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

6.3 Diffusion with Changing Donor Concentrations: The Transient State 185
Diffusion coefficient (m 2 /s)
2.5e-18
2.0e-18
1.5e-18
1.0e-18
5.0e-19
0.0
Thickness of sorption compartment (µm)
2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Thickness of limiting skin (µm)
Fig. 6.19 Effects of thicknesses of cuticular compartments on diffusion coefficient. Total thickness
of cuticle is 3µm, and rate constant was taken to be 1 × 10 −6 s −1
dissolved initially. It is implicitly assumed that desorption from the outer surface is
not limited by diffusion in anticlinal pegs and other portions of the sorption compartment
remote from the limiting skin. This can be taken for granted as long as
diffusion coefficients in the limiting skin is 50–100 times lower than in the sorption
compartment. Whenever this is the case, desorption plots (Fig. 6.18) are linear, and
in this case there is no need to worry.
Using simultaneous bilateral desorption, the diffusion coefficient of 2,4-D in the
sorption compartment was estimated to be 10 −14 to 10 −15 m 2 s −1 (Fig. 6.15). D values
estimated from Fig. 6.18 are 10 −18 to 10 −19 m 2 s −1 , which is lower by more
than three orders of magnitude. We can also estimate D in the sorption compartment
using unilateral desorption. If PCP is desorbed from the inner surface of the
CM a nonlinear plot is obtained with an initial slope of 0.0017s −1 (Fig. 6.20). This
is larger by a factor of 944 than measured for desorption from the outer surface
(Fig. 6.18). Rate constants decrease with time, because the sorption compartment is
heterogeneous, as was seen already in Fig. 6.15. Desorption from the outer surface
of MX-membrane was also non-linear, and linearity lasted only for 2 h (Fig. 6.20).
The initial rate constant was 155 times larger than that measured with Citrus CM.
After 2 h, rate constants decreased because diffusion in the sorption compartment
became rate-limiting. Comparing the initial slope obtained with MX with that of
UDIS demonstrates that PCP mobility in the outer layer of extracted CM is lower
by a factor of 6 than mobility in the sorption compartment (UDIS). This difference
is too small to guarantee linear desorption plots until membranes are empty.
Apparently, diffusion from the anticlinal pegs to the outer layer of the MX was too