Allium ampeloprasum (PROTA)

Plant Resources of Tropical Africa Introduction

List of species

General importance
Geographic coverage Africa
Geographic coverage World
Vegetable
Medicinal
Forage / feed
Food security

distribution in Africa (planted)

1, habit leek plant; 2, habit of flowering bulbous plant.

harvested plants

inflorescence

A. cepa (spring onion, left) and A. ampeloprasum (leek) on the market

Allium ampeloprasum L.

Protologue: Sp. pl. 1: 294 (1753).

Family: Alliaceae

Chromosome number: 2n = 32, 48

Synonyms

• Allium porrum L. (1753).

Vernacular names

Allium ampeloprasum comprises several vegetables of which the most important ones are:

– leek (En), poireau (Fr), alho porro (Po);

– great-headed garlic, elephant garlic (En), ail à grosse tête (Fr);

– pearl onion (En), poireau perpétuel, petit poireau antillais (Fr), alho bravo, alho inglês (Po);

– kurrat (En, Fr).

Origin and geographic distribution

Allium ampeloprasum sensu lato is a wide complex of wild ecotypes and cultivated plants, originating from an area running from Iran to Portugal and northern Africa. Amongst cultivated plants, great-headed garlic seems to be closest to the wild ecotypes; leek has been more modified by domestication.

More or less bulbous leek was well known and cultivated by ancient Egyptians, Greeks and Romans. Modern non-bulbous leek is grown nowadays on all continents, except in areas where high temperatures or heavy rains rule out its cultivation. In Africa leek is produced on a small scale in the Sahel countries during the cool season, and in the East African highlands, mostly around the big cities for Western consumers. Local cultivars of vegetatively propagated leek can be found in the tropics, e.g. in East Africa (Kenya), Indonesia and the West Indies. Great-headed garlic is grown in the highlands in some countries close to the tropics, e.g. Réunion, Saudi Arabia and northern India. Kurrat is grown in the Middle East, pearl onions on a small scale in Europe.

Uses

Allium ampeloprasum comprises several vegetables of which leek is the most important one worldwide. Leek is used as a cooked vegetable. The ‘leek’ is the cylindrical pseudostem, developed by deep planting and earthing-up. The green leaves can be used for soups. Sliced leek is often dehydrated for use in soups. Sessile cloves of great-headed garlic can be used in place of garlic, but their taste is less satisfying when raw. This inconvenience disappears with cooking. The leaves of kurrat are used in salads or as a cooked vegetable to add flavour to dishes. Pearl onions are used for pickling.

Production and international trade

Europe is the most important leek-producing continent, with over 500,000 t annually. France is the largest producer. Most international trade is within the European Union. In tropical Africa leek production is minor and no statistics are available. During winters with severe frost in Europe, some of the leek grown in Kenya has been exported to Europe.

Properties

Leek (trimmed and outer leaves removed, remaining edible portion 57%) contains per 100 g edible portion: water 90.8 g, energy 93 kJ (22 kcal), protein 1.6 g, fat 0.5 g, carbohydrate 2.9 g, fibre 2.8 g, Ca 24 mg, P 44 mg, Fe 1.1 mg, carotene 735 μg, thiamin 0.29 mg, riboflavin 0.05 mg, niacin 0.4 mg, folate 56 μg, ascorbic acid 17 mg (Holland, B., Unwin, I.D. & Buss, D.H., 1991). The major part of the carbohydrates consists of fructans.

The flavour of leek is linked with the presence of S-alkyl-cysteine sulphoxides, propyl- and methyl-cysteine sulphoxides being predominant in modern leeks. This precursor is transformed into a number of S-containing volatiles (e.g. thiosulphinates) when raw leek is cut. The taste of cooked leek, more appreciated by consumers, is also linked with S-containing volatiles, to which a small amount of saponins may add a little bitterness. In great-headed garlic allyl- and propyl-cysteine sulphoxides are present in leaves and cloves, resulting in a taste intermediate between leek and garlic. Some of the saponins found in leek have in-vitro antitumour activity.

Adulterations and substitutes

Leek can be replaced by bunching onions with a long thick pseudostem, such as some cultivars of Allium fistulosum L. or ‘Beltsville Bunching Onion’, a tetraploid seed-propagated cross between Allium cepa L. and Allium fistulosum. In South Africa Allium dregeanum Kunth is collected in the wild by Hottentot people; it is called wild leek, similarly to the wild ecotypes of Allium ampeloprasum on both sides of the Mediterranean.

Description

Robust erect herb up to 150 cm tall; true stem consisting only of a basal plate or disk, with little or no bulb formation, or with bulbs producing two kinds of cloves, small stiped ones and larger sessile ones; roots adventitious.

• Leaves distichously alternate; sheath tubular, forming a pseudostem up to 50 cm long; blade linear, up to 50 cm × 7 cm, V-shaped in cross-section.
• Inflorescence a spherical umbel 4–12 cm in diameter, bearing several hundreds of flowers, on a solid, terete scape up to 150 cm long; umbel subtended by a single, long-pointed spathe shed at flowering.
• Flowers bisexual, campanulate; pedicel 1–5 cm long; tepals 6, in 2 whorls, free, ovate-oblong, 4–6 mm long, purple or white; stamens 6, the inner ones tricuspid; ovary superior, 3-celled.
• Fruit a depressed globose to ovoid capsule 2–4 mm in diameter, up to 6-seeded.
• Seed 2–3 mm × 2 mm, black.
• Seedling with epigeal germination.

Other botanical information

The Allium ampeloprasum complex comprises wild as well as cultivated elements, and the taxonomy is rather confused. In the literature, cultivated leek is often named Allium porrum. Great-headed garlic produces in its bulbs 3–6 very large cloves (up to 60 g). Its stiped cloves, 8–10 mm in diameter, are protected by a hard ‘shell’ and can remain in the soil for more than 1 year before sprouting. Its very large leek-like umbels do not produce seeds. European bulbous leek has sessile cloves covered by a thin silvery-white skin, and sometimes a few shortly stiped cloves without shell. It is found only in old home gardens, e.g. the ‘pearl onion’ grown in Germany and the Netherlands. When tropical vegetatively propagated forms of leek are experimentally grown in temperate countries, they appear to belong to this category, since they produce bulbs, usually with a central spherical clove surrounded by small ones. Some of them never flower under long-day conditions, e.g. the West Indian leek never reached the flowering stage when grown in southern France for 20 years. On the other hand, the Indonesian ‘Anak prei’ flowers in the Netherlands, its seeds giving a heterogeneous progeny. The behaviour of African vegetatively propagated leek under long-day conditions is unknown.

Modern leek has been bred during more than two centuries for elimination of cloves, leading to strictly seed-propagated plants with perfectly cylindrical pseudostems. However, two vestigial cloves may appear at the base of the scape of seed-producing plants. Kurrat (synonym Allium kurrat Schweinf. ex K.Krause), cultivated in the Middle East, is a seed-propagated plant looking like a small leek, but grown for successive harvests of green leaves. ‘Taree Irani’, grown in Iran, is a kurrat-like plant producing in its umbels a mixture of flowers and bulbils.

Seed available in Africa is imported from Western countries, mainly from summer types with pale green leaves and a long soft pseudostem, e.g. ‘Gros Long d'Eté’, ‘Bulgarian’, ‘American Flag’, ‘Carentan’ and ‘Giant Italian’. The winter types with dark green leaves and a short thick hard pseudostem are less often grown and are only suitable for the cool highlands.

Growth and development

Vegetative growth in leek occurs at temperatures of 4–25°C, with an optimum at 16–20°C. Even at optimal temperatures germination is slow (10–12 days) compared with other vegetables. A leafy cotyledon appears first, bearing the seed testa at its apex. At optimal temperatures new leaves are formed every 6 days. Plants reach the harvest stage when 15 leaves have appeared, but continue to grow in thickness.

Floral initiation occurs about 180 days after sowing in temperate conditions, plants exposed to low temperatures (5°C) bolting earlier. Growth of the floral stem is enhanced by long days; consequently bolting and flowering rarely occur under tropical conditions. Leek flowers are pollinated by insects. Both self- and cross-pollination occurs. Besides seed, bulbils (topsets) may be formed in the umbel of some cultivars.

Ecology

A cool season of at least 4 months is needed for good growth of leek. Leek cultivation in hot humid lowlands below 500 m is rarely successful. At higher latitudes during the cool season and at elevations above 1000 m leek is easier to grow. In the lowlands, 130 days are needed (60 days in the seedbed, 70 days in the field) for harvesting leek more than 1 cm in diameter. When temperatures are above the optimum (e.g. 22°C at night, 30°C during the day), leaf senescence can be as rapid as the production of new leaves, resulting in no diameter growth at all. Plants may be harvested from 100–120 days after transplantation, but may be kept in the field longer as they continue to grow thicker (20–25 mm). For leek cultivation, soils must be deep, loose, their pH higher than 5.6, with a good amount of organic matter.

Propagation and planting

In the tropics leek is propagated either by seed or by tillers.

One g contains 300–400 seeds. Leek seed is normally imported. It does not show dormancy. For modern cultivars, about 1000 seeds are sown per m² in a nursery roofed with plastic or with palm leaves, the roofing being removed after emergence of seedlings. Seedlings 15–20 cm tall and pencil-thick are transplanted vertically into holes 10–12 cm deep, at a rate of 200,000/ha, e.g. at a distance of 10–15 cm in the row and 30–45 cm between rows. To obtain long pseudostems, planting is done at the bottom of furrows to facilitate earthing-up. For vegetatively propagated leek, lateral shoots (tillers) are planted in the same way as seedlings.

Management

A harvest of 20 t/ha removes about 65 kg N, 10 kg P, 75 kg K, 35 kg Ca, 3 kg Mg and 15 kg S from the soil. Fertilization with 5–20 t/ha of organic manure and an adequate quantity NPK including sulphates is recommended, as well as liming if necessary.

Diseases and pests

The most important leaf disease is purple blotch (Alternaria porri), to which plants are more susceptible on calcium-deficient acid soils. It can be controlled with dithiocarbamate fungicides or iprodione. In the lowlands, roots are invaded by Pyrenochaeta terrestris in the same way as for garlic or onion. The roots and basal plate can be destroyed by the fungus Sclerotium cepivorum or the nematode Ditylenchus dipsaci, potentially noxious in the tropics if introduced in highlands (e.g. by cultivation of contaminated garlic cloves). Rust (Puccinia spp.) and white tip (Phytophthora porri), which are major diseases in temperate areas, have not been recorded in Africa.

Thrips tabaci can cause serious losses in yield and quality. It can be controlled by insecticides, but also overhead irrigation can decrease thrips infection. This method of irrigation is also beneficial against purple blotch if applied between 11 a.m. and 3 p.m.

Harvesting

Leek is harvested 70–120 days after planting, but may be kept in the field longer. The plants are dug up with a spade, washed, the dried or yellowing leaves eliminated with their sheaths, the remaining leaves trimmed, and roots cut at 1–2 cm. Following consumers’ preference, the green foliage is cut shorter or longer. For vegetatively propagated leek the thicker pseudostems are sold, the thinner lateral shoots kept for own use or for a new planting.

Yield

Under tropical conditions, 15–20 t obtained 70 days after planting in the lowlands during the cool season can be considered a good yield; 40 t/ha might be obtained at elevations of 1500–2000 m if purple blotch is perfectly controlled.

Handling after harvest

Freshly harvested and carefully cleaned leek can be kept 2–3 days, vertically packaged together in baskets or perforated plastic bags. Storage at 0°C is possible for up to 60 days.

Genetic resources

Apart from the vegetatively propagated tropical types (Kenya, Indonesia, West Indies, Haiti), apparently old introductions from temperate countries, there are no tropical leek cultivars. Germplasm collections of European and oriental leek cultivars are preserved as seeds in institutes in Wellesbourne (United Kingdom), Wageningen (Netherlands) and Washington (United States). A number of wild Allium species, more or less closely related to Allium ampeloprasum, are preserved at the Royal Botanic Gardens Kew, United Kingdom.

Breeding

Major breeding objectives in temperate countries are yield, uniformity and disease resistance. Leek can be considered an allogamous autotetraploid with 20–30% self-pollination in seed-production fields. Nowadays seed companies either go on maintaining traditional open-pollinated cultivars under permanent mass selection pressure, or offer F₁ hybrids. Plant vigour loss by selfing is about 30% for the first generation and 40% between S₁ and S₂. F₁ hybrids are based on male sterility. F₁ hybrid leek has 20% higher yields and is more uniform. The rationale behind this is the exclusion of progeny resulting from self-pollination. F₁ cultivars can have a different genetic background but they always have the male sterile parent in common. This line is vegetatively propagated via in-vitro multiplication and/or formation of bulbils, obtained in the umbels by early flower ablation and exposition to photoperiods of 20 hours. Cytoplasmic male sterility has recently been transferred from Allium cepa. No special breeding work for or in tropical countries has been reported.

Prospects

Leek is a productive and highly nutritious vegetable. The need for an Allium used as a cooked vegetable can be satisfied by Allium ampeloprasum or Allium fistulosum. If leek is preferred, breeding for tropical conditions could be an interesting objective. Since there seems to be a general cross fertility between all tetraploid Allium ampeloprasum types, a wide array of subtropical parents would be available. An example of such research is a cross between leek and kurrat realized in the Netherlands to introduce virus resistance. The advantages and disadvantages of vegetatively and seed-propagated leek in tropical conditions should be studied. Breeding for resistance to purple blotch would be interesting too.

Major references

• Brewster, J.L., 1994. Onions and other vegetable Alliums. CAB International, Wallingford, United Kingdom. 236 pp.

• Kik, C., Samoyloo, A.M., Verbeek, W.H.J. & van Raamsdonk, L.W.D., 1997. Mitochondrial DNA variation and crossability of leek (Allium porrum) and its wild relatives from Allium ampeloprasum complex. Theoretical and Applied Genetics 94: 465–471.

• Le Bohec, J., Erard, P. & Le Teinturier, J., 1993. Le Poireau: guide pratique. Centre Technique Interprofessionnel des Fruits et Légumes (CTIFL), Paris, France. 185 pp.

• Mathew, B., 1996. A review of Allium section Allium. IPBGR, Royal Botanic Gardens, Kew, Richmond, United Kingdom. 176 pp.

• Sulistiorini, D. & van der Meer, Q.P., 1993. Allium ampeloprasum L. cv. group Leek. In: Siemonsma, J.S. & Kasem Piluek (Editors). Plant Resources of South-East Asia No 8. Vegetables. Pudoc Scientific Publishers, Wageningen, Netherlands. pp. 62–64.

• van der Meer, Q.P. & Hanelt, P., 1990. Leek. In: Brewster, J.L. & Rabinowitch, H.D. (Editors). Onions and Allied Crops. Volume 3. CRC Press, Boca Ratón, Florida, United States. pp. 179–196.

Other references

• Fattorusso, E., Lanzotti, V., Taglialatela-Scafati, O., Di Rosa, M. & Ianaro, A., 2000. Cytotoxic saponins from bulbs of Allium porrum L. Journal of Agricultural and Food Chemistry 48(8): 3455–3462.

• Holland, B., Unwin, I.D. & Buss, D.H., 1991. Vegetables, herbs and spices. The fifth supplement to McCance & Widdowson’s The Composition of Foods. 4th Edition. Royal Society of Chemistry, Cambridge, United Kingdom. 163 pp.

• Peterka, H., Budahn, H., Schräder, O. & Havey, M.J., 2002. Transfer of male-sterility-inducing cytoplasm from onion to leek. Theoretical and Applied Genetics 12(5): 173–181.

• Schweisguth, B., 1983. Which varietal structure for leek? Proceedings 1st Allium Konferenz, 19–23 July 1983, Fresing, Germany. pp. 264–276.

• Smith, B.M. & Crowther, T.C., 1995. In breeding depression and single cross hybrids in leeks (Allium ampeloprasum ssp. porrum). Euphytica 86: 87–94.

Sources of illustration

• Sulistiorini, D. & van der Meer, Q.P., 1993. Allium ampeloprasum L. cv. group Leek. In: Siemonsma, J.S. & Kasem Piluek (Editors). Plant Resources of South-East Asia No 8. Vegetables. Pudoc Scientific Publishers, Wageningen, Netherlands. pp. 62–64.

• de Wilde-Duyfjes, B.E.E., 1976. A revision of the genus Allium L. (Liliaceae) in Africa. Mededelingen Landbouwhogeschool Wageningen 76–11. Wageningen, Netherlands. 239 pp.

Author(s)

• C.-M. Messiaen, Bat. B 3, Résidence La Guirlande, 75, rue de Fontcarrade, 34070 Montpellier, France

• A. Rouamba, Délégué Régional, INERA, B.P. 49, Tougan, Burkina Faso

Correct citation of this article

Messiaen, C.-M. & Rouamba, A., 2004. Allium ampeloprasum 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 December 2024.