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

216 7 Accelerators Increase Solute Permeability of Cuticles
M t /M o (relative amounts of C 24 Ac desorbed)
0.05
0.04
0.03
0.02
0.01
+ C 12E 8
− C 12 E 8
0.00 0 3 6 9 12 15 18
Square root of time (min −1/2 )
Fig. 7.6 Desorption of tetracosanoic acid (C24Ac) from reconstituted barley wax. Within the
first hour (blue symbols) C24Ac was desorbed using inert borax buffer (pH 9.0) as desorption
medium. During the second hour (red symbols) C24Ac was desorbed by borax buffer containing
C12E8 (10 −2 mol l −1 ). During the final desorption period (blue symbols) borax buffer was used
again. Error bars represent 95% confidence intervals. Data redrawn from Schreiber (1995)
When working with aqueous micellar solutions, micelles scavenge lipophilic
solutes such that their concentration in water remains practically zero. In the present
case this did not occur, because C24Ac was ionised, but when non-ionic solutes are
studied, their sorption in micelles is very useful to maintain maximum driving force.
Using (6.23) (Sect. 6.5.2) D in wax can be calculated from the molar volume.
With the molar volume of C12E 8 (458cm 3 mol −1 ), D is 5.74 × 10 −20 m 2 s −1 .
However, direct measurements using 14 C-labelled C12E8 result in a D of 8.5 ×
10 −18 m 2 s −1 (Schreiber 1995), which is about 150 times higher than D estimated
from (6.23). This large discrepancy between estimated and measured D is direct evidence
that C12E8 not only accelerates diffusion of other solutes but simultaneously
accelerates its own diffusion. This fairly high diffusion coefficient of C12E 8 also
explains the fast induction (Fig. 7.6).
These conclusions are supported by the time course of sorption in wax of
C12E 8 and desorption of C24Ac from reconstituted barley leaf wax (Fig. 7.7). Data
were obtained by independently using either 14 C-labelled C12E8 or 14 C-labelled
C24Ac. It took roughly 1 hour for equilibration of the wax with C12E8, and the
half-time (Mt/M0 = 0.5) of sorption is only about 7 min (Fig. 7.7). With borax
buffer, desorption of C24Ac is linear and very slow (Fig. 7.7). Significantly higher
rates of desorption were obtained when C12E 8 was added to the borax desorption
medium. Rates increased up to 100 min. Afterwards, linear desorption kinetics
were observed when C12E8 concentration in the wax was maximum and constant
(Fig. 7.7). Comparable data were obtained with C12E 8 and pentachlorophenol
(Schreiber 1995).