Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

POLYAMINES AND REGULATION OF RIPENING AND SENESCENCE 327
20 ◦ C. In addition, Put increased shelf life, enhanced fruit quality attributes such as soluble
solids, titratable acid content, and fruit firmness and resulted in either a decrease or a
delay in ethylene production (Perez-Vicente et al., 2002; Serrano et al., 2003). Loss of fruit
firmness in peach during postharvest storage is associated with a decrease in the levels of
PAs and an increase in ethylene biosynthesis (Liu et al., 2006b). Exogenous treatment of
field-grown peaches with PAs and aminoethoxyvinylglycine (AVG), an inhibitor of ethylene
biosynthesis, inhibited ethylene emission, delayed flesh softening, and retained titratable
acidity (Bregoli et al., 2002; Torrigiani et al., 2004). Since these compounds affect ethylene
production and expression of genes involved in ethylene biosynthesis, this hormone is
likely to influence the mode of action by which these PAs affect shelf life (Torrigiani et al.,
2004).
It is important to note that the method of treatment, the cultivar used, and the stage
of ripening can influence the effect of PAs in altering postharvest qualities. Vacuum infiltration
with Put or dipping in a solution of Put decreased ethylene production by kiwi
fruit and delayed flesh firmness in contrast to pressure infiltration (Petkou et al., 2004).
Comparison of Put and Spd treatments showed differential effects on the production of
ethylene and fruit quality attributes between attached fruit and fruit explants in peaches and
nectarines (Bregoli et al., 2006). Treatment of potted miniature roses with various PAs and
methylglyoxal-guanylhydrazone, a PA-synthesis inhibitor, did not modify floral longevity
(Serek and Andersen, 1994).
15.7.1 Polyamines and postharvest fruit firmness
Treatment with exogenous PAs has been shown to increase flesh firmness in several fruits including
“Golden Delicious” and “Mclntosh” apples (Kramer et al., 1991), strawberry slices
(Ponappa et al., 1993), plums (Perez-Vicente et al., 2002), peaches and nectarines (Bregoli
et al., 2006). However, differential effects of Put, Spd, and Spm have been reported. For
example, vacuum infiltration of Spd and Spm significantly increased firmness of strawberry
slices, whereas Put was ineffective (Ponappa et al., 1993).
Some of the effects of PAs on fruit firmness could be due to modification of genes
involved in ethylene biosynthesis, ethylene perception, alteration of cell wall–associated
enzymes, and PA conjugation. Prestorage application of Put suppressed ethylene biosynthesis
probably by lowering ACC oxidase (ACO) enzyme activity and delayed fruit softening
in plum by modulating cell wall–related enzymes involved in fruit softening such as pectin
esterase, exopolygalacturonase and endopolygalacturonase (Khan and Singh, 2007). Also,
in vitro inhibition of pectin methylesterase and polygalacturonase by PAs has been reported
(Leiting and Wicker, 1997; Martinez-Tellez et al., 2002). Calcium and heat-treated plums
stored at lower temperatures also showed increased fruit firmness, which may be due to
the effect of conjugated Put and Spd (Valero et al., 2002b). The role of PAs in delaying
softening in peach and apple can be attributed to their effects on stabilization of cell walls
and membranes (Bonghi et al., 1998; Liu et al., 2006a, b). Higher levels of cell wall–bound
Put and Spd contributed to increased firmness and reduction in mechanical damage in plum
(Perez-Vicente et al., 2002). PAs bind to pectins associated with the cell wall, thus influencing
modifications of cell wall in addition to altering the binding of other ions such as
calcium (Messiaen et al., 1997). Further cross talk between ions such as calcium and PAs
may also influence the activity of pectin esterase (Leiting and Wicker, 1997).