Onion (Allium cepa L.) has been valued as a food and a
medicinal plantsince ancient times. It is widely cultivated,
second only to tomato, and is a vegetable bulb crop known
to most cultures and consumed worldwide (FAO, 2012).
It is a short duration horticultural crop (Brewster, 1990)
grown at low latitudes. It is commonly known as “Queen
of the kitchen,” due to its highly valued flavor, aroma, and
unique taste, and the medicinal properties of its flavor
compounds(Selvaraj, 1976; Griffiths et al., 2002). Onion is
used throughout the year, for example in curries, in the
form of spices, in salads, as a condiment, or cooked with
other vegetables, such as boiled or baked. It is also used in
different forms of processed food, e.g. pickles, powder,
paste, and flakes, and it is known for its medicinal values.
Onions have been used by humanssince the Neolithic age,
and they are still being used all over the world. Over this
long period, there have always been people who have
appreciated the use of onions and used them in considerable quantities, but there have also been those who have
rejected and detested them (Koch and Lawson, 1996).
Onions have been cultivated globally, in at least 175
countries, for around 5000 years. Ancient Egyptians
regarded the spherical bulb as a symbol of the universe.
The name is probably is taken from the Latin unus
meaning “one,” and the Romans introduced the onion
to Britain, from where it may have been carried to the
Americas (Burnie et al., 1999). The first known written
report about the onion comes from the Sumerians and
dates back to 2600–2100 BC.In the Papyrus Ebers,which is
based on ancient Egyptian writings and knowledge,
we discover that leek played an important role in the
kingdom of old Egypt. The great physician Hippocrates
suggested onion as a diuretic, laxative, and emmenagogue. He also used onion for the treatment of pneumonia, and, externally, for healing putrid wounds (Koch and
Onion is an extensively grown biennial bulb crop, with
world production of 74,250,809 tonnes from an area of
4,364,000 hectares (Table 58.1). China and India are the
primary onion growing countries, followed by the USA,
Egypt, Iran, Turkey, Pakistan, Brazil, the Russian Federation, and the Republic of Korea (FAO, 2012). Onion
productivity is highest in the Republic of Korea
(66.16 t/ha), followed by the USA (56.26 t/ha), Spain
(53.31 t/ha), and the Netherlands (51.64 t/ha); the average
productivity across the world is 19.79 t/ha. From an
economic point of view, the international trade in onion
exports is 6.77 million tonnes. The Netherlands is the
highest onion exporter (1.33 million tonnes) followed by
India, China, Egypt, Mexico, USA, Spain, and Argentina.
Bangladesh, Malaysia, the Russian Federation, the UK,
Japan, and Saudi Arabia are the major onion importing
countries in the world.
The genus Allium is very large and consists of many wild
edible species (only a small fraction is cultivated commercially), and is widely distributed over temperate zones
in the northern hemisphere (Hanelt, 1990; Gautam et al.,
1997). The place of origin is purported to be in central
Asia, and the Mediterranean regions are considered to be
the secondary centre of origin (CSIR, 2003). The genus
Allium contains more than 780 species (Burnie et al.,
1999) with large diversities in morphological characters.
The chromosome number of onion is 16 (2n). It has been
classified in hierarchical level as follows:
Super division: Spermatophyta
Species: Allium cepa L.
Onion belongsto the family Amaryllidaceae; the plant is
either biannual or perennial (depending on the cultivar),
and smells when crushed (WHO, 1999). The plant has
shallow adventitious fibrous roots (Ranjitkar, 2003), bulb,
and tubular leaves. The stem grows 100–200 cm tall
during the second year of the plant’s life. The green leaves
of the plant are an extension of the outer food storage
leaves. The inflorescence is umbel-like and develops from
a ring-like apical meristem. The umbel is the aggregation
of flowers at various stages of development, and it contains 200–600 small individual flowers, although this
number can range from 50 to 1000 (Ross, 2001). It is
composed of white or greenish-white small flowers which
grow at the tip of the stem in the second year of the plant.
The onion bulb ranges in shape from flat to globular to
oblong, and the onions are usually of three colors: red,
white, and yellow (Fritsch, 2005). The fruits are capsule
and contain black seeds. The bulb is composed of fleshy
and enlarged leaf bases. The edible onion bulb can grow
up to 10 cm in diameter, and it is composed of several
overlapping layers on a central core. The outer leaf bases
of the bulb lose moisture and become scaly by the time of
harvesting, and the inner leaves thicken as the bulb
develops. The majority of the species of onion grow in
open, sunny, and dry land, mainly in humid climates.
However, the Allium species have been adopted in other
ecological niches of the world (Fritsch and Friesen, 2002).
Cultivars in Major Countries of Production
Many onion cultivars (Table 58.2) have been developed
worldwide. The majority of the cultivars are grown for dry
bulbs, salad purposes, and pickling. These cultivars have
diversities in photoperiod, storage life, skin color, and
bioactive compounds (Brewster, 1994).