Common onions (Allium Cepa) are a widely cultivated food crop. Onions are part of the genus Allium, which includes a wide variety of other plants, several of which are used for food by people. In all there are more than 780 different species in the genus Allium. They include leek, garlic, and, of course, onions. According to Friesen et al. (2006), onions belong to the Cepa section of the subgenus Cepa. The common onion is closely related to the
Green onions are relatives of the common onion. There are a variety of different types of green onions that are known by different names, including Japanese bunching onions, Welsh onions, scallions, escallions, salad onions, and green shallots. Green onions, like the Japanese bunching onion or Welsh onion (Allium fistulosum), lack fully developed bulbs. Instead of the bulbs being used for food, like in the common onions (A. Cepa), the stalks of green onions are usually eaten.
Wild relatives of common onions grow in the Tien Shan and Pamir-Altai mountains of Central Asia. These wild species are collected for food by local people, and scientists believe that they may be used to improve the genes of commercially cultivated onions.
Shallots and Potato Onions:
Shallots are one of the more distinct varieties of A. Cepa onions. They tend to be smaller than other onions, and have a milder flavor. Another distinct type of A. Cepa onion is the potato onion. Potato onions were more popular in the past, but have become less common.
Some varieties of common onions have a milder taste than other varieties. These types of onions are known as sweet onions. Sweet onions varieties grown in the United States were developed in government-sponsored breeding programs in the 19th and 20th centuries. Texas Grano sweet onions, for example, was developed from the hybridization of ‘Bermuda type’ onions, which were of Italian origin, and Babosa type onions, which were imported from Valencia in Spain.
There are a wide range of other varieties of common onions. Over the centuries many different varieties of common onion have been developed in different parts of the world. Regional varieties that developed over the course of centuries are well adapted to temperatures and the growing conditions of specific regions. The greatest diversity of common onion (A. Cepa) varieties in found in the Eastern Mediterranean, Middle East, Central Asia, and the northern parts of the Indian Subcontinent. This is because onions have been cultivated in these regions for particularly long time. Beginning in the 19th century, state-sponsored breeding programs produced new varieties of onions for commercial production.
Commercially cultivated varieties of common onions differ in size, bulb shape, color, and taste. For example, skin colors may be white, yellow, purple, red, brown, or green.
James L. Brewster. Onions and Other Vegetable Alliums. 2nd Ed. Cambridge, Mass.: CABI, 2008., p. 1-14.