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

176 6 Diffusion of Non-Electrolytes
6.3 Diffusion with Changing Donor Concentrations:
The Transient State
Permeance (P) is a composite property (2.18). It is proportional to solubility (K)
and mobility (D) of water and solutes in membrane, and inversely proportional
to thickness of the membrane (ℓ). In homogeneous membranes, solute permeability
is characterised using D and K because they are easily measured. Cuticles are
highly asymmetrical (Sect. 1.4), and both water concentration and mobility vary
with position (Chap. 4). Water transport occurs in aqueous pores, cutin and waxes,
and their mutual arrangements determine water permeability. The question arises
whether this also applies to solutes. In Sect. 6.2 we used permeances to characterise
solute permeability, which can be measured easily in the steady state using various
methods. Unfortunately, since P is a mixed quantity it is not possible to explain why
permeances differ among cuticles from different species. In an attempt to obtain a
better understanding of solute permeability in cuticles, we have developed methods
to estimate solute mobility in CM, MX and in waxes.
6.3.1 Simultaneous Bilateral Desorption
Diffusion coefficients can be estimated from sorption and desorption experiments
(Sect. 2.6). We have used this approach to measure mobility of lipophilic 2,4-D
in CM and MX membranes. Astomatous CM and MX membranes obtained from
leaves and fruits were submerged in aqueous buffer (pH 3) containing 14 C-labelled
2,4-D. After equilibration they were removed from the buffer, blotted dry, flattened
on a piece of Teflon and air-dried. Dry membranes were inserted between the two
half-cells of a transport apparatus made of stainless steel (Fig. 9.4). This made it possible
to desorb 2,4-D separately from the outer and inner surfaces of the membranes.
Borax buffer (pH 9.18), in which 2,4-D is fully ionised, was used as desorption
medium. By changing desorption media quickly and repeatedly, the amounts of
2,4-D desorbed at various times (Mt) were studied.
Desorption was very rapid, and 90% of the 2,4-D initially contained in the CM
and MX membranes were desorbed in 4 h (Fig. 6.15a). With a homogeneous membrane,
50% of solute should be desorbed from either side and the two desorption
plots should be symmetrical. This was clearly not the case, neither with CM and
MX. Only very small amounts (5%) were desorbed from the morphological outer
surfaces of the Citrus CM, while 17% could be desorbed from the outer surface
of the MX membranes. The largest fraction of 2,4-D was desorbed from the inner
surface, as in 240 min 86% (CM) and 71% (MX) left the membranes through the
morphological inner surfaces. Clearly, these membranes are highly asymmetrical,
and this asymmetry can be characterised by comparing the initial rates of desorption
from the inner and outer surfaces respectively. During the first 3 min, 163 times
more 2,4-D was desorbed from the inner surface of the CM than from the outer. With