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

6.2 Steady State Penetration 165
cuticle and eventually into the leaf tissue. When PCP is desorbed from leaves, the
efflux takes place at first from the epicuticular wax and later from the remainder
of the cuticle. Comparing the two rate constants, it is seen that k1 is 23 times
larger than k2, and for this reason diffusion across epicuticular wax was not ratelimiting.
Diffusion across cutin and embedded waxes was the rate-limiting step in
foliar penetration of PCP.
The y-intercept seen in Fig. 6.5 has practical implications. Permeance can only
be calculated if penetration is measured for more than one time interval. From the
slope, rate of penetration and permeance can be calculated. If penetration is measured
using only one time interval, sorption in wax and cuticle is overlooked and
permeance estimated is too high. For instance, after 90 min penetration amounted to
24 × 10 −12 and the y-intercept was 11.3 × 10 −12 mol cm −2 (Fig. 6.5). Subtracting
the y-intercept, the flux is 1.3 × 10 −13 mol cm −2 min −1 , while flux calculated from
the amount penetrated would have been 2.7 × 10 −13 mol cm −2 min −1 . Permeance
calculated without correcting for the y-intercept would be 2.1 times too high. Furthermore,
a desorption experiment provides more information, as rate constants and
compartment sizes can be estimated.
Half-time of desorption for the first compartment, that is, for desorption from
epicuticular waxes is only 82 s. Frequently, adhering donor is rinsed off using
water, buffer, or aqueous acetone. In the experiment shown in Fig. 6.6, this would
have reduced the magnitude of the y-intercept but it would not have eliminated it.
Extended rinsing time will remove some more PCP from the cuticle, but complete
elimination is unlikely because the half-time for the second compartment (the cuticle)
is 31.2 min. If leaves are washed or rinsed after the penetration experiment, a
variable and unknown fraction of solute sorbed in the cuticle is considered to have
penetrated, even though it had not yet reached the leaf tissue. The error is larger
with short experiments. A controlled desorption experiment after blotting leaves
to remove adhering donor solution is clearly the better approach, since it provides
information about number of compartments, compartment sizes and rate constants
by which they drain.
Using the approach outlined above, penetration of other lipophilic solutes into
barley leaves was studied (Fig. 6.7). Permeance increased with partition coefficients.
The approach used with barley leaves was successfully applied to study penetration
of PCP and other lipophilic chemicals into conifer needles (Schreiber and
Schönherr 1992b). With conifer needles amounts penetrated vs time were also
biphasic, but magnitudes of y-intercepts differed greatly among species (Fig. 6.8).
Rates of penetration (slopes) have the dimension amount PCP penetrated per min
and mm needle length. The projected areas of needles were 5mm 2 mm −1 with the
Abies species and 3mm 2 mm −1 with all others. Dividing the slopes of the plots
by projected needle area (Apro) and donor concentration yields the permeance in
m s −1 if SI units were used in calculations (Table 6.4). P differed greatly among the
species.
Desorption analysis as shown for barley leaves in Fig. 6.6 resulted again in two
compartments having different rate constants. Rate constants of desorption of PCP
from the first compartment (k1) ranged from 8 to 11 × 10 −3 s −1 , that is, PCP was