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

164 6 Diffusion of Non-Electrolytes
Table 6.5 Distribution of PCP in the compartments (CPT) of barley leaves as affected by duration
of preloading. CPT1 and CPT2 were obtained by non-linear regression analysis (6.10). CPT3 is the
fraction of PCP unaccounted for by CPT2 and CPT1
Preloading for CPT1 CPT2 CPT3
30 min 0.56 0.10 0.34
90 min 0.41 0.08 0.51
180 min 0.15 0.10 0.75
Desorption kinetics were analysed by non-linear regression. The best fit (r 2 >
0.99) was obtained with a model consisting of two compartments (CPT1 and CPT2)
and two rate constants (k1 and k2):
Mt
M0
= CPT1
� � � �
−k1t −k2t
1 − e +CPT2 1 − e . (6.10)
Rate constants were independent of duration of preloading, and amounted to 8.5 ×
10 −3 and 3.7 ×10 −4 s −1 for k1 and k2 respectively. Rate constants were defined by
(2.6). From the rate constant, the half-time (t 1/2) required for 50% sorption or desorption
(Mt/M0 = 0.5) can be calculated as t 1/2 = ln 0.5/k. Hence, t 1/2 is 81.5 and
1,873 s for compartments 1 and 2 respectively. It takes about five half-times before
the compartments are empty. In the case of barley leaves and PCP this amounted to
6.8 and 156 min for compartments 1 and 2, respectively.
The compartment sizes in (6.10) represent fractions. The absolute size of a
compartment in Bq is obtained by dividing radioactivity in the CPT by total radioactivity
in the leaf. The fractions of PCP contained in the compartments are given in
Table 6.5.
Sizes of CPT1 and CPT3 depended on duration of preloading, while CPT2 was
constant and 8–10% of total PCP was sorbed in this compartment. CPT1 decreased
and CPT3 increased with duration of preloading. PCP in CPT1 and CPT2 was
reversibly sorbed, while PCP in CPT3 increased with time and could not be recovered
from the leaves. This indicates that this fraction of PCP was in the leaf tissue in
dissociated form and was held there by an ion trap mechanism. pH in the apoplast
and symplast are around 5–6, and this favours dissociation.
If the absolute amounts contained irreversibly in CPT3 are plotted vs time, a
straight line is obtained (Fig. 6.5, plot “calculated from desorption”) which intersects
the origin and has the same slope as the plot “amount penetrated” vs time.
Permeance calculated from desorption experiments is 1.4 × 10 −7 m s −1 , and is
identical within experimental error to permeance obtained from the slope of a
penetration experiment.
The parallel displacement is caused by PCP reversibly sorbed on the leaf surface,
in epicuticular wax and in the cuticle underneath (cutin and intracuticular wax).
The donor solution is in contact only with the tips of the epicuticular wax crystallites.
Hence, epicuticular waxes are filled first and serve as intermediate phase
between aqueous donor and cuticle. PCP sorbed in wax continues to diffuse into the