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

7.4 Effects of Plasticisers on Transport in Cuticles 225
7.4.2 Effects of Plasticisers and Temperature on Solute
Mobility in CM
The UDOS method is a valuable experimental tool for studying solute mobility k ∗
in CM (Sect. 6.3). These k ∗ values are directly proportional to diffusion coefficients
D measured with reconstituted wax (see Sect. 6.5). In steady state experiments permeance
P is obtained, which is a composite quantity (2.18) and often difficult to
interpret (see Sect. 6.2). UDOS can be used to study the effect of plasticisers on
mobilities of lipophilic solutes in isolated cuticles (Schönherr 1993a, b; Schönherr
et al. 2001). The 14 C-labelled solute is sorbed in the inner sorption compartment of
the CM. Subsequent desorption of the solute is carried out from the outer side of
the cuticle across the transport-limiting barrier, and this arrangement assures that
plasticisers and 14 C-labelled solutes are spatially separated (Fig. 9.5).
Using isolated Citrus CM, 14 C-labelled 2,4-D and monodisperse alcohol ethoxylates
of the series CxEy (Schönherr 1993a, b), it was shown that the alcohol
ethoxylates increased mobility in Citrus CM. Effects on mobilities k ∗ between 6
and 20 were observed (Fig. 7.15). Amounts of alcohol ethoxylates sorbed in Citrus
MX in equilibrium with the external solutions at concentrations above the cmc can
Effect on mobility k ∗ of 2,4-D in Citrus CM
22
20
18
16
14
12
10
8
6
a b
C 16E 8
C 10E 5
C 14E 7
C 12E 8
C 12E 6
C 8E 4
C 6E 3
50 60 70 80 90 4 5 6 7 8 9 10 11
Concentration (g/kg) of alcohol
ethoxylates in Citrus MX
C 12E 6
C 16E 8 C 14E 7
C 12E 8
C 10 E 5
C 6E 3
C 8E 4
Concentration (g/kg) of alcohol
ethoxylates in barley wax
Fig. 7.15 Correlation between the effects of monodisperse alcohol ethoxylates on mobility k ∗ of
2,4-D in Citrus CM with concentrations of alcohol ethoxylates in (a) Citrus MX (blue symbols)
and in (b) reconstituted barley wax (red symbols). Effects on mobility are taken from Schönherr
(1993a); data for alcohol ethoxylate sorption in Citrus MX were calculated using (7.9), and for
sorption in barley using (7.4)