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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
72 Postharvest Diseases of Fruits and Vegetables
antifungal compound in the ripe fruit, which tends to decay, has been
attributed to the enzymatic activity of hpoxygenase in the fruit. Several
findings supported the suggestion that this enzyme is the cause for diene
decomposition and increased sensitivity of the ripe fruit to decay (Prusky
and Keen, 1993):
(a) The enzyme activity increased considerably during ripening;
(b) Partially purified lipoxygenase oxidized diene under in vitro
conditions;
(c) Treatment with tocopherol acetate, an antioxidant compound that
non-specifically suppresses lipoxygenase, or with an enzymespecific
inhibitor, delayed the reduction in diene concentration
and inhibited the appearance of disease symptoms elicited by
C gloeosporioides.
The activity of lipoxygenase in ripe avocado fruits is affected by the
activity of the epicatechin, a natural antioxidant present in the fruit peel.
The concentration of this compound decreases during ripening thus
allowing the activity of lipoxygenase to increase. A considerable
reduction in the epicatechin concentration in sensitive avocado cultivars
occurs along with the reduction in fruit firmness, and disease symptoms
are expressed only when the concentration is reduced to the lowest levels
(Prusky and Keen, 1993). Thus, epicatechin appears to play a key role in
fruit susceptibility during ripening, by indirectly controlling the level of
the antifungal compound.
A mixture of resorcinols, found in the peel of unripe mango fruits,
showed antifungal activity against A. alternata, the causal agent of the
black spots in harvested fruit. The presence of this mixture was related
to the latent stage of the fungus in young mango fruits (Droby et al.,
1986): during ripening, the resorcinol concentrations in the peel are
reduced, the fruit loses its resistance to the fungus and the quiescent
Alternaria infections become active. Similarly to the preformed
compounds in young avocados, the preformed resorcinols in unripe
mangoes can also be further induced in the fruit. The fact that the
resorcinol concentrations in Kelt mango, which is disease-sensitive, do
not decrease to non-toxic levels during ripening also supports the
assumption that the resorcinols mixture has a role in disease
suppression. Large quantities of resorcinols generally occur in the fruit
peel, whereas the fruit flesh contains only very small quantities.
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