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

226 7 Accelerators Increase Solute Permeability of Cuticles
be calculated from (7.9). A reasonable positive correlation between effects and concentrations
of the respective alcohol ethoxylates sorbed in Citrus MX is obtained
(Fig. 7.15a). Unfortunately, partition coefficients Kww have not been determined for
Citrus wax. Citrus wax is largely amorphous, and we expect Kww to be smaller
than Kmxw. Using (7.4), concentrations of alcohol ethoxylates sorbed in barley wax
instead of Citrus wax can be calculated. A plot of the effects on mobilities in Citrus
as a function of the plasticiser concentration in barley wax gives a reasonable correlation
as well (Fig. 7.15b). This confirms that intrinsic effects of these plasticisers
are similar, and that effects on solute penetration across cuticles are only a function
of the concentration of accelerators sorbed in the transport-limiting barrier of wax.
When comparing the effects of alcohol ethoxylates with those of n-alkyl esters
on diffusion in wax (Sect. 7.3.3), it was shown that n-alkyl esters were five times
more efficient in plasticising waxes at the same internal concentrations in the wax
(Fig. 7.10). A similar tendency can be observed with CM (Schönherr et al. 2001;
Shi et al. 2005b). This is not too surprising, since above we have presented evidence
showing that transport properties of cuticles and effects of plasticisers can be
attributed to sorption and diffusion properties of wax.
Comparing intrinsic effects of alcohol ethoxylates with those of the n-alkyl
esters tributyl phosphate (TBP) and diethyl suberate (DESU) on 2,4-DB (2,4dichlorophenoxybutyric
acid) mobility in Stephanotis cuticles shows that highest
effects were obtained with DESU, intermediate effects with TBP, and lowest but
still significant effects with alcohol ethoxylates (Table 7.4). At 15 ◦ C the n-alkyl ester
DESU is about 4.4 times more effective than alcohol ethoxylates in plasticising the
waxy transport barrier. This effect gradually decreases with increasing temperature
from 15 to 25 ◦ C. At 30 ◦ C the difference disappears completely (Table 7.4).
Activation energies for mobility of 2,4-DB in Stephanotis CM are 105kJ mol −1
in the absence of a plasticiser. They significantly decrease to 26 and 36kJ mol −1 in
the presence of DESU and of the alcohol ethoxylates respectively (Schönherr et al.
2001). TBP shows the same tendency of decreasing effects with increasing temperature
(Table 7.4), and the activation energy decreases to 64kJ mol −1 (Schönherr et al.
2001). Effects of plasticisers are highest at low temperatures. Increasing temperature
or adding plasticisers have similar effects on fluidity of the waxy transport-limiting
barrier of cuticles. ESR experiments (Sect. 7.2) support this conclusion.
Table 7.4 Effect of alcohol ethoxylates (CxEy), tributyl phosphate (TBP) and diethyl suberate
(DESU) on 2,4-DB (2,4-dichlorophenoxybutyric acid) mobility k ∗ in Stephanotis CM at 4
different temperatures
Plasticiser Concentration in CM Effect on k ∗
(g kg −1 ) 15 ◦ C 20 ◦ C 25 ◦ C 30 ◦ C
CxEy 85 11 9 9 9
TBP 90 35 17 17 14
DESU 90 48 25 15 9
Data from Schönherr et al. (2001)