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

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FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE Physiological and Biochemical Changes 107 stronger than responses to wounding alone, and the hgnified zone in wounded, non-infected tissue was much narrower than that in tissue adjacent to the infection (Ray and Hammerschmidt, 1998). The inducement of peroxidase activity and hgnin formation is included among the defense responses of the host to infection (see the chapter on Host Protection and Defense Mechanisms). Polyphenol oxidase activity, which is involved in quinone formation, browning and other reactions in potato tubers, increased following inoculation with F. sambucinum and wounding, but to a lesser extent than peroxidase activity. The greatest polyphenol oxidase activity was found in tissues adjacent to the diseased zone; it seemed to be associated with tuber resistance and was highest in the more resistant potato varieties than in the susceptible ones (Ray and Hammerschmidt, 1988). Furthermore, resistant tubers also browned more quickly than the susceptible ones following infection or wounding. Phenolic compounds were found to increase following potato tuber inoculation or wounding alone in both susceptible and moderately resistant tubers, suggesting that phenolic biosynthesis was induced. Examination of extracts of infected tuber discs revealed the presence of a number of phenolic compounds. Constant amounts of ferulic acid were detected, whereas chlorogenic acid, which is a phytoalexinic compound induced in apple fruit infected by Nectria galligena (Swinburne, 1983), accumulated to maximal levels at 48 h and then decreased. These results indicate that free phenolic acids may be induced by infection or wounding, and are then removed as they are converted into lignin or cross-linked to cell walls (Ray and Hammerschmidt, 1998). For discussion of the inducement of phytoalexins in tuber tissue in response to infection, see the chapter on Host Protection and Defense Mechanisms. http://arab2000.forumpro.fr
FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE CHAPTER 8 MEANS FOR MAINTAINING HOST RESISTANCE It was previously pointed out that fruits become more susceptible to invasion by postharvest pathogens as they ripen (see the chapter on Factors Affecting Disease Development - The Fruit Ripening Stage). Treatments and conditions that lead to delayed ripening and senescence can, therefore, indirectly suppress postharvest disease development. These include low-temperature storage, I0W-O2 and high-C02 atmospheres, ethylene removal from the atmosphere, growth regulators, calcium application; these as well as other treatments or strategies may contribute to maintaining the natural resistance typical of the young fruit or vegetable. A. COLD STORAGE Storage at low temperature is the main method for reducing deterioration of harvested fruits and vegetables. The importance of cold storage in decay suppression is so great that all other control methods are frequently considered as supplements to refrigeration (Eckert and Sommer, 1967). Low temperatures affect both the host and the pathogen simultaneously. They prevent moisture loss from the host tissues and consequent shriveling; they retard metabolic activity and delay physiological changes that lead to ripening and senescence. Since fruits and vegetables become generally more susceptible to pathogens as they mature and approach senescence, the retardation in the physiological activity of the host is accompanied by a delay in decay development after harvest. As with the host, the metabolic activity of the pathogen is also directly influenced by the environmental temperature, and both its growth ability and enzymatic activity can be greatly retarded by low temperatures. Low temperatures can thus delay postharvest disease development in two ways: (a) indirectly, by inhibition of ripening and senescence of the host and extension of the period during which it maintains its resistance to disease; and (b) directly, by inhibition of pathogen development by subjecting it to a temperature unfavorable for its growth. http://arab2000.forumpro.fr
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