Onion

• 1. Introduction

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  The onion, Allium cepa, is the most widely cultivated species of an entire genus of vegetables, yet its ancestral wild form has never been found. Nobody can point to a wild plant in some forgotten valley and say: that is where it all began. Humans have grown and selectively bred onions for at least 7,000 years, and still the origin remains uncertain. Ancient records of its use stretch across both eastern and western Asia. That gap between deep antiquity and botanical mystery is where this story lives. How did a plant with no confirmed wild ancestor become a fixture in the eye sockets of Egyptian pharaohs, the recipes of ancient Mesopotamia, and the cargo holds of ships crossing the Atlantic? And why, in 2008, did scientists in New Zealand genetically modify it to stop making people cry?

• 2. A Plant Without a Wild Ancestor

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  Carl Linnaeus first formally described Allium cepa in his 1753 work Species Plantarum, but the plant was already ancient by then. Related wild species survive in Central Asia and Iran. The most closely related include Allium vavilovii from Turkmenistan and Allium asarense from Iran. Yet A. cepa itself is known exclusively from cultivation. Escapes from farming have established feral populations in some regions, but no truly wild population has ever been confirmed as the original stock. The genus name Allium is the classical Latin word for garlic. Cepa is Latin for onion, and its echoes persist in Spanish cebolla, Italian cipolla, Polish cebula, and the German Zwiebel. Even the English word chive traces back through Old French chive to that same Latin root. Within the genus, the Japanese bunching onion Allium fistulosum and the Canada onion Allium canadense represent distinct cultivated relatives. The shallot, long treated as a separate species, was reclassified as a botanical variety of the common onion as recently as 2011.

• 3. Deep Antiquity

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  Ancient Mesopotamian recipes that featured onions and other Allium species were pressed into clay in cuneiform script on what are now called the Yale culinary tablets. Dating from around 2000 BC, these are the oldest known recipes in the world. The Assyriologist Jean Bottero, who was also known as a gourmet cook, described the cuisine they recorded as one of striking richness, refinement, sophistication, and artistry. Onions appear in the Hebrew Bible, and archaeologists have traced their presence in ancient Israel to Chalcolithic Nahal Mishmar and to Bronze Age Jericho. In Egypt, the bulb carried spiritual weight. Ancient Egyptians viewed the onion's spherical shape and concentric rings as symbols of eternal life. Onion traces were found in the eye sockets of Ramesses IV. Pliny the Elder, writing in the first century AD, recorded Roman beliefs that onions could improve eyesight, aid sleep, and heal conditions ranging from oral sores and toothaches to dog bites, lumbago, and dysentery. Archaeologists excavating Pompeii, buried by volcanic eruption in 79 AD, found gardens that resembled those described in Pliny's detailed accounts. The collection of Roman recipes attributed to Apicius, assembled in the fifth or sixth century AD, drew on onions repeatedly.

• 4. Crossing Oceans

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  In the Age of Discovery, European settlers brought onions to North America as part of the Columbian exchange. They found that close relatives, such as Allium tricoccum, were already widely used in Native American gastronomy. Diaries kept by early English colonists record that the bulb onion was one of the very first crops planted in North America by the Pilgrim fathers. The vegetable took hold quickly in its new land. Between 1883 and 1939, inventors in the United States patented 97 inventions aimed at making onion cultivation more efficient through automation. That wave of mechanical ingenuity over those five decades reflects just how seriously a young agricultural nation took a crop that had travelled so far to reach it. Onion Johnnies, Breton farmers and agricultural labourers from Roscoff in Brittany, made their own trans-channel crossing, travelling first on foot and later on bicycles to sell strings of distinctive pink onions door to door in Britain. And before New York City claimed the nickname the Big Apple, it was known as the Big Onion. Chicago later inherited that name, a reference to the native American word for the city's site, which denotes a type of wild onion.

• 5. The Tear Chemistry

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  Chopping an onion damages its cells, triggering a sequence of reactions that the plant uses as a defence. Released enzymes called alliinases break down amino acid sulfoxides and produce sulfenic acids. One specific sulfenic acid, 1-propenesulfenic acid, is then acted on almost immediately by a second enzyme, lacrimatory factor synthase (LFS), producing a volatile liquid called syn-propanethial-S-oxide. This gas diffuses through the air, reaches the eyes, and activates sensory neurons. The lacrimal glands respond by flooding the eye with tears to flush the irritant out. Cutting onions under running water or submerged in a basin reduces this effect. Leaving the root end intact also helps, because the base of the bulb holds a higher concentration of sulphur compounds than the rest. In 2008, the New Zealand Institute for Crop and Food Research created tearless onions through genetic modification, preventing LFS synthesis entirely. Bayer brought the first commercially available LFS-silenced onions to market in early 2018 under the name Sunions, and they were the product of 30 years of cross-breeding without genetic modification. One study found that consumers preferred the flavour of onions with lower LFS content, though some consider LFS-negative onions inferior because reducing the enzyme also impedes the plant's sulfur uptake. A method based on mass spectrometry has been developed to efficiently distinguish LFS-negative from LFS-positive onions in high-volume production settings.

• 6. Colour, Chemistry, and Cultivation

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  Yellow onions carry the highest total flavonoid content among the common varieties, reaching a level 11 times higher than white onions. Red onions contain considerable anthocyanin pigments, with at least 25 distinct compounds identified, representing 10% of their total flavonoid content. Shallots have the highest polyphenol levels of all, carrying six times the amount found in Vidalia onions. Most onion cultivars are roughly 89% water, with about 9% carbohydrates including 4% sugar and 2% dietary fibre. A 100-gram serving delivers 166 kilojoules (40 kilocalories) of energy. Cultivating onions well depends on understanding day-length. Traditional European varieties are long-day onions that begin bulbing only after 14 or more hours of daylight. Southern European and North African intermediate-day types need only 12 to 13 hours. Short-day varieties, developed more recently, can be planted in mild-winter areas in autumn and require as little as 11 to 12 hours to begin forming bulbs. Bulbing typically takes 12 to 18 weeks. In 2021, China, India, the United States, and Turkey were the world's top producers. In 2022, world production of onions and shallots as green produce reached 5.0 million tonnes, with China accounting for 17% of that total.

• 7. Pests, Rot, and Resilience

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  The onion fly, Delia antiqua, lays its eggs on leaves, stems, and soil near onion plants. It is drawn to the smell of damaged tissue, which means thinning operations can trigger infestations. The larvae tunnel into bulbs, causing foliage to wilt and yellow and bulbs to disfigure and rot, especially in wet weather. The onion eelworm, Ditylenchus dipsaci, is a tiny soil-living nematode that causes swollen, distorted foliage; young plants die, and older ones produce soft bulbs. No cure exists. Affected soil should not be used for onions again for several years, and susceptible crops including carrots, parsnips, and beans must also be avoided. White rot is caused by the soil-borne fungus Sclerotium cepivorum. As roots decay, the bulb bases become covered in fluffy white mycelia that later produce small black structures called sclerotia. These persist in the soil and can reinfect future crops indefinitely. Neck rot, caused by Botrytis allii, attacks bulbs in storage, producing grey mould that eventually dries the bulb into a mummified husk. Preventing physical damage at harvest, followed by careful drying and cool, well-ventilated storage, remains the best defence. Onion oil has been authorised in the European Union as a pesticide against carrot fly in umbelliferous crops.

• 8. Onions in Culture and Verse

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  The 10th-century Exeter Book, written in Old English, contains a riddle that appears to describe an onion in terms laden with sexual overtones. The riddle's "wondrous creature, a joy to women" stands "in a bed"; its column is erect and tall; a woman rubs it to redness; the riddle ends with the line "Wet will be that eye." Homer, in the Odyssey, described the tunic given to the disguised Odysseus as being like "the skin of a dried onion, so soft was it, and it shone in the sun." The scholar R. Drew Griffith noted that the double comparison of the tunic to onion skin and sunlight risks being funny, and recorded that Theopompus found it ridiculous. Griffith proposed that Homer chose the onion deliberately for its capacity to produce tears, hinting at Penelope's grief over Odysseus's long absence. In India, when onion prices spiked sharply in 2015, the Hindustan Times reported that the price surge prompted a wave of public jokes, including the Hindi pun that plays on the words for love and onion: pyar and pyaz. The cross-cultural reach of that moment, from a Bronze Age epic to a 2015 newspaper in India, suggests that the onion's hold on human imagination has never really depended on its botanical classification.