Onions

Taxonomy

Domain	Eukarya
Kingdom	Plantae
Phylum	Magnoliophyta
Class	Magnoliopsida
Superorder	Lilianae
Order	Asparagales
Family	Amaryllidaceae

Scientific Name

Allium

Common Name

onions

Authors: Schwartz, H. F., Mohan, S. K., Havey, M. J., and Crowe, F. J

Introduction

The genus Allium is in the monocot order Asparagales, family Alliaceae, and includes various economically important and cultivated species: the bulb onion (A. cepa L.) and the closely related shallot (A. cepa L. var. ascalonicum Backer) and potato onion (A. cepa L. var. aggregatum G. Don), bunching onion (A. fistulosum L.), chive (A. schoenoprasum L.), Chinese chive (A. tuberosum Rottler ex Spreng.), garlic (A. sativum L.), leek (A. ampeloprasum L. var. porrum (L.) Gay) [syn. A. porrum L.]), and rakkyo (A. chinense G. Don). There are at least 18 other Allium species that are consumed as fresh vegetables, pickled, or used as flavoring. Several alliums, including A. giganteum Regel, A. moly L., and A. caeruleum Pall., are also grown as ornamentals. Allium vineale L. is a weed of pastures.

The distinctive flavor or odor of the genus is produced when plant tissues are bruised or cut and the enzyme alliinase hydrolyzes S alk(en)yl cysteine sulfoxide precursors to form volatile sulfur compounds. Onion, garlic, and their relatives, although primarily grown for food, are also used in traditional medicine, including the treatment of chicken pox, the common cold, influenza, measles, and rheumatism. Antimicrobial characteristics of the alliums are likely the result of sulfur compounds. Research has demonstrated that extracts of onion and garlic decrease sugars, lipids, and platelet aggregation, and enhance fibrinolysis in the blood, indicating that the alliums may help prevent arteriosclerosis and other cardiovascular diseases.

History

According to artifacts recovered in Egyptian tombs, onion and garlic have been part of our food from as far back as 3200 B.C. and are cited as important food staples in the Bible and Koran. The Greeks and Romans wrote about onion and garlic about 400 - 300 B.C., and onion was popular in northern Europe at the start of the Middle Ages. Its medicinal properties were reviewed in the Indian medical treatise Charaka Samhita nearly 2,000 years ago. Garlic is an ancient crop of central Asian origin. It was grown and consumed in Egypt about 2780 - 2100 B.C. during the building of the pyramids. It was recorded in a medical treatise in India about 6 B.C. Today, the bulb onion, with a total world production during 2001-2005 of approximately 50 million metric tons on 3 million hectares in 175 countries, is the most valuable Allium species. On average, worldwide production of garlic is about 10% that of the bulb onion. Leek and bunching onion are the next most valuable species, with production concentrated in Europe and the Orient, respectively. The bunching onion is also important in some areas of the United States, such as California.

Cultivation

The bulb onion exists only in cultivation and likely originated in central Asia. It is now cultivated worldwide and is commonly found in vegetable markets of most countries. Local phenotypes vary in shape (tall, round, or flat), color (white, yellow, pink, or red), firmness, pungency, dormancy, and solids content. Onions produced for dehydration have a much higher solids content than the sweet, fresh market type.

Onion is a diploid (2n=2x=16) herbaceous biennial; i.e., it requires two years per generation. The bulb is a modified stem with fibrous roots and fleshy leaves. Onion forms about 15 leaves per growing season but rarely has more than 10 living leaves at any one time. The plant loses four to five leaves during the growing season to natural senescence and death. Premature leaf death is associated with disease or other damage. Bulb formation in onion is affected by temperature and photoperiod. Onion populations have been classified on the basis of the length of daylight required to initiate bulb formation; short day populations need about 12 hr of daylight and long day types require up to 16 hr. Onion is an insect-pollinated crop. The seed is formed within a three celled capsule, which usually contains two black seeds. A cluster of capsules form in the spherical inflorescence (umbel) at the terminal end of the hollow stalk (scape) which elongates from the biennial bulb. Although most of the world's onions are grown from seed, shallot is commonly grown in tropical areas where seed production is difficult. The viviparous onion, A. x proliferum (Moench) Schrad. (syn. A. cepa var. viviparum (Metzger) Alef.) reproduces by small bulbils that replace all or most of the flowers in the inflorescence.

Garlic is propagated vegetatively by cloves and, in those cultivars that bolt, by inflorescence bulbils. Some modern cultivars may produce flowers mixed with the bulbils, but they rarely set true (or viable) seed. Garlic resembles onion in growth, except that the leaves are thin and flat and the seed stalk (scape), when present, is solid instead of hollow. Cultivars which develop a seed stalk are often called stick garlic. Cloves are bound to the central axis of the plant and are covered with several layers of sheathing leaves. Most cloves, when peeled, are white and have white, tan, pink, reddish purple, or purple clove skins.

The Japanese bunching onion is grown for its edible tops and leaf bases, and has long been the main garden onion of China and Japan. Leek and kurrat (A. ampeloprasum L. var. kurrat) exist primarily as tetraploids (2n=4x=32) and closely resemble wild A. ampeloprasum. Many cultivars of leek have been selected for long, white, edible leaf bases with green tops, and for winter hardiness and resistance to bolting. Kurrat is primarily grown in Egypt and other Near Eastern countries for the green leaves. Hexaploid (2n=6x=48) great headed or elephant garlic (A. ampeloprasum var. holmense (Mill.) Aschers. et Graebn) is propagated vegetatively by its large cloves or smaller ground bulblets. It produces flowers but sets few or no seeds. Chive is cultivated for its leaves. Chinese chive (A. tuberosum) and rakkyo (A. chinense) have been domesticated in the Orient since ancient times. Both are perennials, spread by rhizomes, and are cultivated for the leaves and inflorescences. Cultivated forms of rakkyo are primarily tetraploid and are propagated vegetatively by bulb multiplication.

Original Work

Schwartz, H. F., Mohan, S. K., Havey, M. J., and Crowe, F. J. 2008. The Genus Allium. Pp. 1-4. In, Compendium of Onion and Garlic Diseases and Pests, 2nd ed. Edited by H. F. Schwartz, and S. K. Mohan. APS Press, St. Paul, MN. Available at http://www.shopapspress.org/coofonandgad.html

Selected References

• Agrios, G. N. 2005. Plant Pathology. 5th ed. Academic Press, San Diego, CA.
• Augusti, K. T. 1990. Therapeutic and medicinal values of onions and garlics. Pp. 93-108, in: H. D. Rabinowitch and J. L. Brewster (eds.). Onions and Allied Crops. CRC Press, Boca Raton, FL.
• Brewster J. L. 1994. Onions and Other Vegetable Alliums. CAB International, Wallingford, Oxon, U.K.
• Chase, M., Duvall, M., Hills, H., Conran, J., Cox A., Eguiarte, L., Hatwell, K., Fay, M., Caddick, R., Cameron, K., and Hoot, S. 1995. Molecular systematics of Lilianae. Pp. 109-137, in: Monocotyledons: Systematics and Evolution. P. Rudall, P. Crib, D. Culter, and C. Humphries (eds.). Royal Botanic Gardens, Kew, U. K.
• Currah, L., and Proctor, F. J. 1990. Onions in Tropical Regions. Nat. Res. Inst. (U. K.) Bull. No. 35.
• Dorofeyev, V. F. 1987. Phyto-geographical basis for plant breeding. Pp. 337-344, in: N.I. Vavilov: Origin and Geography of Cultivated Plants. Cambridge University Press, New York, NY.
• Fenwick, G., and Hanley, A. 1985. The Genus Allium. Crit. Rev. Food Sci. and Nutrit. 22:199-271.
• Jones, H. A., and Mann, L. K. 1963. Onions and Their Allies - Botany, Cultivation, and Utilization. Interscience Publishers, Inc., New York, NY.
• McCollum, G. D. 1976. Onions and allies. Pp. 186-190, in: Evolution of Crop Plants. N. Simmonds (ed.). Longman, London, U. K.
• Nonnecke, I. L. 1989. Bulbs. Pp. 294-319 in: Vegetable Production. Van Nostrand Reinhold, New York, NY.
• Rabinowitch, H. D., and Brewster, J. L. (eds.). 1990. Onions and Allied Crops. Vol. I - Botany, Physiology, and Genetics. CRC Press, Boca Raton, FL.
• Rabinowitch, H. D., and Brewster, J. L. (eds.). 1990. Onions and Allied Crops. Vol. II - Agronomy, Biotic Interactions, Pathology, and Crop Protection. CRC Press, Boca Raton, FL.
• Rabinowitch, H.D., and Currah, L. 2002. Allium Crop Science: Recent Advances. CABI Publishing, Wallingford, Oxon, U. K.
• Yamaguchi, M. 1983. Alliums: Onion, garlic, and others. Pp. 184-206, in: World Vegetables. AVI Publishing Co., Westport, CT.