Agriculture Reference
In-Depth Information
the genetic make-up of the cultivar and the conditions of
storage affect the length of dormancy.
Whereas onions are propagated by seed, set or transplant
the situation for shallots is changing. Many traditional
and selected clonal shallot cultivars were developed and
maintained by vegetative propagation, but recently,
commercial firms have started to offer shallot cultivars
which can be propagated by seed. This method avoids the
necessity for storing large quantities of propagating
material, and also frees the crop from viruses which reduce
yields, but growth from seed requires a longer season than
growth from bulblets. The prized culinary qualities of the
French 'Jersey' and 'Grey' shallot cultivars are not always
found in the seed-propagated types (Bufler 1998). In the
tropics, where the short growing season of the shallot is a
useful character, vegetative propagation may retain its
usefulness. Some types of multiplier onion (e.g. certain
cultivars in Sri Lanka and north east Brazil) are already
propagated locally by seed (Currah 2002). In the Côte
d'Ivoire, local seed-propagated shallots have been proposed
as a substitute for imported onions (David
et al
. 1998).
and to provide propagating material to plant for the next
year's crop. Selection of large, good quality cloves for
planting is desirable in order to maintain quality. In
countries where virus-free, or more accurately, virus-tested
material is produced, specialist growers carry out the
production of the early generations cleaned-up by meristem
culture under pest-proof covers, and release them to be
multiplied in 'low-pest' zones for one or two generations
before they are bulked up sufficiently to be sold to growers
(Salomon 2002). In several countries, such as France,
China and Argentina, enterprises specializing in the
production of virus-tested propagating material have been
established during the past 20 years.
In terms of physiology, garlic is affected by cool
treatment of the bulb during the dormant period; storage
temperatures which are very low (near 0°C) affect the
quality of the bulbs in the subsequent season by leading
to the production of 'rough' (irregularly shaped) bulbs.
Therefore, storage conditions for 'seed' garlic should be
quite distinct from those which best preserve the
commercial bulb crop for sale. However, since so many
different clonal varieties exist, their exact temperature
responses are still being determined. Studies on this topic
have been reported from France (Messiaen
et al
. 1993),
Argentina (Pozo
et al
. 1997), Canada (Bandara
et al
. 2000)
and Italy (Miccolis
et al
. 2000). Commercial storage of
garlic for sale can be successful at either warm (about
25°C) or cool (−3°C to 0°C) temperatures. Garlic has a
higher dry matter content than onion (about 40-43% in
recent Indian studies; Singh & Gupta 2002); even so,
storage at −4°C or lower is not regarded as safe, and
many studies suggest that temperatures of about 0°C are
best suited for long-term storage. Some cultivars with very
long dormancy can be stored at atmospheric temperatures
for several months, for example the 'northern' type of
garlic grown in Korea (J.K. Lee, personal communication,
2003) which is dried and safely stored in barns.
GARLIC ANATOMY AND PHYSIOLOGY
Garlic is another important edible bulbous allium. It differs
in several ways from onions and shallots. Garlic is an ancient
cultivated crop which has long been propagated vegeta-
tively, and until recently it was generally thought to be infer-
tile by seed. The garlic bulb of commerce differs in structure
from onions and shallots because it is formed in a different
way. All the leaf bases of the plant become papery at matu-
rity, rather than the inner ones becoming fleshy as in
A.
cepa
: meanwhile, within the leaf axils, one or more whorls
of buds develop which give rise to discrete fleshy garlic
bulblets or 'cloves', all joined together by the basal plate.
Many varieties of garlic also produce a central flower stalk
which may be fully or partially developed at bulb maturity.
The different garlic groups were described and illustrated by
Messiaen
et al
. (1993), and the day-length responses of
these groups are also discussed. Etoh and Simon (2002) also
reviewed garlic from the evolutionary point of view and pre-
sented the recent achievements of breeders who are working
to develop sexually propagated garlic.
Each edible garlic clove consists of an outer papery
membrane, one inner fleshy scale (which is considerably
higher in dry matter and pungency than those of the onion),
and at its centre, an internal leaf bud with shoot initials
which is the germ of the next season's leafy sprout (Takagi
1990). The garlic 'bulb' is therefore a compound structure
which is held together by the outermost papery scales.
Usually it is separated into cloves both for consumption
FACTORS AFFECTING THE POST-HARVEST
PERFORMANCE OF ONIONS AND GARLIC
Pre-harvest factors
These were recently reviewed by Gubb and MacTavish
(2002) in the context of post-harvest biology and by Bosch-
Serra and Currah (2002) in terms of field agronomy. Some
of the key factors affecting onion quality are the choice
of appropriate long-storing varieties for the production
area; careful timing of sowing and (if used) transplanting,
so as to avoid environmental conditions which may lead to
