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

4.6 Water Permeability of Isolated Astomatous Cuticular Membranes 107
CM of most species had permeances which increased with humidity by factors
of 2–3, and Hedera helix belongs to these species This humidity effect has been
attributed to the presence of aqueous pores, which shrink when humidity is lowered
and approaches 0% (Schönherr and Schmidt 1979; Schönherr and Mérida 1981;
Schreiber et al. 2001). Results shown in Table 4.8 were taken as indicative of pore
closure by AgCl crystals, but Hedera helix did not respond. This inconsistency is
difficult to explain. Possibly this lot of CM was not porous, and it would have been
desirable to test humidity effects prior to treatment with AgNO3 and not simply
to determine Pw with zero humidity in the receiver. The same applies to the CM
after treatment. Unfortunately, P AgCl
w was not measured at different humidities. It
was assumed that pores were completely blocked. Thus, the treatment with AgNO3 should have completely eliminated the humidity effect seen in Fig. 4.6 and discussed
in Sect. 4.6.2, subsection: “Effect of Partial Vapour Pressure (Humidity)
on Permeability of CM”. Since this test has not been made, the above considerations
are speculative, although they are plausible. There is another problem with
the interpretation of AgCl effects on permeance. Data in Figs. 4.15–4.17 had been
interpreted as evidence that lipophilic solutes and water diffuse in the waxy pathway,
and no evidence for significant involvement of aqueous pores was detected. Some
of the species were also used in the study with AgCl blockage, and a significant
contribution of aqueous pores to total permeances was calculated.
4.6.3 Diffusion Coefficients of Water in CM and Cuticular Wax
Diffusion and permeation of water across isolated MX membranes, including the
transport of water across polar pores, is discussed in Sects. 4.1–4.5. In the previous
sections we have discussed the nature of the waxy barrier, based on experimental
data obtained with MX and CM. We have also pointed out the lack of data on
wax/water partition coefficients and diffusion coefficients in cuticular waxes. There
is only one study which tried to estimate Dw in CM (Becker et al. 1986) and this
will be considered next.
4.6.3.1 Measurement of Dw for Water in CM from Hold-up Times
Using an amperometric method, Becker et al. (1986) measured Pwv of and D in
CM isolated from a number of plant species. Nitrogen gas saturated with water
vapour served as donor on the morphological inner surface of the CM. The receiver
was dry N2. The calculation of Pwv from the flux and the vapour concentration is
straightforward and requires no assumptions. However, in calculating D using the
hold-up time (te) it must be decided which thickness ℓ should be employed (2.5).
Becker et al. (1986) used the total thickness calculated from the mass per area of the
CM and a specific weight of 1.1gcm 3 . Data are shown in Table 4.9.