Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

200 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
Fig. 9.2 Freeze fracture replica of a young unopened carnation flower petal. The fracture plane exposes the
cell wall (CW), cytoplasm (CY), protoplasmic fracture face of the plasma membrane (PM), and the exoplasmic
fracture face of the endoplasmic reticulum (ER). Arrows indicate the intramembranous particle-free regions (gel
phase) on the ER. Bar = 100 nm. (Reproduced with permission from Paliyath and Thompson, 1990.)
endoplasmic reticulum (ER). The plasma membrane appears turgid and contains numerous
proteins, indicative of an ideal membrane. The endoplasmic reticulum, however, includes
vacant areas indicative of gel-phase lipid. During gel-phase formation, the mobility of the
fatty acyl chains is reduced, making them more rigid (less fluid). Proteins that are excluded
from the gel-phase accumulate in the surrounding liquid crystalline phase. The interface
between the gel phase and the liquid crystalline phase does not pack very well, and this
causes leakiness in the membrane. It is interesting to note that the very first symptoms
of gel-phase formation are noticed on the endoplasmic reticulum and not on the plasma
membrane. However, as the flower develops further (fully open), even the plasma membrane
shows gel-phase areas (Fig. 9.3). The surface of the plasma membrane is frequently
distributed with pitlike structures, which may represent areas containing damaged lipids in
the form of microvesicles that are excluded from the membrane (Yao et al., 1991). Large
vesicular structures (V) are also visible lying close to the plasma membrane.
The degradation of membrane lipids is an essential feature of senescence and signal
transduction pathways that occur in response to hormones and environmental stress (Paliyath
and Droillard, 1992; Chapman, 1998; Wang, 2002; Bargmann and Munnik, 2006; Wang
et al., 2006). After evaluating the pattern of lipid catabolism in microsomes from various
tissues, a pathway for the catabolism of phospholipids was developed in senescing systems
that involves the sequential action of enzymes such as phospholipase D (PLD), phosphatidate
phosphatase, lipolytic acyl hydrolase (LAH), and lipoxygenase (Fig. 9.4). PLD is the
key enzyme of the pathway since none of the following enzymes can directly act on phospholipids,
though there are exceptions such as the potato and carnation LAHs (Galliard,
1980; Hong et al., 2000). PLD is also stimulated by physiologically elevated levels of calcium
and low cytosolic pH. Such conditions can occur during senescence when membrane