Onion

The onion (Allium cepa L.) is a biennial or perennial herb usually grown as an annual and harvested in its first season. The leaves are tubular, hollow and blue-green, arranged in a fan, about 30–60 cm long. The edible part is the bulb — actually a short, flattened stem disc bearing thick, fleshy leaf sheaths (scales) layered around a central bud; normally one bulb forms per plant, with dry outer skin coloured white, yellow, red or purple depending on cultivar. The roots are shallow and fibrous, arising from the stem disc. In the second year the plant sends up a hollow flower scape up to about 1 m tall bearing a spherical umbel of many small white-green flowers; the fruit is a three-valved capsule with black seeds. Bulb size depends on day length: “long-day” cultivars bulb under long days and “short-day” cultivars under short days, so each growing region chooses suitable varieties.

According to Kew POWO the native range of Allium cepa is given as “a cultigen from Central Asia”; it is a bulbous geophyte of the temperate biome, known only from cultivation, with an unknown wild ancestor. Many sources hold that the onion originated in Central Asia and Persia (in what is now Iran, Pakistan and Afghanistan); some phylogenetic work (Yusupov et al., 2020) instead suggests that Central Asian Allium species were not the direct ancestors of the cultivated onion, indicating a complex domestication history in which the wild ancestor may be extinct. Onions have been grown since antiquity, recorded in ancient Egypt, Greece and Rome, and then spread across Europe and Asia. According to POWO the species is now introduced and naturalised in very many regions (Europe, Asia, the Americas, Africa…), with major growing areas including China, India, the USA and many other countries. Dry onion is one of the world's highest-yielding vegetable crops — according to FAO, more than 100 million tonnes a year, led by China and India. In Vietnam onions are grown mainly in the winter–spring crop in the north.

Chemically, the most characteristic feature of the onion is its sulfur compounds. In the intact bulb these are odourless precursors, mainly cysteine sulfoxides (typically isoalliin, trans-S-1-propenyl-L-cysteine sulfoxide). When the onion is cut or crushed, the enzyme alliinase is released and converts isoalliin into sulfenic acids; in onion the enzyme lachrymatory-factor synthase (LFS) then converts part of these into propanethial S-oxide — a volatile compound that makes the eyes water (the lachrymatory factor), while other parts form thiosulfinates and compounds such as zwiebelanes. The LFS enzyme was identified and published in the journal Nature (Imai et al., 2002). As for flavonoids, onions are notably rich in quercetin: the two main glucosides are quercetin 4'-glucoside and quercetin 3,4'-diglucoside (making up most of the total flavonols); quercetin is most concentrated in the dry skin, so peeling greatly reduces it. The bulb also contains kaempferol, isorhamnetin, prebiotic fructans/oligosaccharides (FOS), soluble sugars (sucrose, glucose, fructose), phenolic acids, potassium, selenium, vitamin C, B6 and folate; red/purple onions are additionally rich in anthocyanins (mainly cyanidin derivatives), and there are characteristic sulfur compounds such as onionin A.

Onion is used mainly for its bulb, as a vegetable and flavouring: it forms the aromatic base of many dishes (mirepoix, sofrito) and is stir-fried, souped, stewed, roasted, eaten raw in salads, pickled and put in hamburgers; industry makes onion powder, dried onion, onion oil and blended seasonings. In Vietnam onions are stir-fried with beef, used in soups, salads and bánh mì, and grown mainly in the northern winter–spring crop. In traditional medicine onion is used as an antibacterial, anti-inflammatory, expectorant and circulatory and diaphoretic remedy. In research, the tear-producing LFS enzyme was published in Nature (Imai et al., 2002); several reviews (2020–2021) assess the biological activities of onion and its main constituents: antioxidant, anti-inflammatory and immunomodulatory effects (a PubMed review, 2021) and antibacterial, antiplatelet, anti-diabetic, cardiovascular, bone and neuroprotective effects (the review “Recent Advances in Bioactive Compounds, Health Functions, and Safety Concerns of Onion”, Frontiers in Nutrition, 2021); clinical trials using quercetin-rich onion-skin extract have also been conducted. The flavonol quercetin and the sulfur compounds (thiosulfinates, onionin A) are the two most-studied groups, with quercetin prominent for antioxidant activity and inhibition of inflammatory pathways.

As food, onion is safe for people at normal levels; its bite and eye-watering come from propanethial S-oxide and it may cause stomach irritation or bad breath in sensitive people, and heavy handling can cause contact dermatitis. Crucially, there is toxicity to animals: onions (and garlic) cause haemolytic anaemia in dogs and cats — sulfur compounds such as n-propyl disulfide and S-methylcysteine sulfoxide destroy red blood cells (Heinz bodies), so onions must never be fed to pets, not even cooked onion or onion powder. Some quercetin supplements made from onion skin may interact with medicines, so medical advice is advisable. On conservation, Allium cepa is Not Evaluated (NE) on the IUCN Red List and is not listed in the CITES appendices; the species is known only from cultivation and has naturalised in many places, so it is not threatened in the wild. The main risks are erosion of local variety diversity and dependence on a few commercial hybrids, together with pressure from diseases (downy mildew, onion thrips). Conservation priorities: maintaining local and traditional onion varieties and wild relatives of the genus Allium in genebanks and in situ, because they carry valuable genes for disease resistance, yield and quality.