Mechanical Injury of Fresh Produce 97
that blackspots develop through enzymatic oxidation of phenols by PPO, leading to
spontaneous polymerization and subsequent formation of the dark pigment melanin.
Tubers produced under 10 and 0% potassium regimens had almost twice as much
free tyrosine as those grown at full-strength Hoagland’s solution. The 0% potassium
tubers had significantly higher PPO activity than the control.^48 Many studies have
reported good correlation between free tyrosine and enzymatic browning elicited by
cutting, maceration, or homogenization.177,178
Mechanical damage of peaches increased activity of PAL, PPO, and peroxidase
(POD) and lignin synthesis in cell walls. Application of CaCl 2 at the site of injury
increased the concentration of bound calcium in cell walls, delayed the peaks in
enzyme activity, stimulated synthesis of neutral sugars, and reduced the degree of
esterification of cell wall pectins.^179 In bananas, the peel shows high levels of activity
early in development but activity declines until ripening starts and then remains
constant. PPO activity in fruit does not appear to be substantially induced after
wounding or treatment with 5-methyl jasmonate, suggesting that browning of banana
fruit during ripening results from the release of preexisting PPO enzyme synthesized
very early in fruit development.^180
Red discoloration in chicory leaves may result from mechanical damage. Gillis
et al.^181 noted that red discoloration was induced by applying mechanical loads on
leaves under constant pressure at ambient conditions. Compression and plate contact
induced severe dark pink and brown discolorations as a result of mechanical damage
as well as the development of russet spotting in lettuce.^143 Browning of individual
lettuce heads was observed, although the treatments had no significant effects on
the development of internal tissue browning. Browning was most prominent on the
white midrib tissue. An increase in heat-shock temperature from 20 to 70°C in
excised midrib segments of iceberg lettuce (cv. Salinas) caused a reduced increase
in PAL activity and the accumulation of phenolic compounds in the excised midrib
segments, as well as browning. Synthesis of chlorogenic acid, dicaffeoyl tartaric acid,
and isochlorogenic acid was significantly reduced by these heat-shock treatments. These
treatments also decreased polyphenol oxidase and peroxidase activities.^182
Peroxidase enzymes are also able to contribute to enzymatic browning based on
their ability to accept a wide range of hydrogen donors such as polyphenols.^183 They
are able to oxidize catechins,^184 hydoxycinnamic acid derivatives and flavans,185,186
and flavonoids.^183 The presence of PPO enzyme, however, enhances POD-mediated
browning reactions.^183 The production of large amounts of reactive oxygen species
is one of the earliest defence responses against mechanical damage or pathogen
attack, but it also exposes the plant cells to serious oxidative stress. Morimoto et
al.^187 in a model study to determine H 2 O 2 metabolism using cells of Seutellaria
baicalensis found that in response to an elicitor (such as yeast extract) the cells
immediately initiate the hydrolysis of baicalein 7-O-beta-D-glucuronide by beta-
glucuronidase, and the released baicalein is then quickly oxidized to 6,7-dehydroba-
icalein by peroxidases. Hydrogen peroxide is effectively consumed during the per-
oxidase reaction. The beta-glucuronidase inhibitor saccharic acid 1,4-lactone signif-
icantly reduced the H 2 O 2 -metabolizing ability of the cells, indicating that beta-
glucuronidase, which does not catalyze the H 2 O 2 degradation, plays an important