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

6.3 Diffusion with Changing Donor Concentrations: The Transient State 181
formed, and lipophilic solutes are sorbed in these PLS vesicles. This maintains the
solute concentration in the water surrounding the vesicles at practically zero, and it
ensures good wetting of the waxy cuticle surfaces, because surface tension is lower
than in water. The desorption medium is periodically withdrawn quantitatively and
replaced by fresh medium. At the end of the experiment the cuticle exposed in the
orifice of the lid is cut out, and residual radioactivity in the CM is extracted with
scintillation cocktail. Radioactivity in desorption media and cuticles is determined
with a scintillation counter.
Radioactivity in the desorption media at time t is Mt and the sum of the radioactivity
in desorption media and cuticle is M0, which was routinely compared to the
amount applied; recovery was always 100%. Mt/M0 is the solute fraction desorbed,
and (1 − Mt/M0) is the solute fraction remaining in the CM. Plotting the natural
logarithm of (1 − Mt/M0) vs time always resulted in straight lines (Fig. 6.18). The
slopes of the plots are the rate constants (k ∗ ) as defined by the equation
−ln � � ∗
1 − Mt = k t. (6.13)
� M0
These rate constants are related to permeance (P) as shown in (2.26), which for
convenience is repeated here
−ln (1−M t/M o)
2.0
1.5
1.0
0.5
0.0
ln(Cdonor/C0)
t
= −PA
= k
Vdonor
∗
Capsicum CM
slope = 5.4 x 10 −6 s −1
Citrus CM
slope = 1.8 x 10 −6 s −1
0 20 40 60 80 100
Time (h)
86
78
63
39
0
(6.14)
Fig. 6.18 Unilateral desorption of pentachlorophenol from the outer surface of Citrus and Capsicum
CM. Slopes of the plots are the rate constants (k ∗ ). (Redrawn from Bauer and Schönherr
1992)
Percentage desorbed