Allium (PROSEA)

Plant Resources of South-East Asia
Introduction

Protologue: Sp. pl. 1: 294 (1753); Gen. pl. ed. 5: 143 (1754).

Family: Alliaceae

Chromosome number: x= 8; A. cepa: 2n= 16, A. chinense: 2n= 32, A. fistulosum: 2n= 16, A. sativum: 2n= 16, A. tuberosum: 2n= 32

Major species

Allium cepa L.,
A. sativum L.

Vernacular names

Onion (En)
Indonesia: bawang.

Origin and geographic distribution

Allium comprises 500-700 species and is mainly distributed in the Northern Hemisphere. Centres of diversity are in regions that are seasonally dry, particularly around the Mediterranean in Europe, in central Asia and North America. The Malesian region has no indigenous species, but several introduced species are cultivated.

Uses

Allium species have been used since antiquity for their antiseptic properties. Besides its antibacterial properties, onion (A. cepa) juice is known for its diuretic, lipid- and blood-pressure lowering and anti-asthmatic/anti-allergic properties. Onion is used in traditional medicine to treat boils, wounds, stings and felons externally, and internally to relieve coughs, bronchitis, asthma, gastro-intestinal disorders (e.g. flatulence, diarrhoea) and headache.

Garlic (A. sativum) is valued worldwide as a "panacea" to cure an array of diseases and to strengthen the body. Phytotherapeutical products based on garlic are traditionally used to treat minor vascular disorders. Garlic is recommended against high blood pressure. Oral administration of garlic (and, to a lesser extent, onion) juice or oil has been reported to prevent hyperlipemia, arteriosclerosis and myocardial infarction. Other traditional uses include the treatment of coughs, bronchitis and gastro-intestinal disorders (flatulence). Garlic oil is used as rubefacient (to treat muscle pains, lumbago, arthritis and ischias) and as vermifuge (especially against enterobiasis). Garlic juice is used externally in the treatment of Taenia versicolor, ringworms and chronic wounds. It is also believed that garlic juice promotes longevity and has aphrodisiac properties.

A. chinense ("rakkyo") is used against fever, stomach-ache and eye-infections. In China, it is reputed to be effective in the treatment of stenocardia, angina pectoris and so-called stagnant blood, and it is included in some traditional preparations. A. tuberosum (Chinese chives) is reportedly effective against tumours, toothache and intestinal disorders. Welsh onion (A. fistulosum) is considered to have strong stimulant properties.

Allium species are well known for their worldwide use as a vegetable and condiment, which often goes hand in hand with their attributed medicinal properties.

Production and international trade

Onion (A. cepa) is economically the most important Allium crop in South-East Asia. In 1990 Indonesia produced about 500 000 t onion (mainly shallot) and 100 000 t garlic. In that year the Philippines exported 12 000 t onion and Thailand 18 000 t, but other countries of South-East Asia imported large amounts: Indonesia 16 000 t, Malaysia 125 000 t, Singapore 65 000 t (of which 34 000 t was re-exported), Brunei 2000 t and Papua New Guinea 2000 t. Shallot bulbs are traded fresh, fried or pickled, whereas garlic is sold fresh or dry and in the form of pills, drinks and powders based on extract.

Properties

Allium species release characteristic odours when the tissue is damaged (the "onion odour" or "garlic odour"). They are known for their sulphur-containing compounds, such as S-alkyl-L-cysteine sulphoxides (alkyl is e.g. methyl, propyl, vinyl, allyl) and γ-glutamyl-S-alkyl-cysteines.

The main constituent of fresh undamaged garlic is alliin (S-allyl-L-(+)-cysteine sulphoxide, > 0.3%), which is degraded by the enzyme alliinase (C-S-lyase) to pyruvic acid and 2-propenesulphenic acid upon cutting or bruising the tissue (alliin and alliinase are localized in separated compartments in the undamaged plant material). 2-Propenesulphenic acid is immediately transformed into allicin (diallyldisulphide-mono-S-oxide). Air oxidation of allicin leads to diallyldisulphide (1,7-dithiaocta-4,5-diene), which is the chief constituent of garlic volatile oil, and together with related tri- and oligosulphides is responsible for the characteristic garlic smell. Allicin condensation products such as ajoenes and vinyldithiines are also present in alcoholic garlic extracts. The drug also contains carbohydrates (fructans) and steroidal saponins.

Fresh onion bulbs also contain fructans, and also flavonoids and sulphur compounds (trans-(+)-S-(1-propenyl)-L-cysteine sulphoxide (about 0.2%) and other cysteine derivatives). Upon bruising the bulb, the sulphoxides are degraded by alliinase to release pyruvic acid and alkyl-thiosulphinates, which rapidly turn into disulphides, the predominant compounds in A. cepa extracts. The volatile aliinase split product (Z)-thiopropanal-S-oxide (from trans-(+)-S-(1-propenyl)-L-cysteine sulphoxide) is the well-known lacrimatory factor in onions.

The growing conditions affect the proportion of different thiosulphinates; for instance, garlic grown in cooler climates shows a higher allyl to methyl ratio than garlic grown in warmer climates.

Several effects of A. cepa, A. sativum and other Allium species have been well investigated. In in vitro experiments, garlic, onion and Chinese chives showed antibacterial and antifungal activity against both gram-positive and gram-negative bacteria (including enteropathogens), pathogenic yeasts (Candida spp.) and some skin-pathogenic fungi. In an in vivo study, rabbits and guinea-pigs with experimentally induced dermatophyte infections (Microsporum canis, Trichophyton rubrum) were treated locally with a garlic extract. After 7 days of treatment, it took another 7-10 days for the skin lesions to completely recover.

Tests with rabbits and rats demonstrated that garlic extract lowers blood cholesterol and triglyceride and also has antihypertensive and anti-hyperglycaemic effects. Onion juice has been shown to have anti-hyperglycaemic activity and anti-asthmatic activity in guinea-pigs. Furthermore, the saponin fraction prepared from the methanolic extract of rakkyo bulbs, is reported to exhibit inhibitory activities on cyclic AMP phosphodiesterase and Na⁺/K⁺-ATPase. The action was almost as potent as that of papaverine; this makes the saponins interesting for their cardiotonic effect.

The best investigated activity, however, is the effect on thrombocyte aggregation. Garlic and onion extracts have been found to show in vitro activity against platelet aggregation. Excessive platelet aggregation is recognized to be a dangerous contributory factor to thrombosis and arteriosclerosis, possibly leading to myocardial and cerebral infarctions. Several compounds have been reported as active principles against thrombocyte aggregation; for instance, the activity has been attributed to the ajoenes (inhibiting lipoxygenases), but also to methyl-allyl-trisulphide (the inhibitory effect depends on the content of this compound in the oil). Dimethyl- and diphenylthiosulphinate (from A. cepa) inhibited thromboxane synthesis, whereas the acid amides N-p-coumaroyltyramine and N-trans-feruloyltyramine, lunularic acid and p-coumaric acid, all sulphurless compounds isolated from the ethylacetate-soluble fraction of A. chinense bulbs, were shown to inhibit prostaglandin and thromboxane synthetases. Compared with aspirin, the compounds from A. chinense were more potent. Adenoside was isolated from the n-butanol soluble fraction of A. sativum and A. chinense bulbs; it showed very significant inhibitory activity against the aggregation of human platelets in vitro. Finally, chinenosides (furostanol saponins from A. chinense bulbs) inhibited ADP-induced aggregation of human blood platelets, the effect being comparable to that of aspirin.

Clinical tests have been conducted (inhibition of thrombocyte aggregation, lipid-lowering activity), but the results are usually conflicting or inconsistent, possibly because of the non-standardized preparations or questionable protocols used. In a double-blind, placebo-controlled experiment on the inhibition of blood-platelet aggregation in a group of patients in which this parameter was constantly and/or spontaneously increased the results were significant. After taking a standardized garlic preparation (800 mg daily, containing 1.3% allicin) for a period of 4 weeks, the spontaneous thrombocyte aggregation disappeared, and several other parameters (e.g. microcirculation of the skin and plasma viscosity) improved.

Furthermore, another placebo-controlled, double-blind experiment seemed to demonstrate the effectiveness of a standardized garlic powder (containing 1.3% alliin, 800 mg daily, administered for 4 months) as a blood-cholesterol lowering agent. Other trials seem to indicate a fibrinolytic activity for both garlic and onion.

An inverse correlation has been reported between regular consumption of garlic and onion and the risk of stomach cancer. S-allylcysteine has been found a chemopreventive agent for hepatocarcinogenesis in rats. From tests with mice it was suggested that garlic may provide an effective form of therapy for transitional cell carcinoma of the bladder. Other studies gave evidence of a direct effect of S-allylmercaptocysteine, one of the stable components present in aged garlic extract, on established cancer cells of breast and prostate.

In a test with alloxan diabetic rats, treatment with S-allyl-cysteine sulphoxide isolated from garlic ameliorated the diabetic condition almost to the same extent as did insulin. A garlic extract cured experimentally infected mice of trypanosomiasis in 4 days, with diallyldisulphide as the probable active compound. Garlic may improve arterial oxygenation and symptoms in patients with hepatopulmonary syndrome; this warrants further investigation. Extracts of A. tuberosum have been reported to have anti-tumour activity in vitro and in vivo.

Extracts of welsh onion showed allelopathic effects; in tests, they inhibited the growth of Compositae crops such as lettuce, marigold, Aster and Chrysanthemum. However, a 1% extract was highly effective in stimulating mycelial growth of the edible mushroom Lentinus edodes.

Description

Perennial or biennial herbs, often grown as annual from bulbs or seed, up to 100(-150) cm tall; roots adventitious, up to 30 cm long; bulb usually present and distinct, with papery or chartaceous protective coats (tunics); real stem very short, forming a disk at the base of the bulb; pseudostem on top of bulb formed by the sheathing bases of successive leaves.
Leaves basally concentrated but sometimes sheathing the scape for a considerable distance and then appearing cauline, alternate, often distichous, blades flat, terete, fistulose or angular, usually filiform-linear with acute apex, glabrous.
Inflorescence umbellate, spherical or subspherical, on top of a terete or fistulose scape usually exceeding the leaves, initially surrounded by 2 hyaline bracts which are normally almost fused, composed of flowers, bulbils or both.
Flowers bisexual, often with slender pedicel, actinomorphic, campanulate to urceolate; tepals 6, in 2 whorls, free or almost free; stamens 6, filaments inserted at the base of the tepals, anthers dehiscing with longitudinal slits; ovary superior, 3-locular, style simple, often subgynobasic.
Fruit a globular capsule, splitting loculicidally, several-seeded.
Seeds broad and triangular, often blackish and wrinkled after drying; embryo more or less curved.

Growth and development

Bulbs of Allium species are formed from the lower parts of the leaf-sheaths, as a result of photosynthate mobilization from the leaf-blade to the base of the leaves. A bulb is only formed when the plant has reached a certain stage of growth and when the daylength is long enough and temperature sufficiently high. When the bulb has reached full maturity, the leaf-blades start to wither. The bulbils that are often present in the inflorescence can be useful for vegetative propagation, especially in taxa with poor seed-setting.

Pollination is by insects such as bees, bumble-bees and hover flies. Onion is a facultative cross-pollinator, the percentage of selfing amounting to 10-20%. Flowers are often protandrous. In Chinese chives, more than 90% of the seed develops apomictically. Flower induction is controlled by temperature and daylength, and for several species (particularly rakkyo, welsh onion, Chinese chives) flowering is rare in the tropics.

Other botanical information

There are numerous cultivars, particularly of garlic and onion. Only short-day cultivars are of interest to the tropics. There is no appropriate cultivar classification for South-East Asia. The presumed wild ancestors and some related species of cultivated Allium have been determined, e.g. A. longicuspis Regel which is the presumed wild ancestor of garlic, and A. altaicum Pallas which is a closely related species of welsh onion.

Ecology

Tropical shallot requires an average day temperature of 20-26 °C and a daylength of at least 11 hours, whereas common onion prefers slightly lower temperatures and a daylength of at least 13 hours. In Indonesia, shallot is mainly grown in the lowlands below 450 m altitude, preferably on well-drained alluvial clay soil, whereas Chinese chives, welsh onion and garlic are grown in the highlands up to 2200 m altitude. Most tropical onions are grown during the dry season, as too much rain will result in a high incidence of fungal diseases. They require well-drained soils.

Propagation and planting

Several Allium species (shallot, rakkyo) are commonly propagated by bulbs. To avoid problems of dormancy, the bulbs should first be stored for some months. Garlic is normally propagated by lateral bulbs (cloves) and seldom by bulbils from the inflorescence. In South-East Asia, welsh onion is propagated mainly from basal tillers, and Chinese chives by division of clumps. Propagation by seed may enhance the size and shape of bulbs and minimize diseases. The seed for propagating common onion is produced in the subtropics, where the climatic conditions are more favourable. Some cultivars are direct-seeded, whereas others are transplanted from nurseries where the seed is usually sown under a mulch. Planting distance varies according to species and conditions, but is usually 10-15 cm × 15-20 cm. Intercropping with other vegetables, e.g. hot pepper, carrot, Irish potato and mustard, is common.

Husbandry

Weeds are often a serious problem and weeding should be done regularly; it is mostly done by hand, although chemical weed control is increasing. The crop must be irrigated regularly during dry weather. Organic and/or chemical fertilizers are generally applied. Crop rotation is important to avoid the build-up of diseases and pests such as Fusarium, Sclerotium and nematodes.

Diseases and pests

Fungal diseases are common, particularly during the rainy season. Alternaria, Fusarium, Stemphylium, Aspergillus and Colletotrichum species may cause severe losses in onion. Viruses may also cause problems, which may be overcome by visually inspecting the planting material in the field and destroying any affected plants, or by the use of true-seed cultivars. Nematodes can be very harmful, especially in upland soils at higher altitudes, and without adequate crop rotation. Army worms (Spodoptera exigua) and thrips (Thrips tabaci) are reported as serious pests in shallot, common onion, welsh onion and garlic.

Harvesting

Bulbs are often harvested after the leaves have wilted, usually 2-3 months after planting, but after 3-5 months in common onion and garlic. The bulbs are usually pulled out by hand, tied into bunches, and dried in the sun, usually with the bulbs covered by the leaves to protect them. Often, some bulbs are kept as planting material for the next growing season. Chinese chives is a ratoon crop: leaves are harvested repeatedly from the same plants the year round.

Yield

Yields of common onion in South-East Asia range from 7-20 t/ha; average yields of shallot (6 t/ha), rakkyo, welsh onion (7 t/ha) and garlic (2.5-4.5 t/ha) are generally lower.

Handling after harvest

After drying, the bulbs (or plants of welsh onion) are tied into bunches which are sold directly or stored, often by hanging them on racks in well-ventilated places. For long-distance transport, the dry leaves are cut off and the bulbs are packed in bags or crates. The bulk of rakkyo is steeped in brine and subsequently processed into sweet or sour pickles. Chinese chives leaves are marketed as fresh as possible.

Genetic resources and breeding

Germplasm collections of common onion, garlic and welsh onion are maintained at several institutes in Europe, the United States and Japan. Smaller collections of shallot and garlic are available at research institutes in South-East Asia, e.g. Lembang Horticultural Research Institute (LEHRI), Indonesia. Evaluation of the South-East Asian germplasm collections led to the recommendation of some local cultivars, e.g. of shallot. Breeding objectives are resistance to diseases and improvement of bulb quality and yield. It would be a breakthrough to find seed-producing selections, e.g. in rakkyo and garlic.

Prospects

Allium species have an outstanding reputation as phytotherapeutic. The ability to inhibit thrombosis seems to be of remarkable medicinal value. The medicinal reputation might further stimulate the interest in Allium as vegetable as well. Some medicinal properties have been confirmed by in vitro tests, animal tests and/or clinical tests (e.g. antimicrobial activity, activity against platelet aggregation, blood cholesterol lowering activity), but other attributed properties (e.g. diuretic activity, anti-cancer activity) have not been demonstrated conclusively. Well executed clinical tests with standardized preparations are needed to confirm these properties. Garlic has potential for the preservation of processed foods because of the inhibitory activity against pathogenic bacteria such as Salmonella typhi and Escherichia coli.

Literature

Bruneton, J., 1995. Pharmacognosy, phytochemistry, medicinal plants. Technique & Documentation Lavoisier, Paris, France. pp. 180-183.
Buijsen, J.R.M., 1993. Alliaceae. In: Kalkman, C. et al. (Editors): Flora Malesiana. Series 1, Vol. 11(2). Foundation Flora Malesiana. Rijksherbarium/Hortus Botanicus, Leiden University, the Netherlands. pp. 375-384.
de Padua, L.S. & Pancho, J.V., 1983. Handbook on Philippine medicinal plants. Vol. 4. Technical Bulletin Vol. VI No 1. Documentation and Information Section, Office of the Director of Research, University of the Philippines at Los Baños, the Philippines. pp. 6-8.
Goda, Y., Shibuya, M. & Sankawa, U., 1987. Inhibitors of the arachidonate cascade from Allium chinense and their effect on in vitro platelet aggregation. Chemical and Pharmaceutical Bulletin 35(7): 2668-2674.
Kosuge, T., Yokota, M., Sugiyama, K., Yamamoto, T., Ni, M.Y. & Yan, S.C., 1985. Studies on antitumour activities and antitumour principles of Chinese herbs (in Japanese). Yakugaku Zasshi 105(8): 791-795.
Kuroda, M., Mimaki, Y., Kameyama, A., Sashida, Y. & Nikaido, T., 1995. Steroidal saponins from Allium chinense and their inhibitory activities on cyclic AMP phosphodiesterase and Na⁺/K⁺ATPase. Phytochemistry 40(4): 1071-1076.
Okuyama, T., Fujita, K., Shibata, S., Hoson, M., Kawada, T., Masaki, M. & Yamate, N., 1989. Effects of Chinese drugs "xiebai" and "dasuan" on human platelet aggregation (Allium bakeri, A. sativum). Planta Medica 55(3): 242-244.
Okuyama, T., Shibata, S., Hoson, M., Kawada, T., Osada, H. & Noguchi, T., 1986. Effect of oriental plant drugs on platelet aggregation. III. Effect of Chinese drug "xiebai" on human platelet aggregation. Planta Medica 52(3): 171-175.
Peng, J.-P., Yao, X.-S., Tezuka, Y. & Kikuchi, T., 1996. Furostanol glycosides from bulbs of Allium chinense. Phytochemistry 41(1): 283-285.
Siemonsma, J.S. & Kasem Piluek (Editors), 1993. Plant Resources of South-East Asia No 8. Vegetables. Pudoc Scientific Publishers, Wageningen, the Netherlands. pp. 62-82.

Selection of species

Authors

Diah Sulistiarini, Juliasri Djamal & Iman Raharjo