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

146 6 Diffusion of Non-Electrolytes
1.0 indicates that solubility is the same in both phases. Compounds having partition
coefficients >1 are lipophilic, and they are hydrophilic when Kow < 1. Partition
coefficients vary by several orders of magnitude, and to avoid exponents log values
(e.g., log Kow) are used.
Partition coefficients can also be determined for solid lipid phases such as fats,
waxes or cuticles. Solutes sorbed in a solid constitute a solid solution (Fig. 2.6).
Sorption sites in a solid are finite, and for this reason partition coefficients decrease
with increasing concentration in the aqueous phase (Riederer and Schönherr 1986a).
Partition coefficients CM/water (Kcw) can easily be determined using radiolabelled
solutes. Isolated CM are equilibrated with an aqueous solution of a radiolabelled
compound at constant temperature. After equilibration, the amounts of
radioactivity in water are determined by scintillation counting. The amounts of
radioactivity in the CM can be calculated from the decrease in the concentration
in water after equilibration. If this method is used, the drop in concentration should
be large, especially when radiochemical purity is less than 99%. It is better to determine
radioactivity in both phases after equilibration. Care should be taken to remove
water films adhering to the cuticles by blotting with soft tissue paper. Cuticles are
thin, often only about 3µm thick or less. The inner surface of the cuticles is easy to
wet, and water films cannot be avoided. If partition coefficients are >10, liquid films
of the same mass as the cuticular materials introduce little error. The error can be
estimated and corrected by weighing the CM after blotting and after air drying. With
the weight of both phases (CM and water) known, Kcw can be calculated according
to (2.12). Similarly, partition coefficients can also be measured with MX (Kmxw),
cutin (Kcuw) and cuticular waxes (Kww) as solid lipid phases.
Time needed to establish equilibrium depends on diffusion coefficients in cuticles
and on membrane thickness. Equilibration usually takes only a few hours (Sect. 6.3,
Figs. 6.13 and 6.14). However, with compounds carrying a carboxylic group (e.g.,
2,4-D) and cuticles with epoxyfatty acids (Chap. 2), it was observed that partition
coefficients slowly but constantly increased for many days (Riederer and Schönherr
1986b). This was due to formation of ester bonds between reactive epoxide groups
and carboxyl groups of 2,4-D. Hence, partition coefficients increased with time and
were overestimated. This problem can be avoided by washing the CM in 1.5 M HCl,
which converts epoxy groups in vicinal hydroxyl groups.
6.1.2 Cuticle/Water Partition Coefficients Kcw
Kcw have been measured for 2,4-D and a series of cuticles isolated from ten different
species (Riederer and Schönherr 1984). With leaf CM, partition coefficients ranged
from 240 to 470, and with fruit CM the range was 424–579, depending on plant
species. Mean Kcw of six leaf CM (Clivia, Ficus, Citrus, Hedera, Pyrus and Olea)
was 316. The mean Kcw of four fruit cuticles (Lycopersicon, Capsicum, Solanum and
Cucumis) was 476, which is about 50% higher. Based on this study, leaf CM from