Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

196 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
events. Continued deterioration of the cell membrane eventually leads to the loss of compartmentalization
within the cell. Although we have detailed knowledge of the deleterious
structural changes in the cell membrane and the associated loss of functional properties,
very little attention has been given to developing strategies for the preservation of membrane
structure during ripening and senescence, until recently (Ryu et al., 1997; Paliyath et al.,
2003; Whitaker and Lester, 2006).
9.2 Physicochemical changes in cell membrane
structure and properties
Cell membranes are dynamic entities. Both the protein and lipid components are constantly
being turned over to maintain a functional state suited to the physiological state of the
produce. The biochemical composition, the degree of unsaturation of acyl chains, polar head
groups, and the pH of the medium are all factors that influence and regulate the functional
properties of membranes and the activities of embedded enzymes. The membrane properties
are precisely regulated to maintain cellular homeostasis.
The lipid composition of cell membranes can be quite heterogeneous (Yoshida and
Uemura, 1986; Larsson et al., 1990). In both plasma membrane and tonoplast, phospholipids,
sterols, and ceramide monohexosides were shown to be the major classes of lipids.
The plasma membrane contained relatively higher levels of sterols than the tonoplast, and
thus possessed a higher sterol/phospholipid ratio than the vacuolar membrane. Among the
phospholipids, phosphatidylcholine and phosphatidylethanolamine were the major components,
with smaller amounts of phosphatidylinositol, phosphatidylglycerol, and phosphatidylserine.
Considerable changes occur during the ripening/senescence process that
lead to alteration in the biophysical properties of membranes, including a transition from
predominantly liquid crystalline to gel-phase lipid, a decrease in bulk lipid fluidity or an
increase in microviscosity, an increase in phase transition temperature, and the formation
of nonbilayer lipid structures (Thompson et al., 1987). Such changes occur as a result of the
enzymatic catabolism of phospholipids and the accumulation of degradation products such
as phosphatidic acid (PA), diacylglycerols, free fatty acids, and their oxidation products.
In banana, the total lipid content remained unchanged during the respiratory climacteric
induced by the application of ethylene. The relative proportions of phospholipids, glycolipids,
and neutral lipids remained constant during this period. However, in the phospholipid
fraction, there were considerable changes in fatty acyl unsaturation and composition. The
content of linolenic acid in the phospholipid fraction increased with a concomitant decrease
in the linoleic acid content, thus resulting in a higher level of unsaturation (Wade
and Bishop, 1978). In ripening apple fruit, the microviscosity increased from 3.46 poise in
early climacteric to 4.56 poise in postclimacteric stage. The phospholipid content showed
a slight increase from 6.77 to 8.75 μmol/50 g fresh weight during the same time period.
However, the sterol level increased during the postclimacteric stage, resulting in a higher
sterol/phospholipid ratio. The fatty acyl composition also showed changes, with a decline in
unsaturated (18:3, 18:2) and an increase in saturated (16:0, 18:0) fatty acids. These changes
were also associated with increased leakage of potassium ions from the apple tissue (Lurie
and Ben-Arie, 1983). A decline in total phospholipid content has been observed in carnation
flower petals during senescence (Fobel et al., 1987; Sylvestre et al., 1989) and in cherry
tomato (Güçlü et al., 1989) and tomato (Whitaker, 1994) fruits during ripening. The decline