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

7.3 Effects of Plasticisers on Diffusion of Lipophilic Solutes in Wax 221
log D (m 2 /s)
1e-15
1e-16
1e-17
1e-18
SA
2,4-D
MET
100 120 140 160 180 200 220 240 260
V x (cm 3 /mol)
TRI TB
control
C 8E 4
C 12E 8
Fig. 7.11 Plots of the log D of lipophilic solutes as a function of their molar volumes Vx. D
was determined either using a phospholipid suspension as inert desorption medium (control; blue
symbols) or in the presence of the accelerators octaethylene glycol monododecyl ether (C12E8;
green symbols) and tetraethylene glycol monooctyl ether (C8E 4; red symbols). Salicylic acid (SA),
2,4-dichlorophenoxyacetic acid (2,4-D), metribuzin (MET), triadimenol (TRI), tebuconazole (TB),
bitertanol (BIT). Error bars represent 95% confidence intervals. Data redrawn from Burghardt et al.
(1998)
Table 7.3 Slopes β ′ , y-intercepts D0 and coefficients of determination r 2 of the regression
equations fitted to data of Fig. 7.11
Desorption medium β ′ (mol cm −3 ) log D0(m 2 s −1 ) r 2
PLS −0.0065 −15.94 0.91
C12E8 −0.0023 −15.85 0.76
C8E4 −0.0011 −15.51 0.67
Data from Burghardt et al. (1998)
tortuosity and wax crystallinity were not affected by C12E 8 and C8E 4. This result
is consistent with results of ESR experiments (Fig. 7.4) and reversibility of the
plasticising effect of C12E8 on barley wax (Fig. 7.8) described in Sects 7.2 and 7.3.
Figure 7.11 also indicates that the plasticising effect of the alcohol ethoxylates is
larger with larger solutes having a lower D. This is also seen when the effect of the
plasticisers (D/Dcontrol) is plotted against the reciprocal of Dcontrol (Fig. 7.12). The
largest effects are obtained with bitertanol (which has the lowest Dcontrol) while
the smallest effect is obtained with salicylic acid (which has the highest Dcontrol).
Thus, diffusion of small solutes having a large D is less affected by plasticisers.
All the above experiments were conducted using reconstituted cuticular waxes.
These data provide insights into the molecular architecture of waxes, and show that
plasticisers increase fluidity of amorphous waxes. Can these findings be extrapolated
to cuticles of intact leaves?
BIT