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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
208 Postharvest Diseases of Fruits and Vegetables
treatment may result in renewal of fungal development because of the
progress of fruit ripening (Prusky et al., 1982; Droby et al., 1986). Under
such conditions, decay cannot be suppressed by irradiation.
Since radiation effects are associated with the size of the target
population, one may presume that all environmental factors encouraging
fungal growth, such as appropriate storage temperature or high
humidity, will indirectly affect the dose required for pathogen control.
Several studies indicate that the radiation dose required for pathogen
suppression within host tissues is higher than under in vitro conditions.
This is believed to be the consequence of the chemical protective effect
afforded by the tissues when in contact with the pathogen.
Irradiation may induce various degrees of injury and result in the
formation of mutants among the survivors. Although mutants usually
appear to be less pathogenic than the parent organism, mutants of wider
virulence have also been observed in various plant pathogens (Sommer
and Fortlage, 1966). An irradiated pathogen has the capacity to repair
radiation damage under post-irradiation conditions. Environments found
to be encouraging for recovery are characterized by their ability to slow
down metabolism, such as sub-optimal temperatures, or starvation or
anaerobic conditions (Barkai-Golan, 1992).
Radiation effects on the pathogen may also be associated with the
ability of the host tissue to induce antifungal phytoalexins in response to
irradiation. Riov (1971) and Riov et al. (1971) reported on the
accumulation of the stress metabolites, scopoletin and scopolin in the peel
of mature grapefruits irradiated at 1-4 kGy, and of scoparone following
irradiation at 3 and 4 kGy. These compounds were formed in the
radiosensitive flavedo tissue of the peel in correlation with increased
ethylene production, enhanced phenylalanine ammonia lyase activity
(PAL), and the accumulation of phenolic compounds, which lead to cell
death and peel pitting (Riov, 1975). More than 15 years after the first
isolation of scoparone from irradiated grapefruits, and following isolation
of this compound from other citrus cultivars (Valencia oranges and Eureka
lemons), Dubery et al. (1988) showed its antifungal activity. Another
irradiation-induced non-coumarin metabolite was extracted from damaged
regions of citrus peel and was identified as 4-(3-methyl-2-butenoxy)
isonitrosoacetophenone. This compound, which did not occur in extracts
from non-irradiated fruits, has also been reported to have antifungal
properties (Dubery et al., 1988).
It should be mentioned, however, that in contrast to the radiationinduced
phytoalexins in citrus fruits, El-Sayed (1978) reported that the
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