Maturity, Ripening, and Quality Relationships 65
impact of treatments such as exogenous ethylene application can be considered
positive or negative depending on the reference that is used in the comparison.
Rapid softening is a significant quality problem in apples and can be induced
either by cold storage or by exogenous exposures to ethylene (Johnston et al., 2002a).
However, the response to exogenous ethylene is not the same for all cultivars of
apples, and three categories of response have been identified (Johnston et al., 2001b,
2002a). The first category is that of apples that exhibit rapid softening without
exposure to ethylene. ‘Royal Gala’ and ‘Cox’s Orange Pippin’ have been identified
as belonging to this category. ‘Granny Smith’ softens rapidly in response to exog-
enous ethylene and is hence placed into a second category (Johnston et al., 2002a).
‘Pacific Rose’™ belongs to a third category since it does not show rapid softening
despite exposure to exogenous ethylene production. It is quite clear that the quality
degradation associated with rapid softening in apples is quite variable between
cultivars, and therefore it is essential to understand the specific characteristics of
each cultivar that is being handled.
Exogenous ethylene can also induce production of metabolites in tissue that
directly reduce the acceptability of vegetables. Exposure to ethylene generally
increases phenylpropanoid pathway activities (Saltveit, 1999), and this can lead to
the accumulation of phenolic compounds such as isocoumarins in carrots, which
cause bitterness (Lafuente et al., 1996).
3.3.2.2 Cytokinins
Cytokinins are generally considered to be antisenescent in nature (Downs et al.,
1997) and hence several studies have examined the effects of cytokinin application
on senescence and shelf life in fruit and vegetables. Exogenously applied 6-benzyl-
aminopurine (a cytokinin) has divergent effects on quality. Downs et al. (1997)
reported that 6-benzylaminopurine (BAP) delayed yellowing and senescence-asso-
ciated increases in asparagine, glutamine, and ammonium content of broccoli but
had no effect on the decline in sucrose content. BAP has also been shown to slow
softening and delay titratable acidity losses but had no effect on soluble solids decline
in guava (Sharma and Dashora, 2001). BAP has also been shown to retard cap
opening (a sign of senescence) in mushrooms (Braaksma et al., 2001). Cytokinin
applications generally have a positive effect in terms of slowing deteriorative pro-
cesses in both fruits and vegetables.
3.3.2.3 Gibberellic Acid
The phytoalexin columbianetin has been associated with resistance to Botrytis rot
in celery, and exogenously applied gibbberellic acid (GA) has been shown to slow
its decline in tissue during storage, resulting in up to an nine times increase in
resistance to rot (Afek, 2002). This same application of GA also inhibited the
accumulation of psoralens by 50%. Psoralens are associated with skin rash devel-
opment in humans (Aharoni et al., 1996), so this response suggests that GA treatment
prevents accumulation of compounds in celery that can lead to adverse health effects
in produce handlers. GA treatment of strawberries led to increased phenylalanine