94 Produce Degradation: Reaction Pathways and their Prevention
expression.^137 Ethylene-responsive genes appear to be differentially regulated during
abscission, organ senescence, and wounding. Ruperti et al.^138 isolated four genes
from peaches (Prunus^ persica, Batsch) that were up-regulated during propylene-
induced^ abscission of young fruit. DNA and deduced^ protein sequences of four
selected clones, termed Prunus persica^ Abscission zone (PpAz), revealed homology
to thaumatin-like^ proteins (PpAz8 and PpAz44), to proteins belonging to the PR4
class of pathogenesis-related (PR) proteins (PpAz89), and to^ fungal and plant β-D-
xylosidases (PpAz152).
The potential effects of wound ethylene are dependent on the type and physiol-
ogy of the tissue in question. Fruit maturity influences wound-induced ethylene
response. Wound-generated ethylene, if sufficient, may start the climacteric respira-
tory response that could cause fruit to immediately start to ripen. Wound ethylene,
which is higher in preclimacteric and climacteric than in postclimacteric tissues,^139
may accelerate deterioration and senescence in vegetative tissues and promote rip-
ening of climacteric fruit.^20 In melons, ethylene production was higher in the cut
fruit at the preclimacteric phase,^140 whereas in the postclimacteric phase cutting
resulted in a reduction of ethylene production. The accelerated climacteric phase
caused by ethylene production results in a difference in physiological age between
intact and sliced tissue and contributes to softening and color during storage.114,141,142
Compression levels and number of days in storage were found to have no significant
effect on ethylene levels in lettuce.^142 Quality deterioration may result from increased
ethylene production, which may induce higher cellular metabolism and higher enzy-
matic activity.^144 Changes in ethylene production with storage time and temperature
are also product-specific.61,115,129 Air humidity level may be negatively correlated
with the severity of the symptoms of mechanical damage in bananas, including
respiration and ethylene production.^91 In apricots, ethylene started to rise after
12 hours under the impact area of dropped fruit (cv. San Castrese), and the sound
area on the opposite side produced more ethylene 6 h later.^127 Putrescine-treated
apricot fruits (damaged and undamaged) had a repressed ethylene emission, but
damaged fruits produced higher levels of ethylene.^83
ACC synthase transcript levels could be decreased by ethylene application that
leads to decreased ethylene synthesis.^28 It is also likely that ethylene reduces bruise
severity through another mechanism such as the induction of cell division. Product
pretreatment with ethylene may thus reduce bruise severity. In potatoes, pretreatment
of tubers with ethylene reduced bruising severity that could not be linked to a drop
in polyphenol oxidase (catechol oxidase) activity, tyrosine concentration, or the
biochemical potential for melanin formation. Postimpact treatment of tubers with
ethylene does not appear to reduce bruising damage.^145
4.8.3 PHENOLIC COMPOUNDS
Phenolic compounds appear to be important in a wide range of reactions that result
from injury and a loss of organizational resistance between substrates and enzymes
within the cell. They contribute to resistance of plants to mechanical stress by
participating in lignification of cell walls surrounding the injured zone and inhibition
of microbial growth and germination of spores. Phenolics present in plant tissues