Allium cepa Common Onion (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Allium cepa L. cv. group Common Onion

Protologue: Sp. pl.: 300 (1753). Cv. group Common Onion: Jones & Mann. Onions and their allies. p. 31 (1963).

Family: Liliaceae

Chromosome number: 2n= 16

Synonyms

• Allium cepa L. var. cepa.

Vernacular names

• Onion, common onion, bulb onion (En)
• Oignon (Fr)
• Brunei: bawang besar
• Indonesia: bawang bombay
• Malaysia: bawang besar
• Papua New Guinea: anian
• Philippines: sibuyas (Tagalog), sibulyas (Ilocano), buliyas (Ivatan)
• Cambodia: khtüm barang
• Laos: bwàx fàlangx
• Thailand: hom-yai (central), hom-huayai (peninsular)
• Vietnam: hành củ, hành tây.

Origin and geographic distribution

True wild A. cepa plants are not known. The ancestral group of A. cepa is considered to include A. oschaninii O. Fedtschenko and its allies A. praemixtum Vved. and A. vavilovii M. Popov & Vved. The current natural distribution of this A. oschaninii complex indicates that domestication of A. cepa probably started within contemporary Tadzhikistan, Afghanistan and Iran. South-western Asia can be acknowledged as the primary gene centre of variability. The earliest onion depictions originated in Egypt and date back to about 2700 BC; consequently, domestication must have begun much earlier.

Onion was introduced into western and northern Europe by the Romans about 300 AD. It became widespread in Europe during the Middle Ages and was introduced to the Americas by Columbus. It probably reached Japan in the 19th Century from the United States. It is almost impossible to trace its introduction to the tropics. Onions probably entered tropical East Africa from Egypt or India. Nowadays onions are cultivated almost worldwide at latitudes between 5-60°in both hemispheres. Onion growing is common in the Philippines and of some importance in Papua New Guinea and Thailand, but rare in the rest of South-East Asia. In Indonesia, Malaysia and other areas close to the equator, it is a difficult crop to grow, because of the troublesome storage of sowing seed, the rain damage to the nursery and the difficulty of producing healthy seed locally. Probably for that reason, vegetatively propagated shallots have become popular vegetables. Some shallot cultivars (e.g. "Cipanas", "Bali" and "Bangkok") resemble small onions, but recently all vegetatively propagated onion and shallot types have been lumped together in the cv. group Aggregatum (see separate article); here, only the seed-propagated Common Onion group is described.

Uses

The main characteristic of onion and other Allium crops is their pungency, and because of this they are probably the most indispensable culinary ingredient in the world. In general, they are used for salads (bunching onion or sliced full-grown bulbs), pickling (e.g. silverskin onions), cooking (e.g. in soups and Chinese dishes) and frying (e.g. with meat). In South-East Asia onions are normally eaten in cooked dishes like bakmi (noodles with meat). Onion also plays an important role in traditional medicine (e.g. as a diuretic). Recently its role in suppressing both the blood sugar level and platelet aggregation was revealed.

Production and international trade

In 1989 the total world acreage under onions (including shallots) amounted to 1 852 000 ha producing 27 142 000 t. Over 2 000 000 t were traded internationally. In 1989 the Philippines produced 61 000 t of common onion from 7000 ha, Thailand 41 000 t from 2200 ha in 1988, whereas other South-East Asian countries produced very small amounts. A small part of the Philippine crop is exported to Japan. In 1988, Brunei imported 2100 t, Indonesia 5144 t (partly shallot), Papua New Guinea 2600 t, Malaysia 109 520 t and Singapore 59 376 t.

Properties

The edible portion of mature bulbs is over 90%. The dry matter content varies between 7-20%. The chemical composition per 100 g edible portion is: water 89-93 g, protein 1-2 g and carbohydrates 5-9 g. The energy content varies from 95-150 kJ/100 g. The most important character of onion is the content of S-alk(en)yl cysteine sulphoxides which give rise to flavour and pungency. The pungency is expressed inμmol pyruvate per g fresh weight, the range being at least from 2 ("Imai Early Yellow") to 20 ("Mammoth Red"). The 1000-seed weight is 3-4 g.

Description

• A biennial herb usually grown as an annual from seed. All parts produce a strong onion odour when crushed.
• Roots adventitious, within a radius of 30 cm from the stem in the top 30 cm of the soil.
• Real stem very short, flattened, formed at the base of the plant; pseudostem formed by the sheathing leaf-bases.
• Leaves 3-8, alternate, distichous, glaucous, produced in succession from the broadening stem apex, each arising as a ring which elongates to form the tubular leaf-sheath; leaf-blade cylindrical, at first solid, later becoming hollow, up to 50 cm long, top acute.
• Bulb formed by the thickening of leaf-bases a short distance above the stem; outer leaf-bases thin, fibrous and dry, variously coloured, forming the protective bulb-coat; mature bulb (the onion) depressed globose to ovoid, up to 15 cm in diameter, very variable in shape, size, colour and weight.
• Scape 1-several, 30-100 cm long, usually exceeding the leaves, erect, straight, terete, hollow, often inflated in the middle or in the lower part.
• Inflorescence a spherical umbel, 2-8 cm in diameter with 50-2000 flowers, initially surrounded by a membranous spathe which splits to remain as 2-4 papery bracts; pedicel 1-4 cm long; flowers subcampanulate to urceolate; tepals 6 in 2 whorls, ovate to oblong, 3-5 mm long, greenish-white to purple; stamens 6; ovary 3-locular, style simple, shorter than stamens at anthesis.
• Fruit a globular capsule, 4-6 mm in diameter, splitting loculicidally, containing up to 6 seeds.
• Seed about 6 mm × 4 mm, black, wrinkled after drying.

Growth and development

Onion seeds have hardly any dormancy. On germination the cotyledon functions as a haustorial organ, becoming green and photosynthetic and characteristically forming a sharp bend or knee on the soil. A primary root is produced by the seedling, all other roots are adventitious. After the seedling has established, approximately one new leaf is produced per week. When the plant has reached a certain stage of growth, and when the daylength is long enough and temperatures sufficiently high, a bulb is formed. Bulbing starts from the outer leaf-sheaths and ends up by the formation of some scales, i.e. thickened leaf-sheaths with aborted blades. The mature bulb consists of the disk (real stem), the skin (dry leaf-sheaths), false scales (fleshy sheaths of complete leaves), true scales (fleshy sheaths of leaves without blades), and primordial sprout leaves. When the bulb has reached full maturity (90-150 days after planting), the leaf-blades start to wither. After a dormant period of a few months, provided the temperature is favourable, the primordial sprout leaves emerge, the plants bolt and inflorescences are formed.

Onion is a facultative cross-pollinator, the percentage of selfing amounting to 10-20%. Flowers are protandrous. Pollination is done by bees, bumble-bees and glider flies. When mature, the fruits dehisce, causing shedding of seeds.

Other botanical information

The great variability within A. cepa has led to numerous proposals for infraspecific groupings and hence the taxonomy is quite confusing. Presently, the simple informal classification into two cultivar groups is usually followed:

• cv. group Common Onion: bulbs large, normally single, plants reproduce from seeds or from seed-grown bulbils (sets).
• cv. group Aggregatum: bulbs smaller, several to many forming an aggregated cluster, plants reproduce vegetatively via lateral bulbs (daughter bulbs).

In the cv. group Common Onion numerous cultivars are available. Only the short-day cultivars are of interest for the tropics. In the Philippines "Red Pinoy", "Red Creole" and "Granex" are grown. Thai farmers predominantly use "Granex". In Papua New Guinea "Gladalan Brown", "Awahia" and "Superex" are important. In Malaysia only "Red Creole" and "Granex" are grown to a very limited extent.

Ecology

Bulbing and maturing are earlier and faster under longer days, higher light intensity and, within certain limits, higher temperatures. Short-day onions which show normal bulbing and maturing under tropical conditions, in general exhibit too early bulbing and maturing at intermediate latitudes because of long days, and sometimes thick necks (pseudostem as in leek) because of low temperatures. Most tropical onions are grown during the dry season as too much rain will result in a high incidence of fungal diseases. Fertile alluvial mineral soils are preferred. In the tropics, onions grow well at varying altitudes; however, at elevations above 1000 m growth and development proceed relatively slowly and bolting percentages can increase. Besides the vernalization requirement (exposure of seed bulbs or growing plants to 5-10°C for one to two months), seed crops of tropical cultivars require high temperatures (20-30°C) and low relative humidity (below 70%). Seed production is therefore mainly carried out in the subtropics, especially in California (United States), where the climatic conditions are more favourable.

Propagation and planting

Onion is propagated by seed. In hot humid climates, onion seed deteriorates quickly (within 3 months). In the tropics the seed is usually sown in a nursery under a mulch cover. After emergence, the mulch is removed. About 6-8 weeks after sowing, when the seedling has a base as thick as a pencil and is approximately 15 cm tall, the seedlings are transplanted to the field (transplant system). Occasionally, e.g. in Venezuela, small mature bulblets are planted (set system). A blueprint for onion cultivation at low latitude in South-East Asia is provided by the Philippines. Cultivars "Granex" and "Red Pinoy" are usually direct-seeded, whereas "Red Creole" is transplanted. Direct seeding is carried out on beds at a relatively high density at the end of the rainy season (October-November). The beds, with 2 rows each, are separated by irrigation furrows. As soon as possible, the recommended density is realized by transplanting or thinning.

Husbandry

Organic and/or chemical fertilizers are generally applied. High rates of NH₄⁺-containing fertilizers should be avoided. Nitrogen should be applied at the beginning of the growth period in order to stimulate vegetative growth before bulbing. Watering is usually done by furrow irrigation. Mulching with rice straw is practised in some areas. Mechanization, with the exception of hand-sowing machines, is uncommon. Weeding and harvesting are mostly done by hand, although chemical weed control is increasing. Crop rotation is important to avoid the build-up of pests and diseases such as nematodes, Sclerotium and Fusarium .

Diseases and pests

Severe losses from fungal diseases are very common; well known are Alternaria porri (purple blotch; control by e.g. dithane), Fusarium oxysporum (basal rot; control by crop rotation and resistance), Stemphylium botryosum (leaf-spot; control by e.g. carbamate), Colletotrichum gloeosporioides (anthracnose; control by e.g. benomyl), Colletotrichum circinans (smudge; only white cultivars are susceptible), Aspergillus niger (black mould on stored onions), and Botrytis squamosa (tip burn).

The most important pests in South-East Asia seem to be Spodoptera exigua (army worm) and thrips. Virus diseases, e.g. onion yellow dwarf and aster yellow, are not very common. Bacterial diseases such as Pseudomonas aeruginosa and Erwinia carotovora can cause considerable storage losses. Without adequate crop rotation, nematodes can be very harmful on upland soils at higher altitudes. Sunscald is caused by direct exposure to strong sunshine directly after harvest.

Harvesting

Harvesting takes place 90-150 days after sowing. Open-pollinated cultivars do not mature uniformly, so priming is practised. The crop is pulled out by hand during the dry season (March - April in the Philippines), and remains for some days in the field with the bulbs covered by the leaves (= windrowing). The leaves are then cutt off and the mature bulbs are bagged or packed in crates if they are to be stored.

Yield

For "Granex", yield ranges from 15-20 t/ha and for red cultivars (which have a higher dry matter content) from 10-14 t/ha. These yields roughly correspond with the average world yield (14 t/ha in 1989), but are low in comparison with top yields of more than 50 t/ha at higher latitudes, e.g. in western Europe.

Handling after harvest

The bulbs are normally packed in mesh bags or in crates of 20 kg. Storage for short periods is possible on racks in well-ventilated sheds. In the Philippines, part of the harvest of "Red Creole" is kept in cold stores at 0°C for a maximum period of six months. "Granex" has an inferior quality for storage and export.

Genetic resources

Germplasm collections are maintained at the Institute of Horticultural Research, Wellesbourne (United Kingdom), the National Seed Storage Laboratory, Fort Collins, Colorado (United States), the Research Centre for Agrobotany, Tápiószele (Hungary), the National Institute of Agrobiological Resources, Tsukuba (Japan), and the Centre for Genetic Resources, Wageningen (the Netherlands).

Breeding

Objectives are improvements in yield, quality, uniformity, keepability, resistance, and seed production in the equatorial tropics. Some characters (e.g. keepability) can be introduced from European or American cultivars. Recently improved prospects have appeared for transmitting resistance to downy mildew and anthracnose from A. roylei Stearn to A. cepa. A start has been made with seed production in the tropics, obtaining bulb vernalization in a cold store (Cuba) or by growing at altitudes of 1500-1800 m (Uganda).

Prospects

The popularity of onion as an indispensable and popular culinary ingredient will undoubtedly increase. Its availability largely depends on adaptation to local ecological conditions, disease resistance and keepability. These objectives have not yet been met and as a consequence many tropical countries still import large quantities of onions from subtropical and temperate countries. In the near future, locally produced seed of well-adapted, keepable and disease-resistant cultivars of shallot-like kitchen quality will be in great demand. As soon as such cultivars become available for the equatorial zone, these will partly replace shallots because of the lower cost of the planting material. Close international collaboration between experienced onion researchers and breeders will greatly speed up developments.

Literature

• Currah, L. & Proctor, F.J., 1990. Onions in tropical regions. Bulletin No 35. National Resources Institute, Chatham Maritime, United Kingdom. 232 pp.
• Jones, H.A. & Mann, L.K., 1963. Onions and their allies. Botany, cultivation and utilization. Leonard Hill, London, United Kingdom. 286 pp.
• Rabinowitch, H.D. & Brewster, J.L. (Editors), 1990. Onions and allied crops. Vols. 1 and 2. CRC Press, Boca Raton, Florida, United States. 273 pp. and 320 pp.
• Rismunandar, 1986. Membudidayakan 5 jenis bawang [Cultivation of 5 Allium species]. Sinar Baru, Bandung, Indonesia. 116 pp.
• T√§ckholm, V. & Drar, M., 1954. Flora of Egypt. Vol. 3, Allium. Cairo University Press, Cairo, Egypt. pp. 58-136.
• Tindall, H.D., 1983. Vegetables in the tropics. MacMillan, London, United Kingdom. pp. 20-23.
• Tropical Development & Research Institute, 1986. Pest control in tropical onions. Tropical Development & Research Institute, London, United Kingdom. 109 pp.
• van der Meer, Q.P. & de Vries, J.N., 1990. An interspecific cross between Allium roylei Stearn and Allium cepa L. and its backcross to A. cepa. Euphytica 47: 29-31.

Authors

• Q.P. van der Meer & A.C. Leong