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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
102 Postharvest Diseases of Fruits and Vegetables
(1985a, b) studied P. digitatum ethylene production in several ways:
(a) By comparing an isolate of P. digitatum producing very high levels
of ethylene with an isolate producing very low levels of the gas.
(b) By adding ACC, which is typically connected with the production
of ethylene of plant origin, or glutamic acid, which is probably the
source of fungal ethylene (Chou and Yang, 1973).
(c) By adding AVG (aminoethoxy vinyl glycine), which inhibits plant
ethylene production (Lieberman et al., 1974) and only slightly affects
its production by P. digitatum (Chalutz and Lieberman, 1978).
(d) By the addition of Cu^ ions, which are toxic to the fungus but
stimulate ethylene production in higher plants (Abeles and Abeles,
1972).
The application of this complex of methods to the study of the origin of
the ethylene produced in infected hosts clearly proved that the high level of
ethylene produced in the infected area itself is derived wholly or in part
from the fungus, which is capable of producing it both in culture and in the
fruit. The production of this ethylene was not affected by the presence of
ACC, but was considerably enhanced by the addition of glutamic acid. The
addition of AVG inhibited its production only slightly, whereas the addition
of CuS04 resulted in a marked inhibition of ethylene synthesis. On the
other hand, the ethylene produced by the healthy part of the fruit, at the
periphery of the rot, must be of plant origin, since it was stimulated by ACC
and suppressed by AVG application.
C. PECTOLYTIC ACTIVITY AND ITS SOURCE IN INFECTED
TISSUE
An increase in polygalacturonase (PG) activity in infected tissue during
disease development has generally been accepted as being of fungal origin
(Wood, 1967). However, studies of the origin of pectolytic activity in various
host/pathogen systems demonstrated that the fungus not only 'contributes'
its enzymes to the host, but may also induce enzyme production by the host
itself, as a response to infection (Barkai-Golan et al., 1986). It was thus
found that PG production by avocado fruits following infection with
Colletotrichum gloeosporioides was induced by the fungus, and that
softening of the mesocarp during anthracnose development was primarily
due to the induction of the fruit PG (Barash and Khazzam, 1970).
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