Polyamines and Regulation of Ripening and Senescence - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 324

15.6.1 Role of polyamines in fruit set, growth, and development
Normally, higher levels of PAs are present in flowers but decrease upon fertilization (Pritsa
and Voyiatzis, 2004). Flower opening in damson plum is coincident with increases in free
PA levels in the sepals, petals, and sex organs (Dios et al., 2006). In ovaries, Spm level
decreases following fertilization, with a concomitant increase in Put and Spd in the sepals.
Spd is the most abundant PA during flowering, at S1 and S2 fruit growth phases, whereas Put
levels are highest during S3 and S4 fruit growth phases (Dios et al., 2006). Both arginine
decarboxylase (ADC) and ornithine decarboxylase (ODC) activities decrease following
tomato fruit set. There is also an indication that the ADC pathway may be involved in
cell expansion and the ODC pathway active in cell division process during early fruit
growth of tomato (Cohen et al., 1982; Alabadi et al., 1996; Alabadi and Carbonell, 1998).
Application of Put, Spd, and Spm to wild-type unpollinated ovaries in tomato resulted in
partial parthenocarpy (Fos et al., 2003). Higher PA levels in unpollinated pat -2 ovaries are
correlated with the activation of the ODC pathway, which in turn is influenced by elevated
GA levels found in these ovaries (Fos et al., 2003). Several other studies have also implicated
PAs in the initial stages of fruit development, which include cell division and cell elongation
(Table 15.1).
Table 15.1 shows patterns of changes in free PAs during growth of indicated climacteric
and nonclimacteric fruits. The levels of Put, Spd, and Spm are generally high during early
fruit growth but decline during later phases of fruit development (Valero et al., 2002a).
Higher levels of PAs are observed during the period when growth is primarily accomplished
by cell division. Levels of PAs during early fruit development in tomato, peach, and plum are
higher, with Put being the most abundant PA. Application of ODC inhibitors impairs fruit
growth indicating a role for PAs in regulating cell division and fruit development (Egea-
Cortines and Mizrahi, 1991; Fos et al., 2003). The levels of PAs decline at the later stages of
fruit growth and development (Saftner and Baldi, 1990; Morilla et al., 1996; Shiozaki et al.,
2000; Mehta et al., 2002; Alburquerque et al., 2006; Liu et al., 2006a). In tomato, levels of
Put and Spd declined between immature green and mature green stages (Saftner and Baldi,
1990). Over 100-fold higher Spd levels were present during cell division than at later stages
of fruit growth in tomato (Egea-Cortines et al., 1993). However, there are some exceptions
as increase in Put in muskmelon and in Put and Spd in long-keeping tomato were observed
during fruit growth.
15.6.2 Polyamines and ripening
From a postharvest view point, ripening increases consumer acceptability by rendering fruit
attractive and palatable through enhanced organoleptic characteristics such as flavor, texture,
color, and aroma. Fruit ripening is a genetically programmed process and is accompanied by
a major shift in gene expression leading to desirable metabolic processes (Giovannoni, 2004;
Srivastava and Handa, 2005). Based on respiration patterns at the onset of ripening, fruits are
classified into either climacteric or nonclimacteric fruits. Climacteric fruits exhibit increased
respiration during ripening, whereas in nonclimacteric fruits respiration rate decreases as
fruits ripen. Ethylene is necessary for ripening of climacteric fruits as inhibition of either
ACC synthase or ACC oxidase by reverse genetics impairs progression of the ripening