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

4.5 Diffusion and Viscous Transport of Water 89
4.5.2 Permeability of the Pore and Cutin Pathways
Our new improved model (model II) depicted in Fig. 4.11 shows the limiting skin
of Citrus aurantium polymer matrix. Schönherr (1976a) suggested that the pores
resemble thin tubes. The walls of these tubes were formed by hydroxyl and carboxyl
groups which are surrounded by hydration water. This poses the problem
of the location of the remainder of the polar polymers. In model II, we assume
that polar polymers (peptides, pectins, phenols) constitute a separate continuous
phase, because during synthesis it would be thermodynamically unfavourable to
mix lipophilic cutin with hydrophilic polar polymers. In fact, thin sections of cuticles
show two separate phases. A lipophilic matrix with little contrast is traversed
by a reticulum of heavy contrast (Sect. 1.4). Hydroxyl, carboxyl and amino groups
react with OSO4, MnO4 and heavy metal dyes. These reaction products are visible
in TEM as dark filaments. In this polar polymer phase, aqueous pores arise by
hydration of polar groups and counter ions. These pores are not like tubes and they
are not circular, but the interstitial water phase can still be characterised by equivalent
pore radii. Water can cross the limiting skin by moving either in the pore liquid
or in cutin. Thus, we have two resistances arranged in parallel, the cutin polymer
and aqueous pores within strands of polar cuticular polymers.
Resistance (R) is the reciprocal of permeance (Chap. 2), and according to Ohm’s
law the reciprocal of the total resistance of a group of resistors in parallel is the sum
of the reciprocals of the individual resistances, that is, conductances (permeances)
in parallel are additive
1
Rdiffusion
= 1
+
Rpore
1
= Pdiffusion = Ppore + Pcutin. (4.17)
Rcutin
Fig. 4.11 Schematic drawing of the limiting skin of Citrus MX, composed of the lipophilic cutin
matrix and polar pores arranged in parallel (not to scale)