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

6.3 Diffusion with Changing Donor Concentrations: The Transient State 183
Table 6.7 Comparison of partition coefficients and permeances determined at 25 ◦ C in the steady
state and with UDOS
CM Steady state UDOS
K cuticle/buffer P(m s −1 ) P ∗ (m s −1 ) K soco/buffer P ∗ (m s −1 )
P ∗ UDOS
P ∗ steady state
1 416 1.05 × 10 −8 2.52 × 10 −11 400 1.75 × 10 −11 0.69
2 396 1.49 × 10 −8 3.76 × 10 −11 386 3.02 × 10 −11 0.86
3 326 4.76 × 10 −8 14.60 × 10 −11 325 12.60 × 10 −11 0.86
4 398 11.20 × 10 −8 28.10 × 10 −11 389 24.50 × 10 −11 0.87
Taken from Bauer and Schönherr (1992). Donor solutions had a pH of 3.0
calculated from the mass of the CM, the volume of the donor solution, the decrease
in donor concentration, and the equilibrium concentration of the donor. The donor
solution was removed, and borax buffer was added as receiver and k ∗ was measured
in an UDOS experiment. P ∗ was calculated from (6.16) using the weight average
thickness (ℓCM) of the CM, rather than the unknown thickness of the sorption
compartment (ℓsoco).
Partition coefficients were virtually identical in the two types of experiments
(Table 6.7), except that K soco/buffer tended to be slightly lower, possibly because the
limiting skin was not yet in equilibrium after 3 h of loading. Steady state permeance
(P) varied among CM by a factor of about 10, which is typical for CM of
most species. P ∗ values calculated for steady state experiments varied by the same
factor because K cuticle/buffer varied between CM only slightly. Agreement between
P ∗ obtained from rate constants determined in UDOS experiments (6.16) and those
calculated from steady state data (6.17) is very good, considering that both permeances
varied among CM by a factor of more than 10. This shows that both types of
experiment provide comparable data, and P can be calculated from rate constants
(k ∗ ) measured using UDOS.
P ∗ calculated as ℓCM × k ∗ (UDOS) is consistently smaller than P ∗ calculated
from steady state data (Table 6.7). The factor averaged over all four CM is 0.805,
and this cannot be attributed to differences in partition coefficients, which amount
for only 2% on average. The limiting skin has an unknown but finite thickness,
hence ℓsoco × k ∗ should be smaller than ℓCM × k ∗ . However, P ∗ calculated from
rate constants were smaller than P ∗ calculated from steady state permeance P and
partition coefficients. It appears that ℓCM systematically underestimates the real path
length.
6.3.2.1 Estimating Solute Mobility from Rate Constants
Steady state permeance (P) is proportional to the partition coefficient (2.18). Permeance
of CM is determined by the thickness of the limiting skin, while K reflects
solubility in the sorption compartment. These definitions result in