Agriculture Reference
In-Depth Information
bolting or to splitting and doubling of the bulbs; careful use
of fertilizers such that nitrogen is not scarce in the early
growth stages but is mostly used up by the plant by the time
of harvest; care with the application of irrigation so that the
bulbs are encouraged to dry down fully by the time of
harvest; and the use of a sprouting inhibitor (usually maleic
hydrazide or MH) during the last stages of leaf die-down
but while the foliage is still green. A recent report of the
effect of nitrogen on sweet onions confirmed the need to
keep this element to low levels to reduce rots in storage
(Díaz-Pérez
et al
. 2003). There are also indications that
soil texture can influence storage life: for example, in
the United Kingdom onions from sandy soils seemed to
store better than those from peaty (muck) soils, possibly
because the bulbs dried out more rapidly and thoroughly
once irrigation was withdrawn. In other countries, trace
elements such as copper have been shown to influence skin
quality and hence storage life. It is also important that
sulphur nutrition should be adequate so that full pungency
(if desired) and the correct texture of the cell walls can be
developed (Randle
et al
. 1999; Lancaster
et al
. 2001).
could be detrimental to the environment as there are
concerns over the risk of leaching into drinking water
(Sorensen & Grevsen 2001). Pressure from retailers and
consumers has led to a need for new methods to extend
storage.
Genetics
The genetic component of a cultivar is crucial to its storage
life. Genetically controlled factors which are involved
include the length of dormancy, dry matter content, num-
ber and thickness of outer dry scales after curing and bulb
colour (which can influence susceptibility to disease). The
history of the cultivar in terms of its traditional role (or
that of its immediate open-pollinated progenitors) in the
farming system is important to understand: for example,
in Spain there are traditional onions for several different
seasonal markets, some of which are grown for sale fresh
(the earliest overwintering onions) while others (summer-
grown onions) have been selected as suitable for storage.
The choice of suitable cultivars is therefore an essential
part of the system of storage onion production.
Seed companies provide valuable information on the
storage aspect of cultivars for particular production areas,
but in new areas where onion storage has no local tradition,
experimental validation of storage performance under
local conditions is essential. This should be carried out as
replicated trials on healthy samples of onions which are
examined at regular intervals to check losses from evapora-
tion and respiration (physiological weight loss) as well as
visible losses from rots, sprouts and rooting. Two accounts
by Ko
et al
. (2002a, 2002b) from Taiwan provide good
examples of a multidisciplinary study of this type on short-
day onions, including observations on varietal differences
in black mould resistance.
Currah and Proctor (1990) found that different genetic
groups of short-day onion could be differentiated by their
storage performance. For example, Creole onions stored
longer than Grano and Granex types, while in Egypt very
long storage (up to ten months) was reported under dry hot
conditions with local cultivars. In Europe and Japan, the
majority of storage onions are usually pungent yellow-
brown types, though milder onions of the Spanish summer
storage types are also grown in several countries (western
United States, Argentina and Chile). The physical charac-
ters of the bulbs, and particularly their hardness (which is
related to resistance to stacking damage), may determine
what storage method is best chosen for a particular
group. Softer onions are often stored in bins to limit stack-
ing damage, whereas hard cultivars can be stored in bulk,
sometimes to a height of 3 or even 4 m.
Maleic hydrazide
An important pre-harvest chemical treatment of onions to
prevent early sprouting in store was developed in the
1950s: this was maleic hydrazide (MH) (Isenberg 1956).
The chemical must be applied to the maturing plants before
the foliage has died down and correct timing is crucial -
application at 10% tops down is now recommended in
the United Kingdom (O'Connor 2002) - since the active
ingredient must be absorbed and translocated to the
internal growing points before the foliage has dried up
completely. The chemical acts to prevent cell division at
the growing point by inhibiting cell division - it prevents
spindle formation during mitosis. Eventually during
prolonged storage, the growing points darken and the
onions lose quality by loss of water. MH treatment is
routinely applied to storage onions in the United Kingdom
and the United States. In Germany, however, its use is not
permitted. The use of MH in foodstuffs has been
questioned as studies have shown links between MH and
the production of tumours in mammals (Ribas
et al
. 1996).
It has been estimated that the average adult ingests
approximately 630 mg of MH a day from potatoes, onions
and tobacco from cigarette smoke (USEPA 1999). The
use of MH on onions has been banned by many retailers as
it is the only chemical used on onion that leaves a detect-
able residue. Typically, levels of MH in onion bulbs lie
between 4 and 6 mg kg
−1
with the maximum residue level
(MRL) at 10 mg kg
−1
(Johnson 2006). Additionally, MH
