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

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FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE Novel Approaches for Enhancing Host Resistance 255 postharvest decay, through the possibiHty of using higher and more effective temperatures. Gamma Radiation Ionizing radiation can suppress decay development by directly affecting fungal spores and hyphae. It may also suppress decay indirectly, by dela5dng the ripening and senescence processes of harvested fruits and vegetables and contributing to the maintenance of their natural resistance to pathogen invasion or development. However, gamma radiation may also function as a stress factor, capable of inducing phytoalexin formation. Induction of the phytoalexins, scopoletin and scoparone was reported in citrus fruit following gamma irradiation at 1-4 kGy (Riov, 1971; Riov et al., 1971), and their antifungal activity has been demonstrated (Dubery and Schabort, 1987). However, the part that the induced phytoalexins play in the complex of radiation effects on the pathogen or the defense responses of the host to effective radiation doses has not yet been evaluated. Low-dose Ultraviolet (UV) Light UV illumination is another physical treatment that may induce resistance against pathogen infection (Wilson et al., 1994). UV-C (wavelength below 280 nm) light at low doses was found to induce resistance in a wide array of commodities, such as onions (Lu et al., 1987), carrots and peppers (Mercier et al., 1993), tomatoes (Liu et al., 1993), sweet potatoes (Stevens et al., 1990), peaches (Lu et al., 1991), various citrus fruits (Chalutz et al., 1992; Droby et al., 1993a; Porat et al., 1999; Rodov et al., 1992), and table grapes (Nigro et al., 1998). The enhanced resistance that follows UV treatment may be mediated by the activation of various defense responses in the host tissue. The first such response is the accumulation of phytoalexins to inhibitory levels. In carrot slices, the UV-induced resistance of the tissue to Botrytis cinerea and Sclerotinia sclerotiorum infection coincided with the induction of the phytoalexin, 6-methoxymellein (Mercier et al., 1993). The induced resistance was expressed only after 1 week of storage, when the concentration of 6-methoxymellein in the tissue had reached inhibitory levels. The content of this compound in UV-treated slices that were held at 1 and 4'^C remained elevated for up to 35 days after treatment. Another phytoalexin, scoparone (6,7-dimethoxy-coumarin), has been linked to UV-induced resistance in citrus fruits (Rodov et al., 1992). Its accumulation in kumquat fruits reached its peak (530 \ig gO 11 days http://arab2000.forumpro.fr
FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE 256 Postharvest Diseases of Fruits and Vegetables after illumination (254 nm), but its concentration declined rapidly, returning to trace levels typical of non-illuminated fruit, 1 month after treatment. The accumulation of the phytoalexin was correlated with increased antifungal activity in the flavedo, and resulted in enhanced resistance of the fruit to infection by Penicillium digitatum. Such a defense against infection was achieved when illumination was applied before inoculation with P. digitatum, without direct contact of the pathogen with the light. Moreover, illumination of previously inoculated fruit failed to prevent decay development. The experiments with kumquats clarified that the fruit/pathogen interaction depends on the relative rates of fungal growth and of resistance development by the host. Inoculation of the fruit before UV treatment gives an advantage to the pathogen, while illumination applied 2 days before inoculation was sufficient to improve fruit resistance. Host resistance is thus dependent on the existence of a lag period between illumination and inoculation, during which the phytoalexin can be accumulated and suppress subsequent pathogen development (Rodov et al., 1992). The enhancement by UV treatment of citrus fruit resistance to P. digitatum infection has also been explained in part by the induction of the key enzymes in the secondary metabolite pathway. It was thus found that in citrus fruits, the onset of UV-induced resistance coincided with the induction of phenylalanine ammonia lyase (PAL) (a key enzyme in the phenylpropanoid pathway) and peroxidase activities (Chalutz et al., 1992; Droby et al., 1993a). PAL activity in the peel of grapefruits increased within 24 hours after UV treatment and remained elevated for 72 hours, while peroxidase activity reached its maximum 72 hours after treatment. Both enzymes are considered to play a role in the induced resistance of plants against pathogens. An additional explanation for the induced resistance of citrus fruits following UV treatment has recently been offered by Porat et al. (1999), who found that exposure of grapefruits to UV light (254 nm), followed by inoculation with P. digitatum, resulted in the accumulation of chitinase protein in the peel tissue. Whereas UV treatment or wounding alone had almost no effect on the levels of p-l,3-endoglucanase protein, the combination of the two treatments induced the accumulation of both chitinase and glucanase proteins. Since the two pathogenesis-related proteins are hydrolysers of fungal cell wall polymers and may inhibit fungal development, it was suggested that the resistance of citrus fruit to fungal development induced by UV treatment had been mediated by the http://arab2000.forumpro.fr
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