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

6.1 Sorption in Cuticular Membranes, Polymer Matrix, Cutin and Waxes 149
the Kww values should be doubled (Table 6.2) before they are compared to Kcw or
Kow. After this adjustment, Kww values are still much smaller than Kcw. Thus, even
amorphous barley wax sorbs much less than cuticles. Crystalline wax fractions for
the other species are not known, but they are likely to be smaller than 50%.
6.1.4 Concentration Dependence of Partition Coefficients
At low concentrations in the micromolar and millimolar range, partition coefficients
showed a small but significant decrease with increasing aqueous concentrations of 4nitrophenol
(Riederer and Schönherr 1986a). Thus, for practical reasons, as long as
low concentrations are used it should be sufficient to determine partition coefficients
at one single concentration. However, at high concentrations in the millimolar and
molar range, sorption capacity of the CM can become limiting (Sect. 8.1). It was
estimated for 4-nitrophenol (4-NP) and isolated cuticles of Lycopersicon and Ficus
that about 21% of the volume fraction of the CM are available for the sorption of
molecules like 4-NP (Riederer and Schönherr 1986a). If this is true for all sorbates,
it would imply that only 20 weight percent cuticular waxes can be accommodated
in cutin as embedded waxes.
6.1.5 Prediction of Partition Coefficients
Kow values are frequently used in modelling the environmental fate of chemicals.
Since Kow values for all environmental chemicals are not available and the determination
is time-consuming, there have been many efforts to predict Kow from basic
physicochemical properties such as solubility, fragment group contributions, linear
solvation energy relationships and others (Sangster 1997) Similar concepts have
been adopted to predict Kcw values.
Since log Kcw and log Kow are similar (Table 6.1), attempts were made to predict
cuticle/water partition coefficients from octanol/water partition coefficients. This is
possible with fairly good accuracy (Schönherr and Riederer 1989):
logKcw = 0.057+0.970 × logKow (r 2 = 0.97). (6.2)
This equation is based on 13 different chemicals with log Kcw values varying over
eight orders of magnitude. Using (6.2), cuticle/water partition coefficients in the
range of 10 2 and 10 8 can be estimated for low solute concentrations in water.
Organic solutes with no or few polar groups (i.e., –OH, –COOH, –CHO, –NO2,
–NH2) are hydrophobic and have a low water solubility. Large Kow values are mainly
caused by low water solubility, rather than high solubility in octanol (Sangster
1997). For this reason Kow can be predicted from water solubility. This alternative
approach offers the advantage that log Kcw values can be calculated if log Kow