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

120 4 Water Permeability
P water x 10 10 (m/s)
7
6
5
4
3
2
1
0
LD old CM old LD young CM young
parafilm
Fig. 4.23 Permeance Pw measured with old and young Prunus laurocerasus leaf disks (LD) and
isolated cuticles (CM), using 3 H-labelled water. (Data from Schreiber et al. 2001)
of several months old leaves. This indicates that cuticular permeability increases
slightly when leaves age. A similar observation was also reported for ivy (Fig. 4.14e;
Hauke and Schreiber 1998).
4.9 The Shape of the Water Barrier in Plant Cuticles
The data presented in the preceding Sects. 4.6.1–4.6.4 have to be considered when
trying to establish a model for the structure of the cuticular transpiration barrier.
While there is no doubt that cuticular waxes significantly reduce cuticular transpiration
(Sect. 4.6.1), the question still remains how this is accomplished. In Sect. 4.6.2
three possible models have been suggested, and in Sects. 4.6.2–4.6.4 it was shown
that not all experimental data can be fitted to only one model.
In model III A, all water must diffuse across a cuticular wax layer which is
the transport-limiting barrier. No alternative pathway exists. Data analysing copermeation
and correlation of water permeability with diffusion of stearic acid in
cuticular waxes are consistent with this model. In addition, water transport across
paraffin wax and fatty acid monolayers fits this model.
Experimental results analysing the effect of humidity on cuticular transpiration
and the effect of salt precipitates in cuticles favour model III B, which includes aqueous
pores and provides evidence that some water also diffuses across polar aqueous
pores in parallel to the lipophilic waxy pathway. Model III C is similar to model
III B, but it takes into account the fact that in CM only a fraction of polar pores in
the polymer matrix contributes to water transport because a layer of cuticular wax
covers and closes aqueous pores to some extent.