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

178 6 Diffusion of Non-Electrolytes
MX membranes this asymmetry factor was only 10, because desorption from the
outer surface was higher. This is still a high asymmetry, since with a homogeneous
membrane the factor should have been unity.
This asymmetrical desorption pattern indicates that CM and MX membranes are
composed of at least two compartments. The bulk of the 2,4-D was contained in
the inner volume element, and it was desorbed through the inner surface. We call
this inner domain of the cuticle the sorption compartment (soco), and morphologically
it is identical with the cuticular layer(s) seen in TEM (Sect. 1.4). The outer
domain across which only very small amounts of 2,4-D were desorbed is the cuticle
proper, and we refer to it as limiting skin or limiting layer (Fig. 6.16). Volumes and
thicknesses of these two layers cannot be deduced from desorption plots. As in 4 h
only 5% 2,4-D diffused across the outer surface of the CM, diffusion coefficients
in the limiting skin must have been very low and much lower than in the sorption
compartment. The data do not reveal if it is a wax layer on top of the cuticle or if the
barrier consists of waxes embedded in the outer fraction of the MX. A combination
of both is possible as well. In any event waxes are involved, since extracting them
reduced asymmetry greatly (Fig. 6.15).
When Mt/M0 (desorption through the inner surface) was plotted against the
square root of time (Fig. 6.15b), plots were not linear up to Mt/M0 equal to 0.5, as
would be expected with homogeneous membranes (Fig. 2.10b), but diffusion coefficients
can still be estimated from the initial slope using (2.35). Since both CM and
MX are heterogeneous, these D-values are some kind of average characterising diffusion
of 2,4-D in the sorption compartment (cuticular layer). For these calculations,
Fig. 6.16 Schematic drawing of a cross section of a cuticle showing the thin limiting skin and the
thick sorption compartment (not to scale). Modified from Bauer and Schönherr (1992)