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

6.3 Diffusion with Changing Donor Concentrations: The Transient State 189
Table 6.8 Y-intercepts (−log k ∗
0) and size selectivity (β ′ in mol cm −3 ) measured at 25 ◦ C using
various solutes differing in molar volumes (cm 3 mol −1 )
Species −log k ∗
0 ± CI β ′ ± CI r 2
Populus canescens 2.33 ± 0.62 0.011 ± 0.002 0.98
Pyrus communis cv. Conference 3.66 ± 0.39 0.009 ± 0.002 0.93
Capsicum annuum 3.95 ± 0.38 0.009 ± 0.002 0.89
Stephanotis floribunda 4.11 ± 0.44 0.007 ± 0.001 0.81
Malus domesta cv. Golden Delicious 4.11 ± 0.18 0.010 ± 0.002 0.99
Pyrus communis cv. Bartlett a 4.25 ± 0.36 0.009 ± 0.003 0.96
Pyrus communis MX a 1.99 ± 0.57 0.009 ± 0.003 0.91
Citrus aurantium a 4.28 ± 0.53 0.012 ± 0.002 0.86
Strophantus gratus 4.94 ± 0.36 0.009 ± 0.002 0.93
Hedera helix 5.22 ± 0.41 0.009 ± 0.004 0.91
Ilex paraguariensis 5.27 ± 0.80 0.010 ± 0.003 0.95
Average 0.0095
a Data from Baur et al. (1996b); all others were taken from Buchholz et al. (1998). CI is the 95%
confidence interval
respectively. Doubling molar volume from 100 to 200cm 3 mol −1 reduces solute
mobility by a factor of 8.9, and increasing Vx threefold reduces mobility by a factor
of 79.3. These factors are the same no matter what the numerical value of k ∗ 0 is,
because β ′ is the same for all species.
Using pear leaf cuticles, Baur et al. (1996b) studied the effect of extraction of
waxes on rate constants and size selectivity. This is the only UDOS study of size
selectivity in MX membranes. Extraction increased the y-intercept of (6.21), but
size selectivity was unaffected, and was the same as with CM. It is astounding that
extraction of waxes had no effect on size selectivity, while rate constants and the
y-intercept k ∗ 0 increased by a factor of 182 (Table 6.8).
Solute mobility in plant cuticles at 25 ◦C is completely determined by k ∗ 0 and
β ′ , and this poses the question concerning their physical meaning. Size selectivity
is related to the free volume available to diffusion (Potts and Guy 1992), which is
the reciprocal value of 2.3 × β ′ . In CM and MX the free volume of diffusion is
45.77cm3 mol −1 . Free volume of diffusion is proportional to viscosity, and since it
is the same in CM and MX it appears that viscosity of amorphous waxes and cutin in
the limiting skin of MX-membranes are the same. If waxes embedded in the limiting
skin do not reduce viscosity, which mechanism is then responsible for the effect of
waxes on solute mobilities (k ∗ 0 and k ∗ ) in plant cuticles?
The only examples available are the pear leaf CM and MX. Extracting waxes
increased k ∗ 0 by a factor of 182, and had no effect on size selectivity (Table 6.8).
Baur et al. (1996b) suggested that waxes increase the diffusion path in the CM. The
upper pear-leaf cuticle contains about 30 weight percent of waxes, and a substantial
portion of this occurs as epicuticular wax plates. Baur (1998) compared UDOS rate
constants for bifenox in the CM of Ilex paraguariensis and Pyrus pyrifolia prior
to and after stripping surface wax with cellulose acetate. Stripping did not increase
rate constants, while extracting total waxes with chloroform greatly increased them.