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

236 8 Effects of Temperature on Sorption and Diffusion of Solutes and Penetration of Water
while the CM also contains waxes as additional lipid component. As shown in
Sect. 6.1, wax/water partition coefficients are much smaller than cutin/water partition
coefficients. Thus, cutin is the sole sorbent in the MX and the major sorbent
in CM.
Partition coefficients decrease with increasing temperature and increasing internal
solute concentration. Between 10 −4 and 10 −2 mol kg −1 or 1.39 and 0.014g kg −1 ,
internal concentrations of partition coefficient were nearly constant, but at higher
internal concentrations K decreased. This decrease is more pronounced in rubber
than in tomato CM and MX. With rubber CM and MX, all sorption plots converge
at an internal concentration of about 1mol kg −1 . This indicates that all sorption
sites are occupied and additional sorption is no longer possible. With tomato fruit
cuticles, partition coefficients also decrease at higher 4-NP concentrations but plots
do not converge. The difference in internal concentrations between CM and MX is
highest at low sorbate concentrations, but it decreases as concentration increases.
If waxes sorb little or no 4-NP, partition coefficients for MX would be expected
to be higher by 8% and 25% with tomato and rubber cuticles respectively. However,
differences are larger, indicating that in MX some sorption sites are occupied
by waxes.
At low external concentration of 4-NP, partition coefficients were nearly constant,
even though internal 4-NP concentration increased 100-fold. This indicates
that additional new sorption sites become available when others are being occupied
by 4-NP molecules. The flexible polymer chains open up, and sorbate molecules
squeeze in between. There is a limit to this, however, and when internal concentration
exceeds 10 −2 mol kg −1 additional sorption sites are no longer formed and
partition coefficients decrease. Partition coefficients decrease much more with rubber
leaf cuticles, indicating that flexibility of polymer chains is smaller compared
to tomato fruit cuticles. It appears that cutan in rubber leaf cuticle is more rigid and
provides fewer sorption sites.
The temperature effect on partition coefficients depends on the temperature effect
on activities of the two phases. With substituted phenols, partition coefficients
between inert organic phases which are incapable of hydrogen bonding increase
with temperature, while octanol/water partition coefficients decrease with increasing
temperature (Korenman et al. 1977). Cuticle/water partition coefficients also
decreased with increasing temperature, and this is evidence that hydrogen bonding
between cutin and 4-NP was involved in sorption.
The temperature effect on sorption can be further analysed using a modified
Freundlich isotherm. This makes it possible to distinguish between temperature
effects on the number of sorption sites (solubility in cuticles) and on water solubility.
Cinternal = k ′ ·(Caqueous/C saturated
aqueous ) 1/n
(8.2)
The term in parenthesis is referred to as the reduced concentration. Plotting log
Cinternal vs log of the reduced concentration resulted in straight lines with slopes
k ′ . With Ficus CM and MX, k decreased with increasing temperature (8.1), while
k ′ was independent of temperature (8.2). Hence, the temperature effect on Ficus