Triticum aestivum (PROTA)
|Geographic coverage Africa|
|Geographic coverage World|
|Cereal / pulse|
|Forage / feed|
- Protologue: Sp. pl. 1: 85 (1753).
- Family: Poaceae (Gramineae)
- Chromosome number: 2n = 42
- Triticum vulgare Vill. (1787).
- Bread wheat, common wheat, wheat (En).
- Blé tendre, blé, froment (Fr).
- Trigo mole, trigo (Po).
- Ngano (Sw).
Origin and geographic distribution
Bread wheat arose in the corridor extending from Armenia in Transcaucasia to the south-west coastal areas of the Caspian Sea in Iran. Hybridization of a wild Aegilops species (Aegilops tauschii Coss., with the D-genome) with emmer, an old type of cultivated wheat belonging to Triticum turgidum L., gave rise to the hexaploid wheats, but it is unknown whether bread wheat or spelt wheat (Triticum spelta L.) appeared first. The earliest archaeological finds of spelt wheat are from the southern Caspian area and are dated at around 5000 BC. Finds of bread wheat are difficult to distinguish from durum wheat (Triticum turgidum), but one thinks that those found in the Caucasus, on the anatolian plateau (Turkey), in Central Europe and in Central Asia from the fifth millennium onwards belong to bread wheat. The D-genome in fact conferred to bread wheat and spelt wheat the adaptation to cold winters and humid summers, allowing them to conquer temperate Eurasia, whereas the Mediterranean remained the area of emmer and durum wheat. By the third millennium BC, bread wheat had reached China. In 1529, the Spanish took it to the New World. Bread wheat was introduced into tropical Africa by Arab traders, missionaries and colonial settlers. It is not known exactly when it reached Ethiopia. It was brought from northern Africa to West Africa, where it was already known around 1000 AD. In the early 20th century it was introduced into Kenya and eastern DR Congo.
Bread wheat today is grown in almost all parts of the world. In tropical Africa, it is mainly produced in Nigeria, Sudan, Ethiopia, Kenya, Tanzania, Zambia and Zimbabwe.
Bread wheat flour is made into numerous products including bread (leavened or flat; baked, steamed or deep fried), pastries, crackers, biscuits, pretzels, noodles, farina, breakfast foods, baby foods and food thickeners. It is also used as a brewing ingredient in certain beverages (white beer). Leavened breads are the most popular use of wheat in almost all parts of the world. Increased bread consumption is often linked to increasing urbanization and higher per capita income. Bread wheat utilization has also been adapted to local cuisine. In Ethiopia, for instance, the flour is used to prepare ‘injera’ (pancake-like unleavened bread), porridge and soup. The grain is eaten as a snack and during social gatherings as ‘nifro’ (boiled whole grain often mixed with pulses), ‘kollo’ (roasted grain) and ‘dabo-kollo’ (ground and seasoned dough, shaped and deep fried).
Industrial uses of wheat products centre on the production of glues, alcohol, oil and gluten. By-products of flour milling, particularly the bran, are used almost entirely to feed livestock, poultry or prawns. Wheat germ (from wheat embryos) is sold as a human food supplement. Straw is fed to ruminants or used for bedding material, thatching, wickerwork, newsprint, cardboard, packing material, fuel and as substrate for mushroom production. In many dry parts of the world it is chopped and mixed with clay to produce building material.
Production and international trade
According to FAO estimates, the average world production of wheat grain (bread wheat and durum wheat together) in 1999–2003 amounted to 576 million t/year from 209 million ha. Worldwide, bread wheat constitutes more than 90% of the area under the cultivated wheats. The main wheat producing countries are China (96.8 million t/year from 25.2 million ha), India (71.0 million t/year from 26.4 million ha), the United States (56.9 million t/year from 20.6 million ha), the Russian Federation (39.4 million t/year from 21.7 million ha) and France (35.1 million t/year from 5.0 million ha). Wheat production in tropical Africa in 1999–2003 was 2.5 million t/year from 1.6 million ha, the main producing countries being Ethiopia (1.4 million t/year from 1.1 million ha), Kenya (272,000 t/year from 137,000 ha), Sudan (254,000 t/year from 124,000 ha), Zimbabwe (237,000 t/year from 43,000 ha), Zambia (87,000 t/year from 13,000 ha), Tanzania (82,000 t/year from 60,000 ha) and Nigeria (75,000 t/year from 53,000 ha). In Ethiopia close to 50% of the wheat production consists of bread wheat, the other 50% of durum wheat. From 1961–1965 to 1999–2003 the world production of wheat increased from 248 to 576 million t/year, whereas the harvested area remained stable at around 210 million ha. In the same period the wheat production in tropical Africa increased from 960,000 to 2.5 million t/year, and the harvested area from 1.2 to 1.6 million ha.
Average world export of wheat amounted to 115 million t/year in 1998–2002, the main exporters being the United States (26.7 million t/year), Canada (16.5 million t/year), Australia (15.9 million t/year), France (15.9 million t/year) and Argentina (10.0 million t/year). Main importers are Italy, Brazil, Japan and Iran, each importing more than 5 million t/year. All countries in tropical Africa are net importers. The main importer in tropical Africa is Nigeria (1.9 million t/year in 1998–2002), followed by Ethiopia (770,000 t/year), Sudan (710,000 t/year) and Kenya (570,000 t/year). The share of food aid in wheat imports is as high as 80% for some countries.
The composition of wheat grain is 7–8% coat material, 90% endosperm and 2–3% embryo. The embryo mainly comprises oil and protein, and little starch. The endosperm is starchy, and is surrounded by the aleurone layer which is rich in proteins. When a wheat grain is milled, the outer layers and embryo are separated from the endosperm. The pulverized endosperm becomes wheat flour, while the other parts form the bran. The endosperm varies both in hardness and vitreousness: hard bread wheat grain high in gluten protein tends to be vitreous and low-protein soft wheat grain tends to be opaque. Hard bread wheat grain is best suited for bread making while the soft wheat grain is best for cookies, cakes and pastries. Flour colour varies from white to slightly yellow.
Bread wheat grain (hard red spring type) contains per 100 g edible portion: water 12.8 g, energy 1377 kJ (329 kcal), protein 15.4 g, fat 1.9 g, carbohydrate 68.0 g, dietary fibre 12.2 g, Ca 25 mg, Mg 124 mg, P 332 mg, Fe 3.6 mg, Zn 2.8 mg, vitamin A 9 IU, thiamin 0.50 mg, riboflavin 0.11 mg, niacin 5.7 mg, vitamin B6 0.34 mg, folate 43 μg and ascorbic acid 0 mg. The essential amino-acid composition per 100 g edible portion is: tryptophan 195 mg, lysine 404 mg, methionine 230 mg, phenylalanine 724 mg, threonine 433 mg, valine 679 mg, leucine 1038 mg and isoleucine 541 mg. The principal fatty acids are per 100 g edible portion: linoleic acid 727 mg, palmitic acid 283 mg and oleic acid 236 mg. Soft, white bread wheat grain contains per 100 g edible portion: water 10.4 g, energy 1423 kJ (340 kcal), protein 10.7 g, fat 2.0 g, carbohydrate 75.4 g, dietary fibre 12.7 g, Ca 34 mg, Mg 90 mg, P 402 mg, Fe 5.4 mg, Zn 3.5 mg, vitamin A 9 IU, thiamin 0.41 mg, riboflavin 0.11 mg, niacin 4.8 mg, vitamin B6 0.38 mg, folate 41 μg and ascorbic acid 0 mg (USDA, 2005). Bread wheat grain is deficient in the amino acids lysine and threonine, and somewhat in isoleucine and valine. It is a good source of B-group vitamins and minerals. Wheat grain possesses a unique viscoelastic and insoluble storage protein complex known as gluten, comprising 78–85% of the total wheat endosperm protein. Gluten is composed mainly of glutenin (polymeric) and gliadin (monomeric) proteins. Glutenins confer elasticity and dough strength, while gliadins confer mainly viscous flow and extensibility to the gluten complex. Wheat flour contains roughly equal amounts of glutenins and gliadins, and their imbalance may influence its visco-elastic properties.
- Annual, tufted grass up to 150 cm tall, with 2–5(–40) tillers; stem (culm) cylindrical, smooth, hollow except at nodes.
- Leaves distichously alternate, simple and entire; leaf sheath rounded, auricled; ligule membranous; blade linear, 15–40 cm × 1–2 cm, parallel-veined, flat, glabrous or pubescent.
- Inflorescence a terminal, distichous spike 4–18 cm long, with sessile spikelets borne solitary on zigzag rachis.
- Spikelet 10–15 mm long, laterally compressed, 3– 9-flowered, with bisexual florets, but 1–2 uppermost ones usually rudimentary, sometimes only 1 of the florets bisexual; glumes almost equal, oblong, shorter than spikelet, thinly leathery, keeled towards the tip, apiculate to awned; lemma rounded on back but keeled towards the tip, leathery, awned or blunt; palea 2-keeled, hairy on the keels; lodicules 2, ciliate; stamens 3; ovary superior, tipped by a small fleshy hairy appendage and with 2 plumose stigmas.
- Fruit an ellipsoid caryopsis (grain), at one side with a central groove, reddish brown to yellow or white.
Other botanical information
Triticum is a classic example of allopolyploidy consisting of diploid (2n = 14), tetraploid (2n = 28) and hexaploid (2n = 42) species. Selection at the diploid and tetraploid levels has proceeded from wild species with hulled grain and brittle rachis to the free-threshing species with tough rachis; hexaploid wheats are not known in the wild, they appeared in cultivation. The classification of the genus Triticum and other related genera within the tribe Triticeae was strongly debated. Polyploidy and biphyletic genome differentiation (B vs. G genome) are isolating mechanisms offering adequate species borders. In this approach, Triticum comprises only 5–6 species, including the diploid Triticum monococcum L. (einkorn, grown sporadically in southern Europe and western Asia), the tetraploid Triticum turgidum L. and the hexaploid Triticum aestivum L. (comprising all cultivated hexaploids). Spelt wheat (Triticum spelta L.) is sometimes separated from Triticum aestivum. It is a hexaploid, not free-threshing wheat, with only 2–3 florets per spikelet, cultivated in small quantities in Europe, Africa and on the plateau of western Iran. It can be cultivated under extreme circumstances, not demanding fertile soils, being relatively disease resistant, and having good taste, food and baking qualities. Before 1850 it was a very important wheat in Europe, declining afterwards, especially because it has to be hulled before milling, but is now gaining in popularity in organic wheat cultivation.
Commercially, wheat is classified into distinct categories of grain hardness (soft, medium-hard, and hard) and colour (red, white and amber). Based on growing habit, bread wheat is divided into two subclasses, spring or winter, but facultative types exist. These subclasses in turn may also be divided into grades, which are generally used to adjust prices, based mainly on grain soundness (effects of rain, heat, frost, insect and mould damage), cleanliness, grain protein content and α-amylase activity. In tropical Africa mostly spring wheats are grown.
Hybrids of wheats (tetraploid or hexaploid) and rye called triticale (× Triticosecale) have been developed and these show a mix of characteristics from the parents, combining the hardiness of rye with the high yield and quality of wheat. Triticale is presently grown only locally in tropical Africa, e.g. in Ethiopia, Kenya, Tanzania and Madagascar, and also in northern Africa and South Africa. As a new food crop, it fell short of expectations, but it is becoming increasingly popular as a forage crop.
Growth and development
Germination of wheat occurs at temperatures of 4–37°C, the optimum being 12–25°C. The radicle emerges first and the coleoptile emerges 4–6 days after germination. The primary roots may remain functional for life unless destroyed by disease or mechanical injury, but they constitute only a small portion of the total root system. The first true leaf of the seedling emerges from the coleoptile. Secondary roots start to develop about two weeks after seedling emergence. They arise from the basal nodes and form the permanent root system, which spreads out and may penetrate as deep as 2 m, but normally no more than 1 m. Leaf and tiller production increase rapidly soon after crop emergence. The duration of the vegetative stage may vary from 20–150 days depending on temperature and the cultivar’s vernalization and daylength response. For floral induction, spring types usually require temperatures between 7°C and 18°C for 5–15 days, while winter types require temperatures between 0°C and 7°C for 30–60 days. Flowering begins at the middle third of the spike and continues towards the basal and apical parts in 3–5 days. All spike-bearing tillers eventually flower almost simultaneously. Wheat is normally self-pollinated; cross-pollination is 1–4%. Pollen is largely shed within the floret. Stigmas remain receptive for 4–13 days. Pollen is viable for up to 30 minutes only. Grains in the centre of the spike and in the proximal florets tend to be larger than the other ones. Physiological maturity is reached when the flag leaf (uppermost leaf) and spikes turn yellow and the moisture content of the fully formed grain has dropped to 25–35%. The complete crop cycle of bread wheat varies from 50–200 days in tropical Africa.
Bread wheat can be grown from within the Arctic Circle to near the equator, but it is most successful between 30–60°N and 27–40°S. Optimum temperatures for development are 10–24°C, with minima of 3–4°C and maxima of 30–32°C. An average temperature of about 18°C is optimal for yield. Temperatures above 35°C stop photosynthesis and growth, and at 40°C the heat kills the crop. Wheat does not grow well under very warm conditions with high relative humidity, and in the tropics it is best grown at higher elevations (1200–3000 m) or in the cooler months of the year. Bread wheat requires at least 250 mm water during the growing season for a good crop; it can be grown in areas that receive 250–750 mm rain annually. The sensitivity to daylength differs among genotypes, but most are quantitative long-day plants; they flower earlier at long daylengths, but they do not require a particular daylength to induce flowering.
Soils best suited for bread wheat production are well aerated, well drained, and deep, with 0.5% or more organic matter. Optimum soil pH ranges between 5.5 and 7.5. Wheat is sensitive to soil salinity.
Propagation and planting
Bread wheat is propagated by seed. The 1000-seed weight is 30–50 g. It is advisable to use certified seed that has been treated with fungicides against soil- and seed-borne diseases, but this is rarely practised in tropical Africa. Wheat is sown by hand or machine. When broadcast, the seed is incorporated in the soil using an animal-drawn plough or machine-drawn disc. The seed may also be dibbled directly into a furrow behind a plough and covered, or machine-planted in rows. Common seed rates are 150–200 kg/ha for broadcasting and 75–120 kg/ha for row-planting. The optimum spacing is 10–25 cm between rows, but it may extend up to 35 cm. The sowing depth is 2–5(–12) cm, with deeper planting required in dry conditions. At a sowing depth beyond 10–12 cm seedling emergence is poor. When using a no-till planting machine, sowing can be done straight into the stubble of the previous crop. For rainfed wheat, the seed can be dry-sown, before the start of the rainy season, or when the soil is moist. Bread wheat is usually grown in sole cropping.
Uniform crop stand and early vigour discourage weed growth in bread wheat. In this respect tillering allows the crop to compensate for poor stands and variable weather conditions. Yield losses due to weeds are caused by early competition in the first 4–5 weeks. Hand weeding, tillage practices, stubble management, pre-sowing irrigation, proper crop rotation and herbicides may control weeds. Herbicide use in tropical Africa ranges from little to none in many countries (e.g. Sudan, Rwanda, Burundi, Madagascar) to almost complete coverage in Kenya, Tanzania, Zambia and Zimbabwe.
In tropical Africa bread wheat is produced mainly under rainfed conditions, except in Malawi, Zambia and Zimbabwe where it is grown as an irrigated (flood and sprinkler) ‘winter’ season crop. In Nigeria wheat production is restricted to the river basin irrigation schemes of its northern states. Irrigation has great potential to increase wheat production in Sudan and Somalia. Care must be taken not to over-irrigate since wheat is sensitive to early waterlogging. Irrigation timing is based either on pre-defined crop stages or on estimates of soil moisture depletion.
The mean nutrient removal per 1 t/ha of grain is 40–43 kg N, 5–8 kg P, 25–35 kg K, 2–4 kg S, 3–4 kg Ca, 3–3.5 kg Mg, and smaller amounts of micronutrients. The exact values depend on the available nutrients and water in the soil, the temperature, and the cultivar. Average fertilizer rates in tropical Africa range from 9 kg N and 10 kg P on rainfed wheat in Ethiopia to 180 kg N, 84 kg P and 50 kg K on irrigated wheat in Zimbabwe. Commercial fertilizer application ranges from less than 1% of the wheat area in Burundi to 100% in Kenya and Zimbabwe. Organic manure and compost are not commonly used on wheat, except in Rwanda. Boron deficiency, resulting in grain set failure, can be observed on certain soils; boron is applied to irrigated wheat in Zambia, Zimbabwe and Madagascar. Copper is applied to most rainfed wheat in Kenya, and manganese is needed in certain areas of Tanzania. Soil acidity can be a constraint, e.g. in wheat production areas at lower elevations in Zambia. Liming might raise the pH, but its economic returns are poor for rainfed wheat.
Wheat is best rotated with non-grass crops, particularly with pulses. In the highland areas of East Africa wheat is grown continuously or in rotation with other cereals, pulses or rapeseed (Brassica oilseed crops). In other regions double cropping systems are common, with irrigated wheat grown in the cool dry season and crops such as cotton, sorghum, maize, soya bean and groundnut in the hot rainy season. In Zimbabwe, for instance, double cropping of irrigated wheat and rainfed soya bean is widely adopted, with the same machinery for sowing and harvesting used for both crops.
In tropical Africa wheat is produced in farming systems ranging from small scale, labour-intensive, rainfed systems, e.g. in Kenya and southern Tanzania, to highly mechanized schemes and farms, e.g. in Nigeria, Sudan, northern and central Tanzania and Zimbabwe.
Diseases and pests
Bread wheat is affected by several diseases and pests. In tropical Africa stripe rust or yellow rust (Puccinia striiformis), spread by air-borne uredospores, and Septoria blotches, particularly Septoria leaf blotch (Septoria tritici, synonym: Mycosphaerella graminicola), are the major diseases in the highlands. Stem rust or black rust (Puccinia graminis) can be very damaging in Ethiopia, Kenya and some parts of Sudan; like stripe rust it is spread by air-borne uredospores. Other diseases important in some years are common bunt (Tilletia spp.), loose smut (Ustilago tritici, synonym: Ustilago nuda f.sp. tritici), barley yellow dwarf virus (BYDV) and bacterial leaf streak or black chaff (Xanthomonas translucens). The use of resistant cultivars is the most effective control measure against these diseases. However, resistance breakdown is very frequent for stripe rust. Fungicide application to control stripe rust occurs in Kenya, Uganda and Tanzania.
The most important insect pests in tropical Africa are aphids, which may also transmit viruses. The African migratory locust (Locusta migratoria) is a periodic pest that causes crop damage in northern and eastern Ethiopia. The Hessian fly (Mayetiola destructor) has long been an important pest in regions adjacent to the Mediterranean Sea in northern Africa, southern Europe and western Asia. Pest control with commercial insecticides in tropical Africa is rare, except in Sudan, Zambia and Zimbabwe for aphids. Birds (especially Quelea quelea) are especially important in irrigated wheat.
Important storage insects, e.g. in Ethiopia, include Sitophilus spp. on whole grains, and Tribolium spp. and Ephestia cautella (synonym: Cadra cautella, flower moth) on wheat flour. Clean storage conditions and maintaining grain moisture and temperature at sufficiently low levels inhibit insect activity and development. Rodents, mainly the black rat (Rattus rattus), also damage stored seeds.
In tropical Africa bread wheat is usually harvested with sickles or knives, and on large-scale farms with combines. A crop harvested at physiological maturity (grain moisture content 25–35%) must be dried thoroughly before threshing. Wet weather at harvest time can cause serious losses in grain quality because the grain sprouts readily. Sickle-harvested wheat plants are stacked or spread out to dry in the sun. Threshing is done by trampling animals, by beating bagged spikes, or during combine harvesting. In most parts of tropical Africa wheat stubble is grazed by livestock.
Yields of bread wheat in tropical Africa vary from 400 kg/ha in Somalia and 700 kg/ha in Angola to 5 t/ha in Zambia and 6.3 t/ha in Zimbabwe. The mean yield of wheat in tropical Africa is estimated at about 1.5 t/ha. Lower yields are due to high temperature, high humidity, disease pressure and the low levels of fertilizer applied. Maximum recorded grain yields of irrigated winter and spring wheats are 14 and 9.5 t/ha, respectively; the absolute maximum yield, based on genetic potential, is estimated at 20 t/ha.
Handling after harvest
Threshed grain of bread wheat is winnowed, cleaned and prepared for store or market. Seeds should be dried to a moisture content of 13–14% for safe storage. High temperatures and moist conditions may result in spoilage. Regular re-drying may be necessary to maintain seed viability, if the seed is not stored in an airtight container.
The International Maize and Wheat Improvement Center (CIMMYT), Mexico City, Mexico (60,400 accessions) and the International Center for Agricultural Research in the Dry Areas (ICARDA), Aleppo, Syria (9700 accessions) maintain extensive germplasm collections of Triticum aestivum. Large germplasm collections are also held in the United States (USDA-ARS National Small Grains Germplasm Research Facility, Aberdeen, Idaho, 42,000 accessions), China (Institute of Crop Germplasm Resources (CAAS), Beijing, 35,900 accessions), and the Russian Federation (N.I. Vavilov All-Russian Scientific Research Institute of Plant Industry, St. Petersburg, 25,900 accessions). In tropical Africa the Institute of Biodiversity Conservation (Addis Ababa, Ethiopia) has the largest collection of bread wheat (3400 accessions). Wheat is a priority crop for collection and conservation. More collection needs to be done of its wild and weedy relatives in regions where they are native, of landraces in areas where they have not been collected before, and of new or obsolete improved cultivars with specific traits from breeding programmes around the world for future improvement work.
CIMMYT and ICARDA have large breeding programmes and, upon request, have the international mandate to disseminate bread wheat germplasm to national programmes. In tropical Africa, Ethiopia and Kenya have strong public sector breeding programmes. In Zimbabwe, there is private sector wheat research, and to some extent in Kenya and Zambia too. High grain yield and disease resistance, mainly to stripe rust and Septoria, are the major objectives. Major breeding methods used in tropical Africa are conventional. A number of high-yielding cultivars, mostly spring types derived from CIMMYT germplasm, have been released in tropical African countries. In 1995 their estimated usage ranged from 5% in Malawi to 100% in Zambia and Zimbabwe.
Bread wheat is one of the crops that benefited most from transfer of genes from other species, such as Aegilops, Hordeum and Secale spp., by artificial hybridization, mainly to increase resistance to diseases, especially rusts. Developments in molecular genetics and genetic engineering of wheat have been slower than in cereals such as rice and maize, due to its ploidy level, size and complexity of its genome, the low level of polymorphism and relatively inefficient transformation systems. Consequently, far fewer maps exist in wheat and few QTL (quantitative trait loci) studies have been reported. On the other hand, the hexaploid nature of bread wheat and its amenity to cytogenetic manipulation have offered unique tools for molecular genetic studies. These include the uses of aneuploid stocks to assign molecular markers to specific chromosome arms, of chromosomal deletion stocks for physical mapping and of chromosome substitution lines to map genes of known chromosomal location. The development of improved chemical hybridizing agents, which allows breeders to surmount the problems associated with cytoplasmic male sterile systems, has considerably increased the progress towards the development of economically acceptable hybrid wheat cultivars. Recently, an efficient Agrobacterium-mediated transformation system has been developed for the large-scale production of transgenic wheat plants. Private companies have developed transgenic herbicide-resistant bread wheat cultivars, but these have not yet been produced commercially.
Since bread wheat is the most important food grain source for humans, the need to continuously increase its production cannot be overemphasized. Bread consumption from wheat in tropical Africa is low and varies from country to country; wheat consumption ranges from 2.5 kg/person/year in Uganda to 43.3 kg/person/year in Sudan. However, with the increasing trends of urbanization and income, there is likely to be a concomitant demand for traditional and new convenient, processed wheat-based products. No tropical African countries are 100% self-sufficient for wheat and the region is confronted by rapidly increasing wheat imports. In many of these countries wheat production is constrained by limited usage of high-yielding cultivars, fertilizer, other inputs and irrigation. Increases in wheat production may come from area expansion to non-traditional areas, coupled with social and economic incentives, and further increases in yield by agronomic research and breeding. Since the 1990s, area expansion of bread wheat has been observed in Sudan, Ethiopia, Kenya, Tanzania and Zambia. Research to improve wheat yields at a global scale includes further mixing of germplasm through wide hybridization and synthetic hexaploids, biotechnology tools, hybrid wheat, and basic studies on wheat physiology and host-plant relationships of various diseases and pests. Tolerances to drought, heat, aluminium soil acidity and waterlogging are some of the abiotic factors that require continued research attention.
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Sources of illustration
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Correct citation of this article
Belay, G., 2006. Triticum aestivum L. In: Brink, M. & Belay, G. (Editors). PROTA (Plant Resources of Tropical Africa / Ressources végétales de l’Afrique tropicale), Wageningen, Netherlands. Accessed 7 December 2022.
- See the Prota4U database.