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

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.
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