Allium cepa (PROTA)
|Geographic coverage Africa|
|Geographic coverage World|
|Forage / feed|
Allium cepa L.
- Protologue: Sp. pl. 1: 300 (1753).
- Family: Alliaceae
- Chromosome number: 2n = 16
- Allium ascalonicum auct.
- Onion, bulb onion; shallot (En).
- Oignon; échalote (Fr). Cebola; chalota, cebolete (Po).
- Kitunguu (Sw).
Origin and geographic distribution
Allium cepa (including seed-propagated onions and most shallot types) is only known from cultivation. It probably originates from Central Asia (between Turkmenistan and Afghanistan) where some of its relatives still grow in the wild. The closest among them are Allium vavilovii Popov & Vved. from southern Turkmenistan and northern Iran, with which it gives 100% fertile hybrids, and Allium asarense R.M. Fritsch & Matin from Iran. Allium oschaninii O.Fedtsch. (Uzbekistan and neighbouring countries), which used to be considered the ancestor of Allium cepa cannot be crossed successfully with the cultivated onion, but its domestication seems to be the origin of some European ‘shallots’ (‘échalote grise’ in France, ‘Scalogno di Romagna’ in Italy). From Central Asia the supposed onion ancestor probably migrated first towards Mesopotamia, where onion is mentioned in Sumerian literature (2500 BC), then to Egypt (1600 BC), India and South-East Asia. From Egypt, Allium cepa was introduced into the Mediterranean area and from there to all the Roman empire.
Traditional tropical African cultivars may have been introduced either from southern Egypt, or from India via Sudan to Central and West Africa, as genetically heterogeneous seed or bulb lots, then bred by local farmers into better adapted seed-propagated onions, or selected to become shallots. Allium cepa as bulb onion and/or shallot is probably cultivated in all countries of tropical Africa. Important production areas for bulb onion are Senegal, Mali, Burkina Faso, Ghana, Niger, Nigeria, Chad, Sudan, Ethiopia, Kenya, Tanzania, Uganda, Zambia and Zimbabwe.
In the lowlands between 10°N and 10°S shallots replace onions because the temperature is too high for vernalization and seed production, and the climate too humid. The short vegetative cycle of shallots (60–75 days) gives the possibility of two crops a year, especially in the four-seasons climate along the Gulf of Guinea. Yellow or red/purple shallots are grown in Guinea, Côte d’Ivoire, Ghana, Benin, Nigeria, Sudan, Ethiopia, Uganda, Kenya, Tanzania, and on both banks of the Congo River near Brazzaville (Congo) and Kinshasa (DR Congo). The spicy taste and high dry matter content (15–18%) of shallots have made them attractive for growers farther from the equator, in many areas where common onions are also produced, e.g. by the Dogon in Mali, or in Cape Verde.
Bulbs of Allium cepa are a popular vegetable everywhere. They can be used raw, sliced for seasoning salads, boiled with other vegetables, or fried with other vegetables and meat. They are an essential ingredient in many African sauces and relishes. If consumed in small amounts for their pungency, they can be considered as a condiment. The leaves, whole immature plants (called ‘salad onions’ or ‘spring onions’), or leafy sprouts from germinating bulbs (called ‘cébettes’ in southern France) are used in the same way. Locally, immature flower heads are also a popular food item. In parts of West Africa, leaves still green at bulb harvest are pounded, then used to make sun-dried and fermented balls, which are used later for seasoning dishes. Sliced raw onions have antibiotic properties, which can reduce contamination by bacteria, protozoa or helminths in salads. In traditional medicine onion is used externally to treat boils, felons, wounds and stings, and internally to relieve coughs, bronchitis, asthma, gastro-intestinal disorders and headache.
Production and international trade
FAO statistics report the worldwide annual production of dry onion bulbs in 2001 as 49 million t, continental China being the largest producer with 15 million t; India produced 5 million t and the United States more than 3 million t; Brazil, Indonesia, Iran, Japan, South Korea, Pakistan, Russia, Spain and Turkey each more than 1 million t. Some countries producing more than 500,000 t for a relatively small population are important exporters to Africa, e.g. the Netherlands. Shallots are either included in bulb onion data, or combined with green onions. Mexico reports a production of over 1 million t in the latter category. However, some of these figures should be considered critically, e.g. for Asian countries where ‘onions’ may include Allium fistulosum L. Besides, the data are often incomplete for tropical countries. The production of tropical Africa reaches 1.5 million t, compared with 2.2 million t for non-tropical Africa. It is difficult to know if these ‘dry onions’ are all seed-propagated, or if they include shallots. Most African tropical countries import onion bulbs, either from Niger, which exports an important part of its 200,000 t production, or from Europe or South Africa. Shallots are exported from northern Côte d’Ivoire and Mali to the neighbouring countries.
The dry matter content of onion or shallot bulbs ranges from 7–18%; shallots generally have a higher dry matter content than onions. The composition of the dry matter is rather constant; the variation in the composition of fresh onion and shallot is mainly caused by variations in moisture content. Bulb onions contain per 100 g edible portion (91% of product as purchased): water 89 g, energy 150 kJ (36 kcal), protein 1.2 g, fat 0.2 g, carbohydrate 7.9 g, fibre 1.5 g, K 160 mg, Ca 25 mg, P 30 mg, carotene 10 μg, thiamin 0.13 mg, riboflavin trace, niacin 0.7 mg, folate 17 μg, ascorbic acid 5 mg. The composition of immature plants (young leaves and peeled bulb) per 100 g edible portion (69% of product as purchased) is: water 92.2 g, energy 98 kJ (23 kcal), protein 2.0 g, fat 0.5 g, carbohydrate 3.0 g, fibre 1.5 g, K 160 mg, Ca 75 mg, P 29 mg, carotene 620 μg, thiamin 0.05 mg, riboflavin 0.03 mg, niacin 0.5 mg, folate 54 μg, ascorbic acid 26 mg (Holland, B., Unwin, I.D. & Buss, D.H., 1991). The carbohydrates consist of glucose, fructose, sucrose and fructans. In bulbs with a low dry matter content, glucose and fructose are predominant (75% of the carbohydrates in bulbs with 7.5% dry matter). Fructans on the contrary are predominant when the dry matter content is high (90% of the carbohydrates in bulbs with 17% dry matter).
Fresh onion bulbs contain sulphoxides, the most important of which is propenyl cysteine sulphoxide. Upon bruising the bulb, these are degraded by alliinase to release pyruvic acid and alkyl-thiosulphinates, which rapidly turn into sulphides and disulphides. The volatile alliinase product (Z)-thiopropanal-S-oxide (synonym: propenyl sulphoxide) is the well-known lacrimatory factor in onions. The degradation of sulphoxides can be influenced by external conditions such as boiling and frying, which explains the occurrence of different flavours. The combined taste of sulphides and caramelized sugars gives the flavour to fried onion. The pungency and flavour of onions vary, depending on variety, growing conditions (e.g. temperature and soil nitrogen and sulphur content) and storage conditions.
In in-vitro experiments onion showed antibacterial and antifungal activity against both gram-positive and gram-negative bacteria (including enteropathogens), pathogenic yeast (Candida spp.) and some skin-pathogenic fungi. Onion juice has been shown to have anti-hyperglycaemic and anti-asthmatic properties. The best investigated medicinal activity, however, is the effect on platelet aggregation. Onion extracts have shown in-vitro activity against platelet aggregation, but clinical tests have shown conflicting or inconsistent results. Regular consumption of onion has been reported to reduce the risk of stomach cancer.
- Biennial glabrous herb, usually grown as an annual from seed or bulbs, up to 100 cm tall; real stem very short, formed at the base of the plant in the form of a disk, with adventitious roots at base; bulbs formed by the thickening of leaf-bases a short distance above the true stem, solitary or in clusters, depressed globose to ovoid or oblate, up to 20 cm in diameter, variously coloured.
- Leaves 3–8, distichously alternate, glaucous, with tubular sheath; blade D-shaped in cross section, hollow, up to 50 cm long, acute at apex.
- Inflorescence a spherical umbel up to 8 cm in diameter, on a long, erect, terete, hollow scape up to 100 cm long, usually inflated below the middle; umbel initially surrounded by a membranous spathe splitting into 2–4 papery bracts.
- Flowers bisexual, stellate; pedicel slender, up to 4 cm long; tepals 6, in 2 whorls, free, ovate to oblong, 3–5 mm long, greenish white to purple; stamens 6; ovary superior, 3-celled, style shorter than stamens at anthesis, later elongating.
- Fruit a globular capsule 4–6 mm in diameter, splitting loculicidally, up to 6-seeded.
- Seeds c. 6 mm × 4 mm, black.
Other botanical information
Allium cepa is nowadays considered as divisible into two large cultivar-groups: Common Onion Group, with large, normally solitary bulbs, reproduced from seed or from seed-grown bulblets (‘sets’), and Aggregatum Group, with smaller, several to many bulbs forming an aggregated cluster, originating from a single mother bulb. A cultivar of Allium cepa will be considered as a ‘shallot’ (belonging to Aggregatum Group) if more than 200 buds are present in one kg of mother bulbs, and if, under suitable climatic conditions, most of these buds give new bulbs. This definition does not exclude that, if the sprouts of a common onion grow under conditions unfavourable for floral initiation, they may form smaller bulbs, or, on the contrary, that sprouts from a shallot mother-bulb may produce floral stems when vernalized.
Only short-day cultivars of Common Onion Group, with a critical daylength close to 12 hours, can give bulbs in tropical Africa. They belong to 3 main categories:
– Traditional tropical African cultivars, e.g. ‘Violet de Galmi’, ‘Blanc de Soumarana’ (Niger), ‘Violet de Garango’ (eastern Burkina Faso), ‘Bawku’ (northern Ghana), ‘Red Kano’ (northern Nigeria), ‘Violet d’Abéché’ (eastern Chad), ‘Malanville’ (Benin), ‘Rouge de Tana’ (West Africa) and ‘Tana’ (adaptation from ‘Rouge de Tana’ in Madagascar). Recent selection in this category has given ‘Yaakar’, ‘IRAT 19’ and ‘Noflaye’ (Senegal). Bulbs of these cultivars have a high dry matter content and store well under natural conditions. ‘Noflaye’ is selected for resistance to bolting. Big red onion cultivars traditionally grown in Sudan are more like the West African onions, but white and yellow cultivars may have originated from Egypt.
Red or purple onions are also grown in East Africa, but their names, e.g. ‘Bombay Red’ and ‘Red Globe’ indicate an Indian origin. These cultivars may have been maintained locally for long periods of time (e.g. a ‘Bombay Red’ strain in Tanzania) resulting in varieties adapted to local seed production and differing from the original foreign source material.
– Mediterranean cultivars, either under their original name, e.g. ‘Babosa’, or ‘Valenciana Temprana’ (‘Jaune Hâtif de Valence’) from Spain, or after breeding for increased earliness in the southern United States, e.g. ‘Texas Grano’ cultivars, or ‘Granex’ F1 hybrids. Similar selection work in South Africa gave the open-pollinated cultivar ‘Pyramid’. However, even with the earliest of these cultivars, ‘Texas Early Grano 502’, which can give bulbs at latitude 0°, no satisfactory seed production can be achieved in tropical Africa. These onions are larger than the traditional African ones, but their dry matter content is considerably lower (7–10% versus 12–16%) and their storage life is short under ambient conditions.
– Cultivars of the Creole group. The ‘Red Creole’ type of onion may have been developed in Louisiana from African onions brought to the New World at the time of the slave trade. American catalogues traditionally offer ‘Red Creole’ and more recently ‘Yellow Creole’ and ‘White Creole’; their dry matter content is relatively high, but their seed producing ability in tropical conditions has decreased. Creole onions store better than the Grano/Granex types, but not so well as the West African lines. They tend to be popular in countries of East and southern Africa, where there is no tradition of local seed production, but where red onions are preferred. Their bolting resistance is higher than that of onion cultivars imported from India, hence their suitability for high altitude growing areas, e.g. in Kenya.
Several seed companies worldwide have undertaken breeding work to improve tropical short-day onions, including Technisem in West Africa and East West Seed Company in Zimbabwe and Asia. Improved cultivars resulting from this breeding work are commercially available; these include ‘Noflaye’ (Technisem - later bolting selection from ‘Rouge de Châteauvieux’), and ‘Red Pinoy’ (East West Seed Company - a long storage and late bolting red onion derived from a red polycross). The islands Réunion, Mauritius and Rodrigues have an onion growing tradition too, e.g. ‘Rouge de Châteauvieux’, a red/pink onion bred more than 100 years ago from a fortuitous cross between the French ‘Rouge Pâle de Niort’ and an Indian cultivar. In Mauritius and Rodrigues smaller, very pungent ‘Local Red’ onions are also grown and maintained locally.
Growth and development
The vegetative growth in Allium cepa in temperate areas follows a growth curve with a minimum at 6°C, a relatively flat optimum at 20–28°C, and a maximum at 34°C. Tropical cultivars are possibly adapted to a somewhat higher range of temperatures; research in Sudan has shown that temperatures of 17–22°C at night and 45°C during daytime retards bulbing but that the plants slowly continue to produce leaves, even when the daylength promotes bulbing. At lower temperatures but the same daylength, bulbing proceeded normally.
Allium cepa requires long days for bulb initiation and enlargement, but the critical daylength is relatively short, at less than 12 hours for traditional tropical cultivars, or for the earliest Mediterranean cultivars. There is no true juvenile stage in Allium cepa; bulbing can be initiated in very young plants if days are longer than the critical length and temperatures higher than 20°C. The spectral composition of the light received by the leaves has an important influence too: bulbing is promoted when the red/far-red ratio is lowered by high-density planting, intercropping or even by weeds.
Bulbing starts from the outer, still fleshy leaf-sheaths and ends with the formation of specialized fleshy scales at the centre of the bulb, which are effectively thickened leaf sheaths with undeveloped blades. The mature bulb consists of the disk or base plate, in fact the real stem, the skin (outermost dry leaf sheaths), false scales (fleshy sheaths of complete leaves), true scales (fleshy sheaths of bladeless leaves) and primordial sprout leaves. At maturity, the leaf blades start to wither, while the water and assimilates from them are rapidly transported into the swelling bulb. When the leafy tops have ‘fallen’ by softening of the now hollow pseudo-stem, maturation is complete and the bulbs enter a phase of more or less intense dormancy, which varies in depth between cultivars. This dormancy is most quickly eliminated at cool temperatures, at 7–12°C.
Floral initiation does not depend on daylength, but requires vernalization. Temperatures below 10°C are adequate for temperate or Mediterranean cultivars, and several nights below 15°C for traditional tropical onions, and below 18°C for tropical shallots. Vernalization can be applied to growing plants or to bulbs in storage, provided that the plants are older than a ‘juvenile stage’ (6–7 leaves) or the bulbs more than 2.5 cm in diameter. Floral initiation is characterized by changes in shape at the centre of the meristematic apex, the primordial sprout leaves start to grow, using the food reserves stored in the bulb, and soon the inflorescence buds become visible at the centre of the leafy shoots. Onion is a facultative cross-pollinator, the percentage of selfing amounting to 10–20%. The flowers are protandrous. Pollination is by bees, bumble-bees or flies. When mature, the fruits dehisce, allowing shedding of the seeds.
Optimal vegetative growth and good bulb maturation are generally obtained under dry and cool conditions. Bulb maturity is promoted by a combination of increasing daylength and rising temperatures, whereas withdrawal of irrigation also hastens bulb maturation. Onions and shallots can grow on any soil with pH above 5.6, but adequate calcium nutrition is essential for good vegetative development and disease tolerance. Onion crops in the tropics are often grown under irrigation between rainfed crops of cereals.
Propagation and planting
Three methods are practised for onion production:
– The production of transplants in nurseries sown at a rate of 500–1000 seeds/m2 (1.5–3 g/m2) is convenient for the small farmer. About 40–50 days after sowing the young plants, as thick as a pencil, are transplanted at densities of 16–36 plants/m2. A row spacing of 30–35 cm is suitable to facilitate hand weeding. At higher densities the yield will be higher, but the bulbs smaller.
– Direct sowing is the normal large-scale method in temperate countries. In the tropics this is rarely used, because it needs a high standard of husbandry, precision sowing equipment and considerable skill in the use of herbicides and irrigation. Young seedlings easily perish with heavy rains. Furthermore, a direct-sown crop occupies the plots 50 days earlier.
– Planting ‘sets’ is used in temperate countries for early production. Very small mature bulbs weighing less than 3 g (diameter 1.6 cm) are obtained by a late sowing (days longer than 12 hours, mean temperatures higher than 25°C) at the same density as for transplants; they are harvested and replanted after storage at normal density for the early production of big bulbs. This method is sometimes applied in the tropics (e.g. in Senegal, for the production of early crops), though it could be more widely used with advantage. Bulbils, i.e. the very small bulblets that may develop in the inflorescence, can be used for multiplication, but this is rarely done.
For shallot production, mother bulbs, weighing 3–10 g, are planted at a rate of 25–50 bulbs/m2. Small mother bulbs tend to give a few large shallots, large mother bulbs a larger number of small shallots. Dense planting gives smaller plants and fewer bulbs per plant but a higher total yield. Seed of short-day shallots is now available from some companies. For shallots grown twice a year near the equator, planting should be timed to allow harvesting during one of the dry seasons.
For common onion, the seed yield for 1 ha planted with 5 t of mother bulbs can reach 500 kg. African farmers have developed original techniques for onion seed production. In Senegal, Niger and neighbouring countries, only the lower half of the mother bulb is planted, the upper half being utilized to make sun-dried onion paste. In northern Nigeria, young shoots from the mother bulb are separated and transplanted. Many farmers in West Africa practise seed-to-seed production, because it is easy and not costly. However, this system aggravates the risk of premature bolting in the first year. Local consumers are accustomed to finding dry young scapes inside the bulbs, and sometimes a market exists for the edible scapes and buds. More exigent markets consider the presence of young scapes a defect.
Seed production from Mediterranean cultivars could be obtained under tropical conditions by an adequate vernalization of mother bulbs in refrigerated stores, but such a method would probably be too costly and therefore the seed is usually imported from temperate regions. In Brazil, where electricity is available in rural areas, it is done succesfully.
Onion seed must be harvested under dry conditions, treated gently during cleaning and stored as quickly as possible under dry conditions. It deteriorates rapidly, in only a few months under humid tropical conditions, but can be stored for several years when kept dry in hermetically sealed packages or in containers with a desiccant. An appropriate seed moisture content for long-term storage is 7–8% or less.
For good production onions need fertile soil. The best onion yields in African experiment stations were obtained under high fertilizing with 120–60–150 kg/ha NPK. The fertilizers should also contain enough sulphates. The strong mycorrhization of onion roots by Endogonaceae allows them to exploit the phosphorus reserves of the soil. Nitrate-N is more favourable for onion than ammonium-N, which should not be used alone. Organic matter is useful if fully decomposed, but not when it releases ammonia. Bulbs obtained with excessive N fertilization do not store well, so ideally the N supply should be adequate for the young and bulbing plants but be exhausted by harvest time. For shallots, the vegetative cycle being shorter and the potential harvest lower, recommended fertilizer applications are half of the figures indicated above.
Furrow irrigation is common with onions planted in double rows on beds 40–50 cm wide. Most farmers irrigate onions manually overhead, applying 3–4 l/m2 on dry days. This overhead or sprinkling irrigation is not very appropriate for onions because it stimulates fungal leaf diseases. Drip irrigation is the most suitable method, but is expensive and so far rarely used. Saline water is not well tolerated. Irrigation should be stopped 20 days before the harvest.
During crop establishment much hand weeding is needed because onions are slow growing and do not compete well with weeds. Many weeds may occur, the worst being nut grass (Cyperus rotundus L.).
Diseases and pests
The most important soilborne disease in semi-arid regions is pink root (Pyrenochaeta terrestris); roots rot during bulb enlargement, resulting in small bulbs. The fungus can persist for several years in the soil and invades roots of other Allium species (e.g. garlic) and grasses (e.g. maize). A 5-year rotation without Allium species or cereals is recommended, and if possible solarization, at least for seedbeds. Screening of cultivars from the United States and Brazil resulted in the selection of ‘Texas Early Grano 502’ resistant to pink root. However, its roots were affected by pink root in Senegal, under conditions of monocropping and irrigation with saline water. Breeding for higher levels of resistance is carried out in Brazil. Optimum temperatures for the development of white rot (Sclerotium cepivorum) and stem-and-bulb nematode (Ditylenchus dipsaci) are relatively low, and therefore these diseases have not been frequently reported in tropical Africa. Basal bulb rot caused by Fusarium oxysporum is sporadically reported in Africa. Here too, resistance has been found in plants from a number of countries (Italy, United States, Brazil).
The most important leaf disease under rainy conditions during bulb enlargement is purple blotch (Alternaria porri). Onions grown in acid, calcium-deficient soils are especially susceptible; lime or powdered limestone applied in the planting furrow can be useful. Fungicide such as iprodione, daconil and dithane can be effective. However, the waxy epidermis of onion leaves can retain only small droplets. Glomerella cingulata, the twister disease, has caused heavy losses in onions in the Sahel and northern Nigeria. This fungus is the perfect stage of the asexually propagated fungus Colletotrichum gloeosporioides, which causes anthracnose in shallots in Indonesia and Brazil. Several fungi are seedborne. A powdery mildew (Leveillula taurica) invades onion leaves in coastal areas of West Africa causing chlorotic leaf spots. Downy mildew (Peronospora destructor) is a damaging disease in the East African highlands. Storage rots caused by bacteria or fungi (e.g. Erwinia spp., Pseudomonas aeruginosa, Aspergillus niger) may damage the crop especially if the bulbs have endured too high temperatures during drying on the soil. The principal cause is Aspergillus niger, a common ubiquitous and saprophytic black mould. A correlation was demonstrated between seed infestation by Aspergillus niger and black mould in storage. Therefore clean or disinfected seeds should be used. Disinfection of onion stores is useful too, as well as the destruction of cockroaches, which carry the spores. There is a reciprocal interaction between bulb sprouting and storage rots: bacterial or black mould infection can induce premature sprouting, and the flesh of sprouting bulbs is more susceptible to rot. Virus diseases are of lesser importance in onions and shallots in Africa, but they can be important elsewhere, e.g. on shallots in Indonesia.
Some Spodoptera or Agrotis caterpillars cut the young plants at soil level or eat the leaves. The principal insect pest of onions and shallots is Thrips tabaci, which can be controlled with insecticides, but daily spraying with pure water or overhead irrigation is fairly efficient too. Spraying with insecticides against caterpillars often aggravates thrips damage, because it kills the natural enemies of thrips. Some cultivars are thrips-tolerant, e.g. ‘Yaakar’, bred by the Centre pour le Developpement de l’Horticulture (Senegal). Four lines bred by INA-Cabo Verde (Cape Verde) are also resistant to both thrips and pink root.
African cultivars or ‘Texas Grano’ can be harvested 70–80 days after transplanting (120–130 days after sowing) or planting of sets, and later cultivars 150–180 days after sowing. In coastal areas of Senegal, sets of ‘Violet de Galmi’ planted in October can be harvested in January, ‘Violet de Galmi’ or ‘Texas Grano’ sown between September and January are harvested from February to May, and ‘Jaune de Valence’ sown between January and March is harvested from June to August. This staggering of production is more difficult in more inland parts of Senegal, where most of the crop is sown in December, to be harvested in March. The harvest occurs before daily temperatures are over 38°C, at latitudes of 10–25°.
Shallots have a cycle from planting to harvesting of about 75 days under the shortest days and cooler temperatures, with some inflorescences produced. Under longer days, the cycle is still shorter (60 days, no flowering).
For ‘Violet de Galmi’ 50–60 t/ha has been obtained in experiment stations under optimal fertilization and irrigation, and up to 80 t/ha for ‘Texas Early Grano’. Yields obtained by small farmers reach 10–20 t/ha. Traditional yields of shallots are about 8 t/ha, but they can reach 18 t/ha under optimal conditions.
Handling after harvest
Harvested plants are usually dried for some days in the field, with leaves of one plant covering another; they are then tied in bunches with their leaves, or the leaves are cut off and the bulbs gathered in bags or crates. The necks should be trimmed at about 4 cm above the bulb to avoid damaging the fleshy bulb tissues. When the air temperature is above 38°C, the soil surface temperature can rise higher than 45°C, especially on black soils, so the bulbs are best dried under shade to avoid heat damage.
In equatorial regions, e.g. in Kenya, onions are produced continuously and thus the keeping quality is of less importance. In climates between latitudes 10° and 25° there are at least six months during which freshly harvested onions are not available on the market, and there the choice of cultivars with good keeping quality is important. There are large differences between cultivars for storability; a high dry matter content is linked with good storage characteristics. Notable for poor storage are ‘Texas Grano’ types. Onion bulbs deteriorate most rapidly at temperatures which quickly eliminate their dormancy (5–15°C). They can be kept longer at temperatures close to 0°C, or at 20–30°C, but they should be dried properly, at an optimal relative humidity of 65–80%. Storage rooms need to be only free-ventilating if bulbs are kept as hanging bunches or braids, in stacked, well-ventilated crates or on shelves. Textile bags of 25–50 kg should be placed so that air can circulate around them. Bulk storage in layers more than 40 cm thick or in bins of more than 1 m3 cannot be practised without forced air circulation.
Dormancy of onion bulbs can be enhanced by application of a plant growth inhibitor, maleic hydrazide (MH), 15 days before harvest; it is sprayed on the plants before the green colour has disappeared from the leaves, from where it is transported to the growing points. Imported bulbs can have been produced under MH use, since European legislation does not require mention of this treatment.
Important collections of onion germplasm are maintained in temperate countries: United Kingdom, Netherlands, Hungary, United States and Japan. Collections of tropical cultivars are maintained in India, Brazil, Colombia and at AVRDC (Taiwan). In West Africa, several countries cooperate for collection, maintenance and characterization of traditional onion cultivars, continuing the work initiated during the 1960s and 1970s by CDH and IRAT in Senegal. The largest number of national onion accessions is kept at the Centre Régional de Recherche Agronomique (CERRA), Maradi (Niger) and the largest regional collection is stored at the Centre Régional de Recherches Environnementales et Agricoles (CRREA), Farako-Bâ (Burkina Faso).
In Indonesia, which is the world leader in shallot production (390,000 t), an important shallot collection is preserved at Lembang. In Africa, there are two substantial shallot collections: at Centre de Recherche Agronomique (CRA), Bareng (Guinea) and at the Centre National de Recherche Agronomique (CNRA), Bouaké (Côte d’Ivoire).
Allium cepa is insect-pollinated, allogamous and has a severe inbreeding depression, but there is no system to prevent self-fertilization. Open-pollinated cultivars are therefore composed of plants heterozygous for many genes, except for those involved in bulb characters. Not less than 40–50 plants must be grown together under isolation for maintenance of a cultivar without inbreeding depression, and mass selection pressure must be permanent. In this way it was demonstrated that the propensity of ‘Violet de Galmi’ to premature bolting could be eliminated by saving seeds only from plants that have produced good bulbs without bolting in the first year. Genetic material is available and research is carried out at several national institutes in Africa to reduce onion bolting in the first year.
Three levels can be considered for onion breeding:
– Selection within a cultivar for interesting characters, such as earliness, insect or disease resistance, or male sterility.
– Crosses between onion cultivars. The thrips-resistant ‘Yaakar’ was obtained by crossing ‘Violet de Galmi’ and ‘Roxa do Traviu’. A short critical daylength is usually dominant in crosses between tropical and temperate cultivars.
– Interspecific crosses which are possible with Allium galanthum Kar. & Kir. (for male sterility), Allium fistulosum L. (with great difficulties for gene introgression following a nearly sterile F1 generation), and Allium roylei Stearn (resistance to downy mildew and Botrytis leaf blight). More recently the use of Allium roylei as a bridge species between onion and Allium fistulosum has been proposed for introgression of Allium fistulosum genes into Allium cepa. Allium fistulosum seems to be tolerant to purple blotch and pink root disease, but it remains to be seen whether it offers true resistance genes, or if these tolerances are linked to the non-bulbing habit. A complicating factor is that Allium roylei is resistant to purple blotch, but susceptible to pink root.
Onion F1 hybrids based on cytoplasmic male sterility are promoted by many seed companies. Their value has been contested for tropical countries, since the yield superiority to open pollinated cultivars is not always significant (e.g. ‘Texas Early Grano’ versus ‘Granex’), and their homogeneity is less perfect than in tomato or cabbage hybrids, since the parent lines have been homogenized by only a few (1–2) selfings. Their principal interest for seed-grown crops is the absence of seeds resulting from selfings that give weak plants.
Sowing seed from shallot clones is an easy way to demonstrate their heterozygous nature: purple shallots from Brazzaville gave some white ones in their progeny. Even selfing of a clone will give 2–3% superior plants which in addition will be virus-free. Some European seed companies have recently proposed true-seed shallots for tropical as well as for temperate countries, the seeds giving rise to small shallot-like bulbs. True-seed shallots are not legally recognized everywhere as shallots, in which case they are considered onions. True-seed shallots have phytosanitary advantages. The bulbs obtained in this way could be vegetatively propagated a few times by the farmers, but they would quickly lose the advantage due to new virus infections. In addition, propagation by seed is more economical than vegetative propagation.
In onion and shallot greater progress is expected from breeding than from improved agronomy. There is a great need for African-bred onion cultivars adapted to different ecological zones and consumers. Increased self-sufficiency for onion seed and shallot bulbs in tropical African countries, better yields and quality rewarding the work of African farmers should be the aims of national institutes and that of seed companies. Both the relatively inexpensive experimental work for the preservation and improvement of African onion and shallot germplasm, as well as sharing the benefits of more advanced research, such as marker-assisted breeding and interspecific crosses, should be involved to reach the aforementioned aims.
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- Brewster, J.L., 1994. Onions and other vegetable Alliums. CAB International, Wallingford, United Kingdom. 236 pp.
- Brice, J., Currah, L., Malins, A. & Bancroft, R., 1997. Onion storage in the tropics. NRI, Greenwich, United Kingdom. 118 pp.
- Havey, M.J., 1992. Restriction enzyme analysis of the chloroplast and nuclear 45 S ribosomal DNA of Allium sections Cepa and Phyllodolon. Plant Systematics and Evolution 183: 17–31.
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Sources of illustration
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Correct citation of this article
Messiaen, C.-M. & Rouamba, A., 2004. Allium cepa L. [Internet] Record from PROTA4U. Grubben, G.J.H. & Denton, O.A. (Editors). PROTA (Plant Resources of Tropical Africa / Ressources végétales de l’Afrique tropicale), Wageningen, Netherlands. <http://www.prota4u.org/search.asp>.
Accessed 12 November 2020.