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

6.4 Simulation of Foliar Penetration 191
-ln (1-M t /M 0)
2.5 95
2.0
1.5
1.0
0.5
0.026
0.021
8 x 10
0.0
0
0 10 20 30 40 50 60
-4
0.071
Time (h)
tebuconazole
2,4-D
urea
0.17
0.047
Fig. 6.23 Simulation of foliar penetration at 25 ◦ C using astomatous isolated Stephanotis CM.
Small aqueous droplets (2µl) were pipetted on the centre of the outer surface of the CM, and
during the first 48 h evaporation was prevented by 100% humidity over the donor droplets. At 48 h,
humidity was lowered to allow evaporation of solvent water, and desorption from the inner surface
was continued. Means of 15–20 CM are shown. Numbers on regression lines are rate constants in
s −1 . (Redrawn from Schönherr and Baur 1994)
film (Tesafilm). A phospholipid suspension (PLS) served as receiver solution, and
0.5 ml were pipetted into the receiver compartment of the apparatus. The chambers
were placed into the wells of the aluminium plate maintained at 25 ◦ C. The
receiver solutions were withdrawn and replaced by fresh ones at 12 h intervals, and
radioactivity was determined by scintillation counting. After 48 h the Tesafilm was
removed; the water of the droplets evaporated quickly, and desorption was resumed.
At the end of the experiment the CM was cut out with a scalpel, the radioactive
residues were dissolved in scintillation cocktail and counted. Data analysis was the
same as in UDOS. The natural logarithms of (1 − Mt/M0) was plotted against time.
The slopes of the lines are the rate constants of desorption (k), but their physical
meaning differs from UDOS rate constant (k ∗ ) because in SOFP solutes are not
dissolved in cutin. In the experiment depicted in Fig. 6.23, solutes were initially
dissolved in water or in lactic acid buffer. Their state after droplet drying is uncertain.
2,4-D was probably dissolved in concentrated lactic acid, and most of the very
lipophilic tebuconazole was probably dissolved in epicuticular wax. Urea is hygroscopic,
and all or part of it may have formed a concentrated urea solution. Due to
these uncertainties, SOFP rate constants are more difficult to analyse.
Desorption plots were linear both before and after droplet drying. Slopes measured
with aqueous solutions are the rate constants, and they were constant both
prior to and after droplet drying. Hence, solute concentrations in droplets decreased
92
86
78
63
Percentage penetrated