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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
196 Postharvest Diseases of Fruits and Vegetables
the key enzymes in the production of various antifungal compounds. PAL
has also been suggested to be involved in the enhanced resistance of
kiwifruits to infection, after curing; Ippolito et al. (1994) found that the
gray mold rot in kiwifruits which had been inoculated with Botrytis
cinerea at the pedicel scars or on artificial wounds, was significantly
decreased after curing at 15°C and 95-98% RH for 48 h, before storage at
0°C. The activity of PAL at the sites of infection was enhanced in cured
fruits and, under blue light, an accumulation of phenolic compounds was
recorded.
The accumulation of a phytoalexinic compound in heat-treated citrus
fruits was reported by Kim et al. (1991). Heat treatments applied to
lemon fruits inoculated with Penicillium digitatum resulted in the
accumulation of the phytoalexin scoparone (6, 7-dimethoxy- coumarin) in
the peel during heating and several days later. The increase in the
scoparone concentration in the fruit was directly correlated to the
increase in the antifungal activity of the fruit extracts, as exhibited in
their ability to retard spore germination and germ-tube elongation. From
these results arose the hypothesis that the phytoalexins have an
important role in enhancing the disease resistance of the heated fruits
(Kim et al., 1991).
2. Induction of heat shock proteins
Various varieties of fruits and vegetables may differ in their tolerance
to high temperatures. In general, tropical fruits, such as mangoes and
papayas, are more heat tolerant than fruits from temperate zones
(Couey, 1989). Exposing the host to too high a temperature may result in
heat injury - expressed in changes in color and texture, increased water
loss and enhanced susceptibility to infesting microorganisms.
The exposure of plant tissues to thermal stress was found to result in
the rapid induction of a small set of specific proteins called heat shock
proteins (HSPs) (Key et al., 1981). A correlation was found between the
development of enhanced fruit thermotolerance and the synthesis of
HSPs, as well as between the loss of thermotolerance and the
disappearance of HSPs (Vierling, 1991). The development of
thermotolerance depends on the temperature level: to initiate HSP
synthesis, the temperature should be high enough (35-40'^C), but at
higher temperatures (>42°C), however, HSP synthesis is attenuated and
commodities are likely to suffer heat damage (Ferguson et al., 1994). The
induction of HSPs of differing molecular weights has been described in
various fruits, such as papaya, plum, apple and tomato, or in pear fruit
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