemmed varieties in yield. The color of the straw upon maturation is white, cream, golden yellow, or, in some species, violet. The leaf consists of a sheath that surrounds the stem and a linear leaf blade.
The inflorescence is a compound spike, whose shaft bears spikelets consisting of two glumes and three to five (rarely more) flowers between the glumes. The spike is usually spindle-shaped (most often found in common wheat), prismatic (in durum wheat), or club-shaped; in some species and forms the spike is branched. The spike and the awns are white, red, or black; in varieties with white and red spikes the awns may be black. Wheat is self-pollinated. In most species flowering is closed; however, open flowering characterizes diploid wheat. The fruit, a naked or tunicate caryopsis (usually called a grain), is oval, elliptic, ovate, elongate, or spherical. It has a longitudinal groove on the ventral side and most often is white or red (reddish brown). In consistency the grain may be mealy (common wheat) or glassy (durum wheat and better varieties of common wheat); 1,000 grains weigh 20–50 g (in some species and forms, 70 g or more).
Biological features. Wheat is an annual plant. Perennial forms have been developed through the hybridization of various species and genera. There are winter, spring, semiwinter, and winter-and-spring wheats. Winter wheat has two periods of active vegetation: in the fall, over a period of 45 to 50 days, the vegetative organs develop, and in the spring and summer, in the course of 75 to 100 days, the generative organs form and the plant yields a harvest. Spring wheat is planted in the spring or, in regions with mild winters, in the fall. Its vegetative period is 70 to 110 days.
Wheat seeds sprout at 1°–2°C. The optimum temperature is 12° to 15°C for obtaining uniform sprouts, 16° to 22°C for growth and development, and 22° to 25°C for plumping of the grain. During the vegetative period, winter wheat needs a total of average daily temperatures of about 2100°C, and spring wheat no less than 1300°C. Frost-resistant varieties of winter wheat tolerate winter temperatures to - 20° C and, sometimes, to — 35°C (with normal hardening off and sufficient snow cover). Sprouts of spring wheat can tolerate frosts as low as - 8°C. Resistance to damping off, destruction by wet weather, frost damage, wetting, ice crust, and heaving is of great significance for the success of the winter wheat crop.
Wheat demands a great deal of moisture, especially during grain plumping. It responds well to irrigation, with intensive varieties yielding 80 to 100 quintals of grain per hectare with watering. Spring drought sharply decreases the yield of grain but does not lower the quality. Drought during flowering results in seedless spikes, and during grain plumping, undeveloped grains.
To produce 1 quintal of grain (with straw and chaff), wheat requires 3–3.5 kg of N, 1–1.3 kg of P2O5, and 2–3 kg of K2O. The plant uses the largest quantity of P2O5 and K2O in the period from tillering to flowering and the largest quantity of N in the period from tillering to plumping. The best soils for wheat are chernozems; on peat-podzol soils the plant yields a good harvest with the application of fertilizers. Spring wheat yields especially well when planted in virgin lands or wastelands; the crop does not tolerate acid soils (pH less than 5.0).
History. The USSR (Transcaucasia) is the native land of many species of wheat, including T. araraticum, T. macha, T. timopheevi, T. urartu, and T. persicum. Here many types of common wheat were also discovered. The greatest varietal diversity of durum wheat and poulard wheat (T. turgidum) is found in Azerbaijan and Italy. Wheat culture was known in Southwest Asia (Turkey, Iraq, Syria, and Iran) and in Turkmenia in the seventh millennium B.C., in Greece and Bulgaria in the sixth millennium, and in Egypt earlier than the fourth millennium. At first tunicate species of T. dicoccum and in some places the more ancient T. monococcum were cultivated in these countries. Wheat was first cultivated in China in approximately the third millennium; its cultivation in what is now Hungary, Czechoslovakia, Rumania, and Moldavia originated in about the third millennium. In Transcaucasia wheat was known in about the fifth millennium B.C., in the Northern Caucasus in approximately the first millennium, in Byelorussia, Latvia, and Lithuania in the fourth or fifth century A.D., and in the Cisurals (Perm’ Raion) in the ninth century. Wheat was imported to South America in 1528 arid to North America (territory of the USA) in 1602. It was first cultivated in Canada in 1812 and in Australia in 1788.
Economic significance. Wheat is one of the principal food crops. It constitutes 27 percent of the world’s total grain production. The grain is nourishing and highly caloric, and it contains much protein (10–25 percent in selected varieties, 25–30 percent in wild species). It also contains carbohydrates (60–64 percent), fat (2 percent), vitamins, enzymes, and mineral substances. It is easy to store, transport, and process into flour, froats, and other products. The kernel, bran, and other milling y-products are a valuable concentrated fodder and raw material for the production of mixed feeds. The straw is used as coarse fodder and for bedding; it is also used in the production of paper, cardboard, packing materials, baskets, and hats. The green mass is fed to cattle.
Regions of cultivation. In world agriculture wheat occupies the largest area of all cereal crops. In European countries, soft red winter wheat is the principal class of wheat grown; in the north, for example, in Finland, spring varieties predominate. Hard wheat is cultivated in southern Europe (Spain, Portugal, Italy, Greece, Bulgaria). In Asia wheat cultivation (primarily soft red and white spring varieties) is concentrated in China, India, Turkey, Pakistan, Iran, Syria, Iraq, and the Eastern Mediterranean countries. Hard wheat occupies a considerable area. In India T. dicoccum and T. sphaeracoccum are also cultivated; the latter is also raised in Pakistan.
In the Americas the largest plantings of wheat are in the USA—in the states of Kansas, North Dakota, Colorado, Idaho, Illinois, Indiana, Michigan, and Texas. More than half the land under wheat cultivation is occupied by soft winter varieties, principally red varieties with a glassy grain. Plantings of hard wheat and red and white varieties of soft spring wheat are substantial. In Canada wheat is grown predominantly in the plains of Manitoba, Saskatchewan, and Alberta. Mainly soft red spring varieties with glassy grains are raised; hard wheat is grown on small areas. In Mexico the largest plantings of wheat—soft red spring varieties—are in the state of Sonora.
In Australia white varieties of soft spring wheat are cultivated in all states except the Northern Territory.
|Table 1. World wheat production|
|Area (million ha)||Yield (quintals per ha)||Gross harvest of grain (million tons)|
|1Argentina leads in South American wheat production (almost exclusively soft spring wheat)2Average|
Source: Data from the Food and Agriculture Organization of the United Nations, 1972
|Federal Republic of Germany..........||1.0||1.4||1.6||26.2||33.1||40.6||2.7||4.6||6.6|
|Great Britain .....................||0.9||0.9||1.1||27.2||40.4||42.2||2.4||3.5||4.8|
|World total ...................||173.3||210.9||213.5||9.92||12.12||16.32||171.2||254.3||347.6|
In Africa wheat cultivation is concentrated in the Nile Valley, the northwestern parts of the continent, and the mideastern zone. In Egypt white varieties of soft spring wheat predominate; hard wheat is grown on small areas. In Tunisia, Morocco, and Algeria hard white varieties are grown. In Ethiopia a special species of wheat that is closely related to hard wheat is grown; soft wheat and T. dicoccum are also grown there. In Kenya red and white varieties of soft wheat are raised.
In prerevolutionary Russia winter wheat was grown almost exclusively in the southern part of the country (8.3 million ha in 1913). In the USSR it is cultivated in all the principal agricultural regions, from southern Arkhangel’sk Oblast to the southern regions of Turkmenia. The largest plantings are in the Ukraine, the Northern Caucasus, the central chernozem regions, the Volga Region, and southern Kazakhstan. Red and white varieties of soft wheat, most of which are strong wheats, predominate. Hard winter wheat occupies small land areas, mainly in Azerbaijan.
In Russia in 1913 spring wheat occupied 24.6 million ha; by 1973 this figure had increased nearly twofold. Regions under spring wheat cultivation include Kazakhstan, the Urals, the Volga Region, the forest-steppe and steppe regions of Siberia, the central chernozem regions, and the nonchernozem zone. These regions are occupied principally by soft red and white varieties. In 1973 hard spring wheat occupied about 5 million ha in the Volga Region, the Urals, Kazakhstan, and the central chernozem regions. T. persicum, T. compactant, and T. dicoccum are cultivated on small areas in the USSR.
Varieties. As of 1974, 73 varieties of winter wheat and 107 varieties of spring wheat were regionalized in the USSR. In 1973 the largest areas occupied by soft winter wheat were planted with the variety Bezostaia 1 (developed by P. P. Luk’ianenko, P. A. Luk’ianenko, and N. D. Tarasenko), which occupied 5.5 million ha, and the variety Mironovskaia 808 (developed by V. N. Remeslo), which occupied 5.3 million ha. These varieties are also grown in Bulgaria, Hungary, Poland, Rumania, Yugoslavia, the German Democratic Republic, Czechoslovakia, and other countries. There are extensive plantings of Odessa 16 and Surkhak 5688. Intensive varieties include Kavkaz, Avrora, Mironovskaia Iubileinaia, Odessa 51, and Il’ichevka. Such new varieties as Krasnodar 39, Orbita, and Polesskaia 70 combine high yield with increased winter-hardiness.
Widespread varieties of hard winter wheat are Shark, Ak-Bugda 13, Arandany, and Dzhafari. The most widely cultivated variety of soft spring wheat is Saratov 29 (developed by A. P. Shekhurdin, V. N. Mamontova, and N. N. Kulikov), which occupied more than 16 million ha in 1973. Also important are Bezenchuk 98, Albidum 43, Skala, Liutestsens 758, Milturum 553, and Saratov 210. Kharkov 46 (developed by P. V. Kuchu-mov and E. E. Vatulia) is a popular variety of hard spring wheat, occupying 4 million ha in 1973; other widespread hard spring wheats are Melianopus 26, Narodnaia, and Raketa.
Wheat selection in the USSR is based on high-quality initial stock. Methods used in the development of varieties include in-tergeneric and interspecific hybridization; physical, chemical, and natural mutagenesis; and the transformation of spring varieties into winter ones. The use of ancient varieties of Russian wheat is reflected in the genealogy of many modern varieties. The soft winter glassy wheat raised in the USA is in considerable measure based on varieties imported from the Ukraine, especially the variety Krymka. Krymka was also used in selection of the Japanese variety Norin 10, which was the parent of the best dwarf wheats developed in Mexico, the USA, and India.
The development and cultivation of dwarf and semidwarf varieties (known as the green revolution) has sharply increased grain yields. For example, grain production in Mexico increased threefold between 1952 and 1972 (from 8.8 to 27.2 quintals/ha); in India production doubled over the same period. Dwarf and semidwarf varieties are also used in selection in many countries to develop nonlodging qualities and responsiveness to irrigation and to increase productivity.
In winter-hardiness, drought resistance, grain quality, spike production, and disease resistance, the best varieties of the USSR are superior to those of other countries. Plant breeders in the USSR seek to develop short varieties that are resistant to disease and responsive to irrigation and large doses of fertilizers. They are involved in the development of intensive varieties of spring wheat that maximally approach winter varieties in yield. Also being developed is hard wheat with a yield nearly as great as that of soft wheat. In addition, varieties with grain having increased protein content, especially of the esential amino acids tryptophan and lysine, are being developed.
Technology of cultivation. Winter wheat is planted on bare or occupied fallows (perennial grasses), after lupine, a mixture of vetch and oats, peas, early potatoes, or corn for green feed. The best precursors for spring wheat are cleared fallow, corn, sunflower, pulse crops, potatoes, perennial grasses, and winter cereals.
Preparation of the soil involves autumn fallow and autumn tillage. The principal fertilizers used for winter wheat are manure and composts (20–60 tons/ha), which are especially effective in the nonchernozem zone. Effective doses of mineral fertilizers are 40–80 kg/ha of P2O5, up to 60 kg/ha of K2O, and 40–100 kg/ha of N. While sowing the wheat, 40 kg/ha of P2O5 (granulated superphosphate) is applied in the rows; 30–60 kg/ha of N and 30 kg/ha of P2O5 are added later. Foliar application is used. Spring wheat is treated mainly with mineral fertilizers: 30–45 kg/ha of P2O5, 20–35 kg/ha of K2O, and 20–30 kg/ha of N. Upon planting the wheat, an additional 10–15 kg/ha of P2O5 is applied. Under irrigation conditions and when growing intensive varieties, the dose of fertilization is increased.
Wheat is sown in rows spaced 15 cm or 7–8 cm apart. On 1 ha, 4 to 7.5 million seeds are planted (1.8–2.5 quintals/ha). The sowing depth is from 3 to 8 cm. Snow retention on the fields is important, and in the spring the plantings are harrowed to destroy soil crust. Herbicides are used to control weeds. In irrigated regions, the wheat is watered two to five times (500–800 cu m/ha of water).
Swath harvesting is used during the phase of wax ripeness; combine harvesting can be used when the grain is completely ripe. Soil preparation is done by general-purpose machines. Wheat is sown with grain drills and is harvested with grain-harvesting combines and reapers.
Insects injurious to wheat include Apamea sordens, the Hessian fly, the green-eyed fly, the frit fly, Eurygaster integiceps, and the wheat-stem sawfly. Diseases include loose smut, bunt smut, brown rust, yellow rust, and powdery mildew.
REFERENCESVavilov, N. I. Mirovye resursy sortov khlebnykh zlakov, zernovykh, bobovykh, l’na i ikh ispol’zovanie ν selektsii: Pshenitsa. Moscow-Leningrad, 1964.
Luk’ianenko, P. P. Izbrannye trudy: Selektsiia i semenovodstvo pshenitsy. Moscow, 1973.
Tsitsin, N. V. Otdalennaia gibridizatsiia rastenii. Moscow, 1954.
Mironovskiepshenitsy. Edited by V. N. Remeslo. Moscow, 1972.
Prutskov, F. M. Ozimaia pshenitsa. Moscow, 1970.
Pshenitsa i ee uluchshenie. Edited by M. M. Iakubtsiner, N. P. Koz’-mina, and L. N. Liubarskii. Moscow, 1970. (Translated from English.)
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M. M. IAKUBTSINER