Agriculture Reference
In-Depth Information
The genetics of onion storage performance, which
involves the study of quantitative trait loci (QTLs), have
been studied (e.g. Galmarini
et al
. 2001). The performance
of progenies from crosses between contrasting onion
lines showed that there were significant genotypic and
phenotypic correlations between the traits soluble solids
concentration (SSC), dry matter, pungency, and
in vitro
anti-(blood) platelet activity. A chromosome region on the
onion linkage group E seemed to account for a substantial
amount of the phenotypic variation for these traits.
However, Havey
et al
. (2003) later discovered that when
analyzing family means adjusted to average dry weights
(DW), linkage group E showed no significant effect on
these traits. This region may therefore be related to
onion water content. Havey
et al
. (2003) found QTLs on
linkage groups A and D associated with fructose and
sucrose concentrations which Galmarini
et al
. (2001) had
previously associated with SSC or DW. The close genetic
association of these characters may explain why the
linkages between them are fairly hard to break in practical
breeding terms.
Garlic genetics is more obscure, since no well-known
clones have been characterized genetically, and crossing
between them for experimental purposes is not yet possible.
However, the ancestral origins of garlic (
A. sativum
L.) and
its relationships to other species of
Allium
are gradually
being explored using the methods of molecular biology
(Etoh & Simon, 2002; Klaas & Friesen 2002). However,
these studies have not yet contributed to our understanding
of post-harvest performance in garlic and much remains
to be discovered about this species in the future. Different
clones of garlic certainly do differ in their storage ability:
for example, in Korea, 'northern' types store well for many
months at ambient temperatures while 'southern' lines
introduced from China more recently have shorter
dormancy and require storage at low temperatures to keep
for several months (J. K. Lee, personal communication,
2003). In Argentina, 13 clones were compared and the
influence of temperature, photoperiod and cold storage on
them was defined (Pozo
et al
. 1997). In India, several
clones were recently compared for dehydration potential
and significant differences were found (Singh & Gupta
2002). In Syria, mutations induced by gamma irradiation
have been used to try to improve resistance of garlic to the
fungal disease white rot (caused by
Sclerotium cepivorum
)
and its storage performance, with encouraging results
(Al-Safady
et al
. 2000); some lines from the mutation
breeding scheme showed reduced storage losses. Work on
the genetic transformation of garlic is beginning to be
reported (Park
et al
. 2002).
HARVESTING TECHNIQUES
In areas where onions can be stored with minimal structures
or even out of doors, methods of harvesting tend to be
rather rough and ready. Simply by giving better instruc-
tions to the labourers who harvest the onions, mechanical
damage and other unnecessary losses can be reduced. For
example, it is often poorly understood by the workers that
a substantial neck (minimum about 4 cm) should be left
on the onion before curing. Often, labourers trim onions at
the easiest level, that of the neck, causing wounds to the
not yet dry fleshy tissues which can lead to pathogens
becoming established.
If onion sowing or the planting of sets is slightly mis-
timed, this may result in too many doubled and split bulbs
which are unsuitable for storage. Hence, a whole-crop
approach which emphasizes optimum timing, appropriate
choice of variety, a fertilizer regime to avoid excessive N
being present in the crop at maturity, and good control
of pests and diseases, all contribute to the production of
a storage crop at harvest time with what Guerber-
Cahuzac (1996) described as 'quality capital'. The art of
the store manager is to retain this quality capital for as
long as is needed until the time is right to sell the crop at
a profit.
While many onions are still harvested by hand, either
where labour is cheap or where holdings or individual
fields are too small to allow a mechanized approach, in
northern Europe and other areas of extensive farming,
modified potato or carrot lifters have long been used to
mechanise the harvesting of hard storage onions. In the
'direct harvest' system the onions are topped, allowed to
dry for a short time then lifted and transported immediately
to bulk stores where the drying process begins immediately.
However, studies progressed in the United States to
develop an understanding of the physical factors, and to
improve the mechanisation of harvesting and handling
of sweet, soft onions (e.g. Maw
et al
. 2002a, 2002b). It has
been known for many years that it is important to treat
onion bulbs gently throughout the harvest and handling
processes (Isenberg 1955), and recent work using a
'synthetic' onion in Germany has helped to define the
points at which stresses and potential for damage occur
during harvesting and grading (Herold
et al
. 1998). The
addition of padding to equipment, and attention to dis-
tances that onions fall during store or bin loading all count
towards reducing damage and hence later losses. Novel
handling methods involving the use of very large bins con-
taining up to 18 t, to minimise handling, are now being
used commercially in the United Kingdom (P. Garrod,
2003, personal communication).