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

118 4 Water Permeability
about 31 carbon atoms (Jenks et al. 1996), and using (4.21) the average thickness
of a monolayer would be 4.0 nm. With a wax amount of 1.6µgcm −2 at most, 4–5
monomolecular layers of wax molecules could be established.
4.7 Permeances of Adaxial and Abaxial Cuticles
The analysis of water permeance of cuticles presented in this chapter relies on
data obtained with isolated astomatous cuticular membranes and with reconstituted
waxes. However, most leaves have stomata at least on one side, and due to this lateral
heterogeneity we have at least three parallel pathways of water: (1) diffusion
across stomatal pores, (2) diffusion across cuticles over guard cells and accessory
cells, and (3) diffusion across cuticles over ordinary pavement cells.
A more sophisticated approach permits the quantification of the fluxes of water
vapour across cuticles and across stomatal pores (Santrucek et al. 2004). It is based
on the fact that water diffusion in the vapour phase depends on density of the vapour,
whereas the water diffusion through the solid phase of the cuticle remains unaffected.
The total flux Ftotal across an isolated cuticle with stomatal pores is the sum
of the two fluxes occurring across the solid cuticle Fcuticle and the stomatal pore
Fstoma
Ftotal = Fcuticle + Fstoma. (4.22)
Transpiration experiments were carried out using the experimental setup depicted in
Fig. 4.22 and 3 H-labelled water vapour in either a pure helium (He) or pure nitrogen
(N) atmosphere at ambient pressure. Radio-labelled water diffuses from the donor
across the membrane into the receiver, where it is trapped and sampled in regular
time intervals. Different gas phases can be obtained by flushing the receiver chamber
with either helium of nitrogen gas.
The two fluxes of water FtotalHe in helium and FtotalN in nitrogen can be measured
using isolated stomatous cuticles
FtotalHe = Fcuticle + FstomaHe, (4.23)
FtotalN = Fcuticle + FstomaN. (4.24)
This experimental approach is based on the fact that the diffusion coefficient of
water in helium DwHe is 3.6 times higher than the diffusion coefficient of water in
nitrogen DwN (Cussler 1984). Thus, using membranes with pores (e.g., stomatous
cuticles) higher fluxes will be measured in a helium atmosphere compared to a nitrogen
atmosphere (4.25). However, no differences occur with a non-porous membrane
(e.g., astomatous cuticle) and fluxes in both gases (helium and nitrogen) will be the
same
FstomaHe
=
FstomaN
DwHe
= 3.6. (4.25)
DwN