Role of Cuticles in Produce Quality and Preservation 29
[62]. Veraverbeke [30] measured water diffusion coefficients of apple flesh, cutin,
and wax. Wax had the lowest values at 0.192 to 3.03 × 10 –14 m^2 sec–1, followed by
cutin at 4.03 to 7.16 × 10 –14 m^2 sec–1 and apple tissue at 433 to 1,120 × 10 –14 m^2
sec–1. Although the diffusion of water per unit area of cuticle is very low, diffusion
through the cuticle is still an important mode of penetration since the cuticle encom-
passes a large surface area compared to structures such as lenticels [26]. For instance,
Maguire et al. [63] reported that 5- to 10-fold more water is lost by diffusion through
the cuticle than through lenticels. The rate of cuticular diffusion can also be affected
by environmental conditions. Chamel [37] found greater cuticular penetration of
herbicides at higher relative humidity and temperature.
2.4.2 BARRIER TO PATHOGENS
Plant cells provide a substrate for the growth of number of fungi and bacteria. The
cuticle can protect plants from pathogen infestation and can act as a formidable
physical barrier to many microorganisms. For instance, fruit mummification in
blueberries occurs when the pathogen Monilinia vaccinii-corymbosi infects the plant.
The pathogen cannot penetrate the cuticular layer on the fruit, but it can infect the
fruit by attacking the flower parts instead [64]. Conidia produced on blighted shoots
are transported to the floral stigmas by wind, rain, or insects. The conidia germinate
on the stigma and the hyphae grow within the stylar canal until it reaches the base
of the style. The fungal mycelium then invades and begins to colonize the ovary.
This colonization lasts throughout the growing season [64].
Some pathogens are able to breach the cuticle and infect the produce. The typical
infection process for pathogenic fungi begins when spores attach to the cuticle
surface. Fungal spores generally must first adhere to the cuticular surface of produce
before they can penetrate it [65]. The pathogen adheres to the cuticle surface via
intermolecular forces until a bulblike structure called the appressorium begins to
develop in close contact with the cuticle surface and then provides additional bonding
strength. From the appressorium, a small growing point called the penetration peg
develops at the cuticle surface. As the penetration peg grows, it pierces the cuticle
and epidermal cell wall and commences the infection process [65]. However, the
penetration peg may not be able to pierce the cuticle if the penetration force required
exceeds the adhesive strength of the pathogen to the cuticle surface. Thus, produce
with a relatively thick or otherwise difficult-to-penetrate cuticle have greater resis-
tance to infection.
Pathogens commonly secrete cutinases and other enzymes at the penetration site
that may soften the cuticle and facilitate spore adhesion and penetration [66]. There
are no known pathogens that can produce enzymes to degrade epicuticular waxes
[65]. However, phytopathogenic fungi produce cutinases that can soften and partially
degrade the cutin layer that lies beneath the epicuticular wax layer [67].
Invading pathogens apparently penetrate the epicuticular wax layer solely by
mechanical force (e.g., penetration peg). Once the pathogens gain access to the cutin
layers through the penetration site, they secrete cutinases. The cutinases soften the
cuticle and facilitate further penetration by the penetration peg. It should be noted
that the role cutinase plays in the infection process remains controversial, since some