Agriculture Reference
In-Depth Information
are normally grown in the tropics, a survey was made to
find out more about the nature of tropical landrace and
popular imported types of SD onions. Information was
obtained on cultivars and their yields and storage
performance, seed sources, growing methods and methods
of curing and storage. Several different genetically distinct
types of short-day onions were being grown, and their
storage performance was related to their ancestry and
traditional uses (Currah & Proctor 1990).
Biological aspects of onion post-harvest have also
been thoroughly reviewed in recent years. Komochi (1990)
gave a good summary of biological findings on onion
dormancy up to that date and Maude (1990) reviewed
storage diseases and their control. Storage and transport
diseases and disorders of onions were illustrated and
described by Snowdon (1991). Brice
et al
. (1997) presented
the options for choosing various types of storage installa-
tions and provided decision trees to help readers to
define their needs and the choices which would be
feasible for their own environments and economic
situations. Gubb and MacTavish (2002) reviewed recent
literature on the biological aspects of storage. The
findings on bacterial diseases of onions were reviewed by
Mark
et al
. (2002). For garlic, Messiaen
et al
. (1993) dealt
briefly with garlic storage from the European perspective
after consulting growers in France. However, garlic storage
has not been as extensively reviewed as the onion story,
and many of the most useful papers published are in
French or Spanish, with many from Latin America.
cylindrical leaf bases, which enclose a number of fleshy
bladeless leaves or bulb scales. All the thin and swollen
scales alike are joined at their lower ends to the 'base plate',
a flattened disc-like stem which provides all the vascular
connections between the different layers of the bulb and the
roots. 'Dormancy' is the term used to refer to the state of
the onion at harvest time, when there is little cell division
taking place, inhibitory growth substances have been
exported from the leaves to the bulb and biological activity
is at a low ebb. The art of the store manager is to keep the
onion in a dormant state for as long as possible, until the
time is right for it to be sold. Dormancy normally wears off
gradually and is replaced by a stage known as 'rest' (period
of sprout suppression) in which the onion can resume
internal activity under the influence of suitable temperatures
and in the presence of water or of high atmospheric
humidity, eventually producing visible shoots.
Shallots are a morphologically distinct subgroup of
A. cepa
: instead of a single bulb, they form a cluster of
relatively small dry bulblets which remain attached together
at the base when they are mature. Traditionally, shallots
have been multiplied vegetatively by separating and
planting out the small bulblets, each of which produces a
new shallot cluster in the next growing season. Intermediate
forms between shallots and onions, known as potato onions
or multiplier onions, are grown in countries such as Russia,
Finland, southern India and Sri Lanka. A review of liter-
ature on shallots, including some experimental results, was
published by Rabinowitch and Kamenetsky (2002).
The outer scales of onions and shallots, formed from the
lower parts of leaf sheaths which expand actively around
the inflating fleshy scales during the bulbing process, dry
out during bulb maturation and curing, to produce one or
more layers of dry papery scales. These dry or semi-dry
scales (the 'skins' of the onion) tightly envelop and protect
the bulbs during the dormant period. Their integrity is
important for maintaining bulb dormancy (Apeland 1971),
and at least one undamaged layer of skin over the bulb is
also needed when the onions are prepared for sale, since
the presence of the intact skin is an important quality
feature of the onion (Hole
et al
. 2000). During 'curing', an
important stage in preparing onions for long-term storage,
simple compounds present in the skins polymerize: some
produce chemicals with antibiotic properties which are
effective against some fungi attacking onion bulbs
(Takahama & Hirota 2000). It may be that the gradual
process of the withdrawal of water from the skins
underlying the outermost dry one contributes to keeping
the bulb turgid. However, little research has been done to
trace the exact changes which take place at this time. Both
ONION ANATOMY AND PHYSIOLOGY FROM
A STORAGE VIEWPOINT
The cultivated onion is a biennial and forms its storage
organ, the bulb, at the end of the first season of growth.
The bulb stores water and nutrients for the next season of
growth, when the onion would naturally mobilise its reser-
ves to sprout rapidly and produce flowering shoots and
eventually seeds. In its initial season of growth, the onion
first produces a substantial vegetative (leafy) plant and then
switches into 'bulbing' mode under the influence of several
factors, most notably photoperiod (day length). The onset
and rate of bulbing are also influenced by a number of other
factors such as temperature, spacing, nutrient status, and
biotic and abiotic stresses (Brewster 1994, 1997, 2008).
Bulbing is the process by which water and assimilates from
the foliage leaves are stored in the fleshy leaf bases and in
special bladeless storage scales which are formed at the
heart of the bulb after bulbing is initiated.
The dry bulb onion of commerce is therefore a simple
layered structure formed by the swelling of the initially
