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

104 4 Water Permeability
4.6.2.4 Effect of Partial Vapour Pressure (Humidity) on Permeability of CM
All polymers sorb water when exposed to water vapour or liquid water. Sorption
is greater in polar polymers than in non-polar polymers (Table 4.1). When dealing
with effects of water vapour pressure on permeability of membranes, effects on
fluxes and permeance must be distinguished. With lipophilic polymers (polyethylene,
silicon rubber) sorption isotherms are linear, and permeance is the same at all
partial pressures, while permeance of polar polymers increases with partial pressure
(Figs. 4.5 and 4.6). When measuring the effect of humidity on permeance of CM,
the donor faced the inner side of the CM while partial pressure was varied in the
receiver facing the outer surface of the CM. With increasing partial pressure of the
receiver, the driving force (∆aw or ∆Cwv) decreases, and with 100% humidity in
the receiver, the driving force is zero and no net flux of water can be observed. For
this reason, the effect of humidity on Pw was studied using tritiated water (THO)
as donor, and the flux of the tritiated water was measured. The driving force was
the concentration of tritiated water in the donor, because THO concentration in the
receiver was practically zero. In this way, water fluxes at very high humidity in the
donor can be measured with high accuracy.
Extracting cuticular waxes increases permeance by orders of magnitude
(Table 4.6), and the question arises whether waxes modify the effect of p/p0 on
permeance of MX as seen in Fig. 4.6. In several investigations, it was shown that
an effect of partial water vapour pressure (p/p0) on permeance can also be seen
in CM (Schönherr and Schmidt 1979; Schönherr and Mérida 1981; Schreiber et al.
2001). Water permeability of CM increased by a factor of 2.3 for Vinca major, 3.2
for Citrus aurantium with humidity increasing from 2% to 100%, and factors of
2.4, 2.8 and 2.9 were found for Hedera helix, Prunus laurocerasus and Forsythia
intermedia respectively (Schreiber et al. 2001). There was a weak increase in permeability
of up to 70% humidity, and a much more pronounced increase between
70% and 100% humidity (Fig. 4.18). Similar effects of humidity on permeability
were obtained with MX (Fig. 4.18b).
The effect of humidity on permeance demonstrates the involvement of polar
functional groups in cuticular wax and/or the MX (Sects. 4.1 and 4.5). In waxes,
polar groups are mainly contributed by fatty acids and alcohols, but compared to
MX cuticular waxes have very few polar groups (Riederer and Schneider 1990).
Methylation of carboxylic groups significantly decreased the effect of humidity
on permeability of MX by 50%. This demonstrates the involvement of carboxylic
groups of the MX. Since the effect of humidity did not totally disappear after methylation,
other functional polar groups (i.e., –OH) linked to proteins, phenolics and
carbohydrates which cannot be methylated apparently contributed to the effect of
p/p0 on Pw. There was no effect on Pw after methylation of Prunus laurocerasus
CM. This can be interpreted as evidence that penetration of water was limited by
cuticular waxes lacking carboxyl groups, or in which carboxyl groups did not contribute
to water permeability. Perhaps carboxyl groups in waxes were not accessible
to diazomethane. As there was a significant effect of partial pressure on Pw of CM,
hydroxyl groups which formed water clusters and polar pores may have caused it.