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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
74 Postharvest Diseases of Fruits and Vegetables
attack or presence, as well as to other stresses. Induction of existing
preformed compounds can take place in the tissue in which they are
already present, or in a different tissue (Prusky and Keen, 1995).
An example of a preformed compound being induced in the same
tissue where fungitoxic concentrations are naturally accumulated is the
antifungal diene in the peel of unripe avocado fruits. Prusky et al. (1990)
found that inoculation of unharvested or freshly harvested avocado fruits
with C. gloeosporioides, but not with the stem-end fungus Diplodia
natalensis, resulted in a temporarily enhanced level of this compound.
The response to this challenge doubled the amount of the preformed
diene after 1 day, and the effect persisted for 3 days, suggesting
persistence of the elicitor (Prusky et al., 1990). A significant increase in
the concentration of the antifungal diene, with longer persistence of the
elicitors but without symptom expression, has been induced by a
nonpathogenic mutant of Colletotrichum magna (Prusky et al., 1994).
The inducement of preformed antifungal compounds by two different
Colletotrichum species suggests that nonspecific eliciting factors are
probably present in the hyphae of both species (Prusky, 1996).
Wounding is an abiotic factor that may enhance diene concentration.
However, whereas wounding of freshly harvested fruit resulted in a
temporarily enhanced diene accumulation in the fruit, inducement did
not occur in fruits 3-4 days after harvest (Prusky et al., 1990). Gamma
irradiation is another abiotic factor capable of inducing diene
accumulation. Following irradiation at 5 or 20 krad, the diene
concentration was doubled, but a day later its level was lower than in the
untreated controls. This phenomenon was probably due to the fact that
irradiation also enhanced fruit ripening and, in turn, the decline in the
diene concentration (Prusky and Keen, 1995).
An inducement of the antifungal diene also followed a high-C02
application. Exposing freshly harvested avocado fruits to 30% CO2
resulted in increased concentration of the diene upon removal from the
controlled atmosphere storage. The concentration of diene then decreased
but this decrease was followed by a second increase which correlated
with suppressed decay development (Prusky et al., 1991). The increased
concentration of the antifungal diene in the fruit could result from direct
synthesis or from inhibition of its breakdown during fruit ripening.
Several findings have pointed to the importance of lipoxygenase activity
in the breakdown of the diene, and to its being affected by the activity of
epicatechin in the fruit peel. In the flesh of avocado fruits both diene and
monoene antifungal compounds are among the compounds that
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