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

234 8 Effects of Temperature on Sorption and Diffusion of Solutes and Penetration of Water
Tomato fruit cuticles (Lycopersicon esculentum) contain 7% waxes and 62%
cutin. Polar polymers amount to 24% (Table 1.1). The ester cutin is of the C16
type, and dihydroxyfatty acids are the major constituents. Adaxial rubber leaf cuticles
(Ficus decora) are composed of 56% cutin, 19% polar polymers and 25% waxes
(Table 1.1). The cutin contains C16 and C18 hydroxyfatty acids, and 18-hydroxy-
9,10-epoxystearic acid is a major constituent, which results in the formation of
non-ester cutin (cutan) similarly to what happens in Clivia cutin (Fig. 1.2).
8.1.1 Sorption Isotherms and Partition Coefficients
Small pieces of cuticular material were equilibrated at constant temperature as
described in Sect. 6.1. At equilibrium, the molal 4-NP concentrations (mol kg −1 )
in the aqueous phase (Caqueous) and in the CM or MX (internal phase Cinternal) were
determined. The partition coefficients K were calculated as the ratio of the molal
concentrations Cinternal/Caqueous. When 4-nitrophenol concentration in the cuticle
(Cinternal) was plotted against concentration in water (Caqueous), linear sorption
isotherms were obtained at all temperatures and concentrations of 10 −3 –10 −6 mol
kg −1 . Alternatively, the natural logarithms of these concentrations can be plotted.
The slopes of these plots (k) are related to the intensity of sorption and the number
of sorption sites. The parameter n characterises the slope of the isotherm. The linear
portions of the plots fit the Freundlich isotherm
or in logarithmic form
Cinternal = kC 1/n
aqueous
logCinternal = logk+ 1
n logCaqueous
(8.1)
(8.1a)
Freundlich isotherms are frequently encountered when solutes interact with heterogeneous
substrates. At low concentrations the parameter n is not far from 1.0, but at
higher sorbate concentrations the plots become increasingly convex to the Caqueous
axis. At low sorbate concentrations this type of isotherm can be classified as constant
partitioning, while at higher concentrations (>10 −3 mol kg −1 ) it resembles a
Langmuir type isotherm. Linear isotherms are obtained under the following conditions:
(1) the substrate consists of flexible molecules and has regions of differing
accessibility to the sorbate, (2) the solute has a higher affinity to the substrate than to
the solvent water, and (3) the solute is able to penetrate into initially less accessible
(highly ordered) regions of the solid.
At all concentrations and temperatures, partition coefficients (K) and the
Freundlich parameter k were >1. K ranged from about 15 to 250 (Fig. 8.1), and
k varied between 14 and 190 (Riederer and Schönherr 1986a); that is, 4-NP is better
soluble in CM and MX than in water. At low internal concentration, K is greater
with the MX than with the CM. The only lipid compartment of the MX is cutin,