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

6.2 Steady State Penetration 153
on the surfaces of the cuticles containing dissolved sugars are most likely the cause
for this difference.
The above two model calculations leave little doubt that partition coefficients for
highly water soluble compounds and very thin cuticles cannot be determined with
sufficient accuracy, and they should be looked at with suspicion.
6.2 Steady State Penetration
Non-electrolytes are neutral solutes which carry no electrical charges. Weak acids
and bases can be treated as non-electrolytes if the pH is adjusted such that dissociation
is suppressed. For instance, with weak acids 99% of the molecules are
undissociated when the pH is 2 units below the pKa. In this section, we restrict our
attention to solutes which are sufficiently water-soluble and not too volatile, such
that working with open donor and receptor compartments is possible.
Solute permeability of cuticles can be characterised by permeance. Combining
(2.3) and (2.18), we obtain
J = P(Cdonor −Creceiver) = KD
ℓ (Cdonor −Creceiver). (6.6)
Permeance (P) is calculated by dividing the steady state flux (J) of a solute by the
driving force, which is the difference of the solute concentration between donor
and receiver. In the steady state the concentration in the receiver can be maintained
negligibly small, and driving force is simply the donor concentration. P can be
determined using isolated CM, leaf disks or detached leaves. We shall demonstrate
application of (6.6) using examples taken from the literature.
6.2.1 Permeance of Isolated Cuticular Membranes
Riederer and Schönherr (1985) measured 2,4-D permeability of CM isolated enzymatically
from astomatous leaf surfaces and fruits, and these data are suitable to
demonstrate variability of P among plant species. 2,4-D is a weak acid (pKa is
2.77) and it was 14 C-labelled. Donor and receiver solutions were buffered at pH
2, and donor concentrations ranged from 0.2 to 1 × 10 −3 mol l −1 . At a pH of 2.0,
85.5% of the molecules are non-ionised (Sect. 6.1). Plotting the amount diffused
into the receiver vs time, linear plots are obtained and their slopes represent the flow
rate (F in mol h −1 ). Dividing F by membrane area (1cm 2 ) and concentration of
non-dissociated 2,4-D in the donor gives the permeance (P). Non-dissociated 2,4-
D is lipophilic, as the partition coefficients CM/water(Kcwr) range from 240 to 579,
depending on species (Riederer and Schönherr 1984). It follows that 2,4-D is sorbed
in cuticular lipids, while 2,4-D concentration in the polar cuticular polymer phase is
negligible.