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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Host Protection and Defense Mechanisms 75
accumulate within special oil cells named idioblasts (Kobiler et al., 1994).
It is not clear whether induction of the antifungal diene involves de novo
synthesis or a release of the compound from the idioblasts. However, the
fact that about 85% of all antifungal compounds within the fruit
mesocarp are located in idioblasts suggests that their synthesis might
occur in these specific cells (Kobiler et al., 1994).
Inducement of preformed antifungal compounds has also been
described in mango fruits. A mixture of resorcinolic compounds normally
occurs in fungitoxic concentrations (154-232 \ig ml^ fresh weight) in the
peel of unripe mangoes, whereas only very low concentrations are
present in the flesh of the fruits (Droby et al., 1986). Peeling of unripe
fruits and exposing them to atmospheric conditions for 48 h enhanced the
concentrations of resorcinols in the outer layers of the flesh to fungitoxic
levels (Droby et al., 1987). This enhancement was accompanied by an
increase in fruit resistance to fungal attack. Fruit peeling resulted in
browning of the flesh accompanied by enhanced activity of phenylalanine
ammonia lyase (PAL). There are indications that PAL activity is
connected with production of phytoalexins and other compounds involved
in the defense mechanisms of the plant (Kuc, 1972). However, it was
further found that cycloheximide application inhibited both the
enhancement of PAL activity and the browning of the tissue, but did not
interfere with the production of resorcinols and the resistance of the fruit
to Alternaria alternata (Droby et al., 1987). These results confirm the
assumption that the resorcinols that accumulate in the fruit flesh
following peeling are responsible for the decay resistance of the peeled
fruit.
Exposure of the fruit to a controlled atmosphere containing up to 75%
CO2 was found to enhance the level of resorcinols in the peel itself, where
they are normally present; this enhancement was accompanied by decay
retardation, as indicated by a delay in the appearance of the symptoms of
A. alternata infection (Prusky and Keen, 1995). These results showed
that it is possible to induce the preformed resorcinols both in the tissue
in which they are already present, and in a different tissue.
In carrot roots, high concentrations of the antifungal polyacetylene
compound, falcarindiol, were recorded. This compound is found in
extracellular oil droplets within the root periderm and the pericyclic
areas (Garrod and Lewis, 1979). The high concentrations of the
antifungal compound were suggested to result from the continuous
contact of the carrots with organisms in the rhizosphere or with various
pathogens. One of the important antifungal compounds in carrot roots is
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