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

66 4 Water Permeability
of an ionic species is defined as the number of equivalents of ions transferred by 1
faraday of electricity.
The first term on the right hand side of the equation gives the thermodynamic
limiting value of the concentration potential, and the second term is the deviation
due to co-ion flux. With an ideal permselective membrane (¯t of the co-ion is zero)
the second term vanishes, and (4.1) reduces to the so-called Nernst equation. For an
ideal cation permselectivity, we have
Emembrane = − RT
F z+
and for an ideal anion permselective membrane
Emembrane = − RT
F(−z−)
ln aoutside +
ainside +
aoutside −
ln
ainside −
(4.2)
(4.3)
is obtained. For a monovalent ion (z = 1) and 25 ◦ C, the term RT/F z amounts
to 25.7 mV. Neglecting activity coefficients, the membrane potential of an ideal
permselective membrane is 17.81 mV when the concentration ratio is 2.
Between pH 2 and 9 and at 25 ◦ C, membrane potentials measured with identical
buffers and a KCl concentration of 4 × 10 −3 moll −1 on both sides were smaller
than 0.5 mV with all cuticles (apricot, pear and Citrus CM and Citrus MX) tested.
When salt concentrations are the same on both sides of the membranes, there is no
difference in electrochemical gradient and no driving force. Hence the transference
number for anions and the diffusion potential is zero, and any electrical potential
measured would be the difference of the Donnan potentials (4.1) on the two surfaces
of the membrane. This difference is called asymmetry potential. As it was close to
zero at all pH values, it appears that the concentration of fixed charges was the same
with all cuticles tested. This is a surprising result in view of the gradient of polarity
seen in most TEM pictures (Sect. 1.4).
Membrane potentials across CM and MX membranes were measured with
buffered KCl solutions of 4 × 10 −3 moll −1 and 2 × 10 −3 moll −1 in contact with the
inner and outer surfaces of the membranes respectively. Membrane potentials were
strongly pH dependent (Fig. 4.1). At pH 9, potentials between −14 and −16mV
were measured. Potentials decreased with pH, at first slowly but below pH 6 more
rapidly, and they assumed positive values at pH 2 ranging from 2 to 13 mV. pH values
resulting in zero mV were 2.9 (pear), 3.2 (Citrus) and 3.4 (apricot). These pH
values mark the isoelectric point of the cuticles. If the pH is higher the membranes
carry a net negative charge, at pH values below the isoelectric point membranes
have a net positive charge. At the isoelectric point, the membranes have no net
charge. This does not mean that they have no fixed charges, rather that the number
of positive and negative fixed charges is equal.
Above the isoelectric point, cuticles carry net negative charges which are neutralised
by cations, in the present case by K + . Below the isoelectric point they are
positively charged, and charges are neutralised by anions (Cl − ). At pH 9 membrane