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

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FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE Physiological and Biochemical Changes 99 nor tomatoes Days after harvest Fig. 22. Ethylene evolution from nor mutant tomato fruit, at its mature-green (A) and mature (B) stages of maturity, following inoculation with Botrytis cinerea (A), or following treatment prior to inoculation (•) as compared to uninfected fruit (•). (Reproduced from Barkai-Golan et al., 1989 with permission of Blackwell Wissenschafts-Verlag GmbH). Ethylene synthesis is also stimulated by mechanical wounding, such as occurs while introducing the inoculum into the host. However, comparison between rates of ethylene production elicited by wounds of certain dimensions and those elicited by fungal lesions of similar size, indicates that ethylene production in fungus-infected fruits is considerably greater than that in wounded ones. These results may suggest that, in addition to the mechanical stress produced during pathogenesis, which is responsible for the 'wound-ethylene', another factor is involved in the synthesis of 'infection-ethylene'. This 'biological factor' seems to differ for different fungi (Barkai-Golan et al., 1989b). http://arab2000.forumpro.fr
FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE 100 Postharvest Diseases of Fruits and Vegetables B. ETHYLENE SOURCE IN INFECTED TISSUE The marked increase in ethylene evolution during infection stimulated the questions: What is the source of ethylene in the infected fruit? Is ethylene produced by the attacking pathogen or by the attacked host? It is known that various fungi do produce ethylene in vitro as a metabolic product during their development (Hag and Curtis, 1968). However, with the exception of a few species, such as Aspergillus clavatus, A, flavus, Penicillium digitatum and P. corylophilum, which produce considerable levels of ethylene, the production of ethylene by most species is low, reaching 0.5-1.0 ppm/g dry weight. Determination of ethylene production in vitro may be complicated due to the fact that different strains or even different isolates, of the same species, may differ in their capacity to produce ethylene. Moreover, when no ethylene is detected for a given fungus in culture, it is still possible that the pathogen can synthesize it in the fruit, because of the special substrate supplied by the living tissue. However, the healthy tissue is also capable of producing and emitting ethylene but, in contrast to climacteric fruits, in which ethylene evolution may reach high peaks during ripening, most plant tissues produce very low levels of this gas when healthy. The increased ethylene produced by the diseased fruit may, therefore, be from the damaged host tissue, from the activity of the pathogen within the host, or both. Examination of the source of ethylene in apples infected by the brown rot fungus, Sclerotinia fructigena (Hislop et al., 1973), and in tomatoes infected by Rhizopus stolonifer or Botrytis cinerea (Barkai-Golan and Kopeliovitch, 1983; Barkai-Golan et al., 1989b) showed that the rotted tissues themselves, which contained an abundance of actively growing mycelium, contributed only slightly or not at all to the production of ethylene by the infected tissue. On the other hand, the healthy fruit tissue at the periphery of the rot, where only a few hyphae could be seen penetrating the healthy tissue, were producing considerably higher levels of ethylene (Table 5). The induction of ethylene production by the host tissues in response to infection has also been exhibited by the non-ripening, non-ethylene-producing nor tomato mutant at various stages of maturity (Table 5). Moreover, none of the fungi mentioned produce ethylene in vitro. These results suggested that the ethylene recorded in the various host/pathogen systems is produced by the host in response to fungal infection. http://arab2000.forumpro.fr
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