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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
218 Postharvest Diseases of Fruits and Vegetables
also been found to retard ripening of several harvested commodities, such
as tomato and peach (Liu et al., 1993; Lu et al., 1991), thus leading
indirectly to reduction in their susceptibility to infection.
Optimum UV-C doses for induced resistance in the various
commodities fall within narrow ranges, specific for each commodity. Also
the maximum protection provided by UV treatments occurs at varying
times after treatment and depends on the commodity. Studies with
UV-treated grapefruits showed that fruits picked at different times
during the growing season responded differently to UV treatments. The
temperature at which the fruit was stored following treatment was
another factor affecting resistance development (Droby et al., 1993a).
Maximum resistance of peaches against Monilinia fructicola and
tomatoes against Rhizopus stolonifer was observed 48-72 h after UV
treatment, whereas sweet potatoes exhibited maximum protection
against Diplodia tubericola 1-7 days after treatment (Wilson et al.,
1997a).
Ultraviolet Effects
In UV-illuminated apples and peaches the induced resistance to decay
has been attributed to the inhibition of ripening and the maintenance of
the natural resistance of the young fruits to infection (Lu et al., 1991).
Studying the changes found in UV-treated citrus fruits indicated that
induced resistance occurs in concomitance with the induced activity of
the enzymes phenylalanine ammonia lyase (PAL) and peroxidase
(Chalutz et al., 1992; Droby et al., 1993a). These findings led to the
hypothesis that induced activity of the two enzymes plays a role in the
enhanced resistance to decay that follows UV treatment.
Several investigations indicated the ability of UV illumination to
induce phytoalexin production in various host tissues. UV illumination
(254 nm) of citrus fruits was found to induce the phytoalexin scoparon
(Rodov et al., 1995b; Rodov et al., 1992), the production of which is
enhanced both by increasing the UV dose and by raising the storage
temperature (Rodov et al., 1992). Its accumulation in kumquat fruit
reached its peak 11 days after illumination, but the amount declined
rapidly, returning to trace levels, typical of non-illuminated fruit, 1
month after treatment. The accumulation of the phytoalexin was
correlated with an increase in the antifungal activity of the flavedo which
led to improved resistance of the fruit to infection by Penicillium
digitatum. Such a defense against infection was achieved when
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