Post harvest diseases fruits and vegetables - Xavier University ...

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Host Protection and Defense Mechanisms 83
Several studies with citrus fruits have also described gamma
irradiation as a stress factor leading to the induction of antifungal
phytoalexinic compounds in the treated fruit tissues. (See the chapter on
Physical Means - Ionizing Radiation).
Biosynthesis of toxic compounds as a result of wounding or other
stress conditions, is a ubiquitous phenomenon in various plant tissues.
An example of such a synthesis is the production of the toxic compound
6-methoxymellein in carrot roots in response to wounding or to ethylene
application (Chalutz et al., 1969; Coxon et al., 1973); the application of B.
cinerea conidia and other fungal spores to the wounded area was found to
stimulate the formation of this compound (Coxon et al., 1973). A similar
result is also achieved by the application of fungal-produced pectinase, in
spite of the fact that this enzyme does not affect cell vitality (Kurosaki et
al., 1986b). This toxic compound probably has an important role in the
resistance of fresh carrots to infection. Carrots that have been stored for
a long period at a low temperature lose the ability to produce this
compound and, in parallel, their susceptibility to pathogens increases.
Enhanced resistance of carrots can also be induced by application of dead
spores; carrot discs treated with J3. cinerea spores which had previously
been killed by heating developed a marked resistance to living spores of
the fungus, which was much greater than that of the control discs. The
increased resistance of the tissue to active fungal spores, as
evaluated by the inhibition of germ-tube elongation on the treated carrot,
is shown in Fig. 15. The most effective inhibitors found in the tissues
after the induction of resistance, as well as in the control tissues, were
methoxymellein, p-hydroxybenzoic acid, and polyacetylene falcarinol
(Harding and Heale, 1980).
In spite of the demonstration that many plant organs produce
low-molecular-weight antibiotic compounds in response to infection,
there remain substantial questions as to the role of phytoalexins in
disease interactions. The lack of clear answers to questions such as
whether phytoalexin accumulates in suitable concentrations at the
right place and the right time, has led to the conclusion that
phytoalexin synthesis is but one of a vast array of phenomena that,
taken together, contribute to the expression of resistance (Nicholson
and Hammerschmidt, 1992).
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