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

4.6 Water Permeability of Isolated Astomatous Cuticular Membranes 111
could be responsible for the difference. Unfortunately, in the two sets of experiments
different species were used. Nevertheless, it is interesting that these two independent
approaches yield estimates for Dw of the same order of magnitude. It is an
advantage of these approaches that solubility of water in cuticular wax (wax/water
partition coefficients) are not needed for estimating Dw. Unfortunately, assumptions
concerning the thickness of the limiting skin must be made, and these assumptions
enter as the square.
4.6.4 Water Permeability of Paraffin Waxes
Model III A postulates that wax in and on top of the CM completely determine water
permeability, while aqueous pores are not involved. This model is difficult to test,
because there are no data on water permeability of reconstituted waxes. The evidence
presented is indirect, and relies on co-penetration in CM of lipophilic solutes
and water (Figs. 4.16 and 4.17). The observation that cuticular transpiration is correlated
with diffusion of stearic acid in cuticular wax (Fig. 4.15) agrees with model
III A, and the correlation between water and benzoic acid permeances is also consistent
with model III A, even though (4.17) indicates that, in CM of species with
relatively high Pw, water transport in aqueous pores may have been involved. On
the other hand, data of Table 4.8 are consistent with model IIIC, which assumes
that a significant amount of water crosses the CM via aqueous pores, which can be
blocked by silver chloride precipitates. There is no reason why CM of all species
investigated so far should meet model III A, B or C criteria. The question arises
which thickness of a wax barrier could account for observed water permeances.
Since we have no data for reconstituted wax, we shall take a look at data obtained
with paraffin waxes.
4.6.4.1 Water Permeance of Polyethylene and Paraffin Wax
In Table 4.7, data on wax coverage for CM of several species are given. They
range from 11.8µgcm −2 (Citrus aurantium) to 1,317µgcm −2 (Malus). For Allium
(Table 4.6), only 1µgcm −2 has been determined using gas chromatography. Multiplying
the wax amounts by specific weight (0.9gcm −3 ), we obtain wax layers
having a thickness from 11 nm (Allium), 131 nm (Citrus) and 14.6µm (Malus). For
most species in Table 4.7, wax amounts varied from 30 to 100µgcm −2 , resulting in
wax layers of 333 nm to 1.1µm. Can we estimate Pwv for such wax layers?
Waxes are partially crystalline, hydrophobic and they consist mainly of CH2
and CH3 groups. This is also true for polyethylene, which is partially crystalline
but exclusively composed of CH2 and CH3 groups. Pwv of polyethylene is 3.3 ×
10 −11 m 2 s −1 (Table 4.1). A membrane of a comparable physico-chemical structure
is parafilm (polyethylene covered with paraffin) and water permeability Pwv of
127µm-thick parafilm is 3.33 × 10 −7 ms −1 (Santrucek et al. 2004), which results in