Agriculture Reference
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isolated from the florets prior to and after storing for 2, 4, 6, and 8 days. The membrane was
incubated with radiolabeled PC, and catabolites in each fraction were determined. It can be
seen that initially (0 day) the formation of PA, indicative of phospholipase D activity, was
quite low. However, with prolonged storage, the PA level increased, followed by increases in
the formation of diacylglycerols (DG), free fatty acids (FFA), and hydroperoxide products
of free fatty acids (FAOOH). Broccoli is very difficult to store, and often over 50% of
the produce is damaged during shipping. Broccoli is immediately washed and cooled with
an ice-water mixture after harvest and is generally shipped with ice for long distances.
However, by employing controlled atmosphere storage (5% CO 2 ,3%O 2 , 92% N 2 at 5 C
and a relative humidity of 80%), the storage life of broccoli can be extended for up to
2 months (Deschene et al., 1991). These studies reflect the changes in lipid content observed
in vivo during senescence including a decline in phospholipid content and an increase in
neutral lipids.
The accumulation of phospholipid degradation products in the membrane can affect its
biophysical properties. Membrane lipids adopt liquid crystalline, gel, micellar, and hexago-
nal phases based on the composition, hydration, and presence of ions (Lafleur et al., 1990).
Accumulation of lipid degradation products in the membrane can cause localized transfor-
mation of membrane lipid bilayer into destabilized bilayer and nonbilayer lipid structures
such as the micellar and hexagonal-phase lipid. Lipid catabolites such as alkanes, fatty
alcohols, and fatty acids can alter the physicochemical properties of the lipid bilayer and,
thereby, the functional state of a biological membrane (Lohner, 1991). Alkanes can induce
the formation of nonbilayer structures in the membrane. Diacylglycerols induce different
forms of perturbations in the bilayer depending on whether they possess saturated or unsat-
urated fatty acids. Saturated diacylglycerols such as dipalmitin and distearin induce lateral-
phase separation of the lipids into diacylglycerol-enriched, gel-like domains and relatively
diacylglycerol-free regions in the liquid crystalline phase (DeBoeck and Zidowski, 1989).
Diacylglycerols are also known to cause formation of micelles and vesicles in the membrane
(Allan et al., 1976). The accumulation of free fatty acids and their metabolites can induce
formation of gel-phase lipid (Katsaras and Stinson, 1990). As well, a decline in phospho-
lipid content and a relative enrichment in sterols can cause the formation of gel phase and a
decrease in bulk lipid fluidity (Duxbury et al., 1991). The formation of gel-phase lipid during
senescence is a widely noted structural alteration. Formation of gel phase and accumula-
tion of gel-phase forming lipids were initially demonstrated in senescing bean cotyledons
(McKersie et al., 1978; Pauls and Thompson, 1984) by wide-angle X-ray diffraction and
electron paramagnetic resonance techniques. These studies were conducted using isolated
microsomes or with reconstituted lipids after their isolation from the membrane. Freeze
fracture studies on carnation petals also revealed the formation of gel phase during flower
development (Paliyath and Thompson, 1990). During such studies, petal fragments were
frozen in liquid propane and fractured in a specialized apparatus maintained at
180 C, by
circulating liquid nitrogen. With this procedure, the fracture plane passes through regions
with the weakest structural interactions, which most often occur in between the membrane
bilayers. Thus, the fracture exposes the surface of the interior of the bilayer. A replica of
the fracture face is made using evaporated gold and carbon, and the replica is cleaned and
observed under an electron microscope to reveal surface property details. A freeze fracture
image of a partially open carnation flower petal is shown in Fig. 9.2. The fracture plane
reveals cellulose microfibrils of the cell wall (CW), the plasma membrane (PM), and the
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