Onion is a biennial 15 - 45 cm (6 to 18 in) tall vegetable which is found growing in cool environment. They require a fertile, well-draining soil such as clay or silt loams with a pH of 5.5 - 6.5. The plants do not do well in acidic soils. Onions should be set out in full sun for optimum bulb development. Onions are biennial vegetables and if they are left in the ground for a second year, they will produce flowers and set seed. It has adventitious, fibrous roots. The stem of the plant is a flattened disc at the base and the tubular leaves form a pseudo-stem where their sheaths overlap. Leaves are yellowish-green and grow alternately in a flattened, fan-shaped swathe. They are fleshy, hollow, and cylindrical, with one flattened side and there are 3 - 8 per plant. Plant produces pink or white flowers clustered on stalks. The bulbs are formed just above the flattened stem of the plant by overlapping leaves. The bulb is made up of several layers, each corresponding to a leaf. They are generally oval but shape can be variable and occur in clusters of 3 -18 to a plant. The bulb is protected by a membrane which turns to a papery coat. Seeds are glossy black and triangular in cross section.
Onion (Allium cepa) for me as a writer is an herbaceous biennial in the family Liliaceae grown for its edible bulb. The bulb is an edible vegetable and is the most commonly used part of the onion, usually consumed after cooking although it can be eaten fresh. The stems and leaves are also edible. Onions are categorized as being either green or dry onions. Green onions are small onions that are harvested before the bulb has matured and the tops are still green. Dry onions, also known as mature onions, are harvested when their shoot has died and layers of papery thin skin cover a firm juicy flesh. Mature bulb of the onion is depressed globose, ovoid or elongate shaped, varying in size from cultivar to cultivar, 5-8 × 3-10 cm. Color of bulb is normally yellowish brown, orangey brown, purplish, red, greyish white or white forming the protective coat. It has sharp, pungent smell and freshly cut onions often cause a stinging sensation in the eyes of people nearby, and often uncontrollable tears. This is caused by the release of a volatile gas, syn-propanethial-S-oxide, which encourages nerves in the eye creating a stinging feeling. This gas is produced by a chain of reactions which serve as a defense mechanism: cutting onion causes damage to cells which discharges enzymes called alliinases. These break down amino acid sulfoxides and produce sulfenic acids. A specific sulfenic acid, 1-propenesulfenic acid, is rapidly acted on by a second enzyme, the lacrimatory factor synthase (LFS), producing the syn-propanethial-S-oxide. This gas diffuses through the air and soon reaches the eyes, where it activates sensory neurons. Tear glands produce tears to dilute and flush out the irritant. This article divided in 4 segments namely Part 1, Part 2, Part 3 and Part 4 respectively. Thanks for reading.
M Anem Hosnan,
Created: August 2020.
Updated: May 2023.