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

88 4 Water Permeability
We have demonstrated the presence of aqueous pores in some cuticles for the first
time. Theories and methods are available. For a better understanding as to how cuticles
function, we need systematic studies using cuticles from many more species.
Occurrence of aqueous pores in MX and CM of plant cuticles is not an academic
problem. Aqueous pores have been speculated about during the last 5 decades, but
the issue was not approached experimentally. Aqueous pores are involved in humidity
effects on transpiration, and they are a prerequisite for penetration of hydrated
ionic species, which are insoluble in cutin and waxes and require aqueous pores to
cross cuticles (Schönherr 2006). We shall return to this topic in Chap. 5.
4.5.1 Lipophilic and Hydrophilic Pathways in the Polymer Matrix
The pore model discussed in the previous section postulated that the polymer matrix
is composed of aqueous pores surrounded by a lipophilic matrix. Diffusion of water,
solutes and viscous flow is limited to aqueous pores, while the lipophilic matrix was
assumed to be impermeable to water and solutes. In this section, we shall evaluate
this assumption and discuss consequences.
According to our model I, most polar functional groups lined the pore walls.
Riederer and Schönherr (1984) estimated cutin composition of CM for various plant
species (Table 1.1). Correcting for the weight fraction of waxes, the fraction of cutin
in the MX of Citrus aurantium was estimated as 0.77, the remainder (0.23) being
the hydrolysable fraction (polysaccharides, peptides, phenolic constituents). Most
of these polar polymers should be located in the pores, while cutin would be filling
the volume between pores.
Citrus cutin is composed of esterified C16 dihydroxyfatty acids, and in a linear
polyester one of the two hydroxyl groups is free. With a molecular weight of
around 300gmol −1 this hydroxyl group amounts to about 5.7% by weight, while
the remainder (94.3%) is made up of ester bonds, CH2 and CH3 groups. The permeances
(Pwv) for synthetic polymers having 3µm thickness (Table 4.1) range from
6.3 × 10 −6 ms −1 (polypropylene) to 6.3 × 10 −3 ms −1 (ethyl cellulose). We have no
data for permeance of ester cutin, but since it has a similar composition to the synthetic
polymers in Table 4.1 it is not very likely that it is completely impermeable
to water, as was assumed in model I (Sect. 4.5). Water transport in cutin polyester
would be by diffusion, and polar solutes are excluded in all probability. If this is
accepted, we can conclude that Pdiffusion across the MX membrane is the sum of two
independent permeances, while Pviscous is not affected. What would be the magnitude
of the contribution of the polymeric pathway to total diffusional flux of water,
and what would be the consequences for estimating pore size?