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

100 4 Water Permeability
P w (m/s)
2.5e-9
2.0e-9
1.5e-9
1.0e-9
5.0e-10
P w = 6.63 x 10 7 (1/m) D SA - 4.49 x 10 −11 (m/s)(r 2 = 0.90)
0.0 0.0 5.0e-18 1.0e-17 1.5e-17 2.0e-17 2.5e-17 3.0e-17 3.5e-17
D SA (m 2 /s)
Fig. 4.15 Water permeances Pw of isolated CM plotted as a function of DSA (diffusion coefficient
of stearic acid) in reconstituted cuticular waxes of 24 plant species. (Data from Table 4.7)
the amorphous phase (Riederer and Schreiber 1995), whereas the crystalline phase
is not accessible. With an increasing amorphous fraction in the wax, sorption is
expected to increase. Since diffusion of water and stearic acid takes place in the
same amorphous wax phase, path length of diffusion might increase as well, and
thus the ratio of Kww/ℓτ could be very similar for all investigated species. This
hypothesis could help to explain why Pw is correlated with DSA.
Diffusion coefficients for lipophilic solutes in CM and wax (Sect. 6.3.2.3: “Variability
of Solute Mobility with Size of Solutes” and Sect. 6.5.2) differ among species
because tortuosity of the diffusion path differs. This implies that differences in DSA
(Table 4.7) are caused by differences in length of the diffusion paths (ℓτ). Since
water in CM of all species diffuses in the waxy pathway, the path lengths of stearic
acid and water should be the same for both. Hence, partition coefficients (Kww) may
vary little among species, and since Kww/ℓτ is constant (6.6 × 10 7 m −1 ) differences
in Pw should mainly be due to differences in lengths of the diffusion path.
Data on sorption and diffusion of water in cuticular waxes could help us to better
understand the waxy barrier. There are data on water concentration in polyethylene
and liquid hydrocarbons. Water concentration in polyethylene at 25 ◦ C and
100% humidity is about 7 × 10 −4 gg −1 (Table 4.1). According to Schatzberg (1965)
the solubility of water in a liquid alkane (hexamethyl tetracosane) at 25 ◦ C is
5.5 × 10 −5 gg −1 or 55mgkg −1 . Sorption of water in isolated and reconstituted solid
cuticular waxes has not been studied, for good reasons as the following example
will show. Starting with the assumption that the above paraffin/water partition
coefficient (5.5 × 10 −5 gg −1 ) is valid for cuticular waxes, we can estimate the concentration
of tritiated water (THO) needed to detect THO in a wax sample of 1 mg.
For a reasonable counting statistic, we aim at 100Bqmg −1 wax, and we can calculate
the concentration of THO in water from the definition of K (2.12). The ratio