Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

PHOSPHOLIPASE D, MEMBRANE DETERIORATION, AND SENESCENCE 197
in total phospholipids in ripening tomato fruit pericarp tissue was associated with increases
in the levels of phosphatidic acid and free fatty acids, and in the sterol/phospholipid and
glucocerebroside/phospholipid ratios (Whitaker, 1991, 1992, 1993, 1994). There were also
marked ripening-specific changes in sterol lipid content and composition, including an increase
in total sterols, higher proportions of sterol esters, free sterols, and sterol glycosides
relative to acylated sterol glycosides, and a dramatic increase in the stigmasterol/sitosterol
ratio (Whitaker, 1988, 1994).
Surprisingly, there was no loss of phospholipids during chilling or after subsequent
warming of mature green tomato fruit (Whitaker, 1991, 1992, 1994; Bergevin et al., 1993),
but profound effects on sterol lipid metabolism were observed. Most notably, a large increase
in the proportion of free sterols occurred during chilling, whereas after warming of chilled
fruit there was a rapid return to at-harvest levels of free and conjugated sterol classes
plus a sharp increase in the stigmasterol/sitosterol ratio (Whitaker, 1991, 1993, 1994).
Palta et al. (1993) have also reported differences in phospholipid metabolism in leaves of
freezing-tolerant wild-type potato species and freezing-susceptible cultivated species. Cold
acclimation of both the species resulted in similar changes in plasma membrane lipids that
included a decrease in palmitic acid (16:0), an increase in unsaturated to saturated fatty
acid ratio, an increase in free sterol levels, especially sitosterol, and a small decrease in
cerebrosides. Lipid changes specific to the freezing-tolerant species included an increase in
phosphatidylethanolamine, a decrease in sterols, an increase in linoleic acid with a decrease
in linolenic acid, and an increase in the acylated sterol glycoside to sterol glycoside ratio.
The lipid compositions of chloroplasts and mitochondria are quite different from that of
the plasma membrane (Schwertner and Biale, 1973). Phospholipids amounted to 50–56% of
total lipids in mitochondrial preparations from avocado fruits, cauliflower buds, and potato
tubers. As in most cell membranes, the major phospholipids were phosphatidylcholine and
phosphatidylethanolamine, but the unusual phospholipid cardiolipin (diphosphatidylglycerol)
was also abundant. The glycolipids included monogalactosyl and digalactosyl diacylglycerols.
Potato mitochondria had a relatively low content of phospholipids. In contrast to
mitochondria, chloroplasts are highly enriched in the monogalactosyl and digalactosyl diacylglycerols
relative to phospholipids, and phosphatidylglycerol is the major phospholipid.
Irrespective of their composition, all membranous structures undergo enzymatic and active
oxygen-mediated catabolism during senescence.
9.3 Membrane lipid catabolism during senescence
The pathway of membrane lipid degradation that occurs during ripening or senescence was
delineated using several systems that included bean cotyledons (Paliyath and Thompson,
1987), carnation flower petals (Paliyath et al., 1987), broccoli florets (Deschene et al., 1991),
and tomato fruit (Todd et al., 1990; McCormac et al., 1993). In most of these studies, a microsomal
membrane fraction comprising the plasma membrane, endoplasmic reticulum, and
tonoplast membranes was isolated by differential centrifugation of the tissue homogenate.
Microsomal membranes were incubated in an enzyme assay mixture containing a radiolabeled
phospholipid substrate consisting of uniformly labeled phosphatidylcholine (PC), a
specific molecular species of PC (e.g., 16:0/16:0, 16:0/18:2, and 18:0/20:4), or other phospholipid
classes. After a period of incubation, the reaction was terminated by acidification
and the addition of chloroform/methanol (2:1 v/v), effecting a phase separation. The heavier