The onion (Allium cepa) is a widely cultivated bulb vegetable belonging to the Amaryllidaceae family, valued both for its culinary versatility and its biological complexity. It develops as a layered bulb formed from swollen leaf bases that store carbohydrates, allowing the plant to survive adverse conditions and regrow. Onions are rich in sulfur-containing compounds, which are responsible for their pungent aroma and the tearing reaction they cause when cut; these compounds form when cell damage activates enzymes like alliinase, producing volatile sulfur gases.
Nutritionally, onions provide dietary fiber, vitamin C, vitamin B6, folate, and antioxidants such as quercetin, which has been studied for anti-inflammatory and cardiovascular benefits. Onions also exhibit antimicrobial properties and have played roles in traditional medicine for centuries. Agriculturally, they are a cool-season biennial grown as an annual, highly sensitive to day length, which determines bulb formation and makes specific varieties suitable for different latitudes.