Microstructure of Produce Degradation 535
that they are fully hydrated or have normal turgor pressure. The vascular bundles
appear normal and there are no wrinkles or folds in the tissue. In contrast, the adaxial
surface of the aged leaf (Figure 18.4b) has an irregular appearance, areas where
epidermal cells appear to be missing, and irregular dark spots. Although the vascular
bundles look normal, epidermal cells are not as plump as in the fresh sample. More
pronounced changes are evident on the bottom (abaxial) leaf surface, where the fresh
leaf (Figure 18.4c) shows a distinct vascular bundle, stomata and plump epidermal
cells; the aged leaf (Figure 18.4d) shows a vascular bundle that blends in with the
rest of the extensive folds in the epidermis. In the fresh sample, the epidermis appears
flat, the stomata are nicely arranged and the epidermal cells are plump (Figure 18.4e);
however, early signs of degradation are also apparent in the fresh leaf in areas where
the epicuticular wax has started to peel and crack (Figure 18.4f). The aged leaf
(Figure 18.4g) shows deformities of the epidermis, which has shriveled and wrinkled
due to dehydration; some of the epidermal cells have collapsed and others appear
to have maintained some internal turgor pressure. In the region of the vascular bundle
of the aged leaf (Figure 18.4h), dark spots, which are not stomata, are evident. One
of the dark spots on the outside of the vascular bundle proved to be a bacterial
colony (Figure 18.4i and j). The colony was associated with a fibrous substance,
probably a biofilm, which some bacteria exude in order to adhere to a substrate.
Biofilms are typically composed of highly hydrated polysaccharides [10,11].
Because the polysaccharides have a high water content, they are difficult to preserve
using aqueous fixation techniques typically used for scanning electron microscopy.
During fixation and subsequent dehydration, portions of the polysaccharide dissolve
and the remaining insoluble portions lose definition and tend to clump together,
forming what looks like fibers under the microscope. The larger fibrous components
are part of the parsley leaf that is disintegrating due to microbial activity (Figure
18.4i and j) that develops readily on parsley due to its short shelf life.
FIGURE 18.4 (Opposite page)Italian parsley. Scanning electron micrographs show the outer
epidermal surfaces of leaves. Adaxial, upper leaf surfaces showing the vascular bundles and
epidermal cells in the fresh (a) and aged (b) leaves. (a) Fresh leaf showing regular, hydrated
epidermal cells. (b) Aged leaf with shrunken epidermal cells, areas where epidermal cells appear
to be missing (oval) and irregular spots. Abaxial, lower, leaf surfaces show the outer epidermis
of fresh (c, e, f) and aged (d, g, h, i) leaves. (c) Fresh leaf has distinct vasuclar bundles, epidermal
cells and stomata. (d) Aged leaf has extensive wrinkling or folding, epidermal cells appear to
have lost turgor pressure, and the vascular bundle is indistinct. (e) Fresh leaf showing open
stomata (arrow) and epidermal cells that have adequate turgor. (f) Fresh leaf showing a single
stomatal complex (arrow) and epidermal cells with rounded shapes indicative of adequate turgor;
the “flaky” appearance is due to epicuticular wax that has started to peel and crack. (g) Aged
leaf showing shrunken stomata (arrow) and folds. Arrows, stomatal complexes. (h) Aged leaf
showing a distinct view of the vascular bundle and stomata; the epidermal cells have lost turgor
pressure and wrinkles are apparent; darkened areas are apparent (box). (i) High magnification
of the boxed area in h showing a bacterial colony associated with a thin, filmy substance,
probably a biofilm (arrows) composed of polysaccharides that microorganisms exude to allow
them to adhere to a substrate; fibers (*) are plant material that is decomposing. (j) Bacterial
colony showing greater detail; arrows indicate possible biofilm. VB, vascular bundle. Magnifi-
cation bars: a–d, 500 μm; e–g, 50 μm; h, 200 μm; i, 10 μm, j, 2 μm.