Role of Cuticles in Produce Quality and Preservation 27
claims are supported by studies comparing moisture permeation rates of intact cuticle
to cuticle with the epicuticular waxes removed by solvents. Moreover, a study using
a sorghum mutant with a much thinner epicuticular wax layer than normal showed
significantly higher water vapor permeability [43].
The water vapor permeation rate of the plant cuticle is important in controlling
moisture loss and, indirectly, in modulating the supply of minerals, such as calcium,
to the plant via the xylem [44]. High transpiration rates coincide with higher calcium
levels in plant tissue [45]. In areas with high humidity, transpiration rates may be
too low to provide adequate calcium to the produce, resulting in various physiological
disorders [34,44,45]. Consequently, calcium supplementation may be necessary to
control physiological disorders and sustain critical levels. The most effective method
of achieving higher calcium levels may be to spray the surface of the produce with
a dilute calcium solution [46]. Calcium sprays must come in direct contact with the
deficient plant tissue since little, if any, calcium can translocate to deficient tissue
[47–49]. In apples, several seasonal sprays of calcium may be needed to significantly
increase the calcium content of the fruit [50–53].
One of the first requirements for cuticular penetration of spray solutions involves
adequate wetting of the cuticle surface. The epicuticular wax layer is difficult to wet
with aqueous solutions. Spray droplets tend to bead up and simply fall from the
cuticle surface. Wetting difficulties may be compounded by the presence of crystal-
line wax structures that can catch spray droplets and prevent adequate contact with
the plant surface. Wetting difficulties may also occur on plant surfaces with a dense
distribution of trichomes or epidermal hairs [54]. To mitigate the wetting difficulties,
surfactants can be added to the spray solution [55]. The surfactants lower the surface
tension of spray materials and facilitate wetting of the cuticle surface. In addition,
a study showed that the cuticular permeability increased even when the cuticle
surface was fully saturated with a solution containing surfactants [56].
Materials can directly permeate through the cuticle itself. However, the cuticle
also contains natural pathways such as stomates, lenticels, and cracks that may
facilitate permeation. Glenn et al. [48] studied the cuticular pathways of calcium
through enzyme-isolated apple cuticle. The cuticle from apple fruit harvested approx-
imately 8 weeks after full bloom had a dense distribution of stoma on the fruit
surface, especially at the calyx end. By 12 weeks, the stoma appeared distorted and
open, and by 16 weeks the stoma had mostly developed into lenticels. Schlegel and
Schonherr [50] studied the penetration of^45 CaCl 2 through the cuticle surface of apple
sections and found that the penetration rate depended on the stage of fruit develop-
ment. During early stages of fruit development, the penetration reached its highest
rate as essentially all of the calcium penetrated within 24 hours. The penetration
rate dropped markedly when stomates turned to lenticels and trichomes dropped off.
However, preferential sites for penetration continued to be lenticels, stomates, and
trichomes.
Another study examined the different penetration pathways through the cuticle
by using enzymatically isolated cuticles mounted on molten agar containing oxalic
acid [26]. After the sample was cooled and became solid, a calcium solution was
applied to the cuticle surface. As the calcium penetrated the cuticle, it came in contact
with the agar containing oxalic acid and formed calcium oxylate crystals that were