Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

THE BREAKDOWN OF CELL WALL COMPONENTS 185
in both fruits. XTH reduction during the infection might be related with the fungus attack
mechanism. Decrease in activity and the consequent lower xyloglucan endotransglucosylation,
together with the increase in endoglucanases, would permit fungal access to the
cellulose-xyloglucan network, increase the efficiency of cellulose hydrolysis, and thus facilitate
the progress of the fungal infection. Hemicellulose degradation is important in the
breakdown of plant cell walls, causing cell wall loosening, increasing the porosity of the
wall, and allowing the colonization of plant tissue (Miedes and Lorences, 2004).
In bell pepper fruit tissue, massive fungal colonization was followed by extensive degradation
of the pectin component of host walls and middle lamella due to the necrotrophic
growth of Botrytis cinerea. Cellulose breakdown was limited to small wall areas. The disruption
of host walls and the reduction of pectin labeling appeared to parallel levels of cell
wall–macerating enzymes isolated from B. cinerea-infected tissue. High levels of PG and
trace amounts of cellulase were detected in B. cinerea-infected tissue. In chitosan-treated
tissue, the preservation of pectin-binding sites and the intense and regular cellulose distribution
over host walls suggested that chitosan might have prevented the maceration of host
tissue by B. cinerea. Chitosan not only was effective in reducing the production of PGs
by B. cinerea, but also caused severe cytological damage to invading hyphae, which may
be responsible for the limited ability of the pathogen to colonize tissues in the presence of
chitosan (Elghaouth et al., 1997).
Pear PG inhibitor protein (PGIP) caused partial inhibition of the crude mixture of
Botrytis enzymes and increased the ratio of dimeric to monomeric uronide products. However,
no accumulation of larger oligomeric breakdown intermediates was detected, and no
impact on ethylene elicitor activity of the digestion products was observed. Differential inhibition
of the B. cinerea PG isozymes by pear PGIP was observed (Sharrock and Labavitch,
1994).
8.12.7 Irradiation
The biological effect of gamma rays is based on the interaction with atoms or molecules in
the cell, particularly water, to produce free radicals, which can damage different important
compounds of plant cell. The UV-B/C photons have enough energy to destroy chemical
bounds, causing a photochemical reaction. Gamma rays accelerate the softening of fruits,
causing the breakdown of middle lamella in cell wall. They also influence the plastid
development and function, such as starch–sugar interconversion. The penetration of UV-
B light into the cell is limited, while gamma rays penetrate through the cells. For this
reason, UV-B light has a strong effect on surface or near-to-surface area in plant cells. Plant
pigments, such as carotenoids and flavonoids, save plant cells against UV-B and gamma
irradiation (Kovacs and Keresztes, 2002).
UV light has been used as a postharvest treatment to enhance shelf life of various fruits
and vegetables (Liu et al., 1993; Maharaj et al., 1993). The beneficial doses of UV-C are
reported to induce the accumulation of phytoalexins (Devlin and Gustine, 1992) and activate
genes, encoding pathogenesis-related proteins (Green and Fluhr, 1995). Barka et al. (2000)
demonstrated that UV-C treatment resulted in a reduction in softening and the lowered
activities of cell wall–degrading enzymes (PG, PME, cellulase, xylanase, β-D-galactosidase
and protease) in tomato fruit. They proposed that cell wall–degrading enzymes are one
of the targets of UV-C irradiation by inducing their proteolysis or the reduction of their