Agriculture Reference
In-Depth Information
Onions are valued for their pungency and give flavour (Randle and Lancaster,
2002) and texture to foods. In addition, both onion and garlic (
Allium sativum
) have
medicinal effects which may be important in the prevention and amelioration of
coronary problems (Augusti, 1990). In a recent review on the effect of
Alliums
(onion and garlic)
on health (Keusgen, 2002) it is suggested that consumption of
Allium
vegetables may lower the risk of gastrointestinal cancers. Also, it is
suggested that daily intake of garlic (
A. sativum
) significantly reduces the incidence
of diseases that are caused by atherosclerosis (Keusgen, 2002).
Physiologically, the onion is a long day plant and a certain length of day must
occur before bulbing is induced. Bulbing in high northern latitudes, such as UK, is
initiated when days are 14-16 h long. In low latitudes within the tropics and sub-
tropics, daylength varies little and onions have adapted to respond to shorter days;
these genotypes may initiate and form bulbs under daylengths of 13 h or less (Currah
and Proctor, 1990). Onion cultivars may be classified by daylength sensitivity: very
long day (VLD) (northern Europe), long day (LD) (as characterised by US 'long day'
onions), intermediate day (ID) and short day (SD) types (Bosch
et al
., 2002).
The onion bulb consists of a series of concentric fleshy leaf bases that are
enclosed in thin dry protective wrapper leaves. Because of this anatomy, onion bulbs
can be stored for considerable periods.
Brown-skinned onions of the 'Rijnsburger' type predominate in the UK and the
Netherlands. The crop is drilled directly in early spring as seeds or planted as sets
(small bulbils) to gain crop growth advantage. Two weeks before harvest, crops are
sprayed with a sprout suppressant, the onion foliage is removed mechanically and
the bulbs are direct harvested and transferred to bulk stores. The onions are surface
dried in forced air at 425 m
3
h t at 30
o
C and this is followed by gentler drying
with recirculated air until the onion necks (pseudostems) are fully dry (Anon.,
1977). The bulk stores are maintained overwinter at ambient temperatures for up to
-
1
-
1
five months; the storage life of the crop may be extended further by cold storage.
Gubb and MacTavish (2002) discuss the various factors that affect the storage
capability of different onion cultivars in different countries.
There are many different types of sub-tropical and tropical onions. Texas Grano
and New Mexico Yellow Grano are typical cultivars grown in the southern states of
North America (Corgan and Kedar, 1990). Similar cultivars are grown in Israel and
hybrid cultivars are widely used. About 80% of crops are direct seeded but sets and
transplants are also used. Generally the bulbs are harvested by hand.
In the tropics red-skinned types are common, for example, Red Creole and
Bombay Red; brown- and yellow-skinned onions also are grown. Texas Grano is a
cultivar common to many of the countries. Seeds are sown into flat beds and the
seedlings are transplanted into the field. Sometimes plants are grown on a ridge
system to enable irrigation during the growing period (Uzo and Currah, 1990). Bulbs
are harvested by hand and may be sold immediately. Also, bulbs may be field-cured
at high temperatures, after which some are stored in well-ventilated thatched houses
(Uzo and Currah, 1990).
Bosch
et al
. (2002) list the principal commercial types of onions for Europe,
Asia, USA, Canada, South America, Australasia, Africa and Russia. Similarly,
