Water and Solute Permeability of Plant Cuticles: Measurement and ...

1.4 Fine Structure of Cuticles 15
structure and permeability. From the 372 studies reviewed, only two explicitly dealt
with diffusion. Wattendorff and Holloway (1984) used potassium permanganate as
tracer. Schmidt et al. (1981) attempted to find a correlation between water permeability
and fine structure of Clivia CM at different stages of development. All
other workers rationalised their work by alluding to the barrier function of cuticles,
but they simply used standard procedures to generate pictures, while permeability
was not estimated. Nevertheless, some useful terminology and information about
structure–permeability relationships may be obtained from some of these studies.
Extracting waxes increases permeability by 1–3 orders of magnitude (Chap. 4
and 6). This shows that cuticular waxes play a decisive role in water and solute
permeability, and both localisation and structure of waxes are important in understanding
structure–property relationships. The presence of polar paths in lipophilic
cuticles is another topic of importance, because it is a prerequisite for penetration of
hydrated ionic solutes but not necessarily of water (Schönherr 2006).
1.4.1 Nomenclature
We adopt the definitions and nomenclature of Jeffree (2006), which is also used
by most of the workers in the field. The cuticle is a polymeric membrane located
on the epidermal wall of primary organs. It has a layered structure. The outermost
layer is called cuticle proper (CP), and the layer underneath is the cuticular layer
(CL). In many species, an external cuticular layer (ECL) located under the CP and
an internal cuticular layer (ICL) facing the epidermal wall can be distinguished.
Soluble cuticular lipids or waxes occur as epicuticular waxes and as embedded or
intracuticular waxes. CP, CL and waxes constitute the cuticle (CM), which in some
species can be isolated enzymatically. Due to its layered structure, the cuticle is a
heterogeneous membrane. We distinguish transversal heterogeneity which is apparent
in cross-sections, and lateral heterogeneity which arises due to the presence of
trichomes and stomata.
1.4.2 Transversal Heterogeneity
1.4.2.1 Light Microscopy
In the light microscope, cross-sections of cuticles appear homogeneous. When
Clivia cuticles are stained with Sudan III, transversal heterogeneity is not visible
(cf. Fig. 2.6). Sudan III is a non-ionic lipophilic dye, but it does not stain solid
paraffin or carnauba wax (Sitte and Rennier 1963). The dye is sorbed in polymeric
cutin, but wax is not stained. Prominent cuticular pegs extend deep between the anticlinal
walls of the epidermal cells. The thick epidermal wall is stained at pH4 with
toluidine blue, which is an anionic dye that binds to carboxyl groups of pectins in