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

192 6 Diffusion of Non-Electrolytes
exponentially with time. Rate constants were higher after droplet drying, but the
effect of drying differed among compounds. The effect was spectacular with urea,
where the two slopes differ by a factor of 93. Urea is a highly water soluble compound
(1 kg urea dissolves in 1 kg water), and above 90% humidity it deliquesces.
On droplet drying, it was most likely present as concentrated aqueous solution, and
an increase in urea concentration by a factor of 93 implies that the volume of the
donor decreased by the same factor, because according to (2.19) Cdonor = M/Vdonor.
If permeance is constant and independent on urea concentration, rate constants
should be proportional to Adroplet/Vdroplet (2.26).
Tebuconazole and 2,4-D are both lipophilic (Appendix B), and their initial rate
constants were relatively high. These two compounds dissolve in the surface wax,
and the situation resembles that discussed in Sect. 6.2 when we dealt with the submersion
technique. Surface waxes served as intermediate donor phase. With these
lipophilic solutes, equilibration between water and surface wax must have been fast,
and during the first 48 h the surface wax was the donor phase, and 60% (tebuconazole)
to 70% (2,4-D) of the amount in the donor droplets penetrated into the receiver
in 48 h. Penetration of 2,4-D was a little faster than penetration of tebuconazole,
which has a greater molar volume (308cm 3 mol −1 ) than 2,4-D (241cm 3 mol −1 ).
Why rate constants of these two compounds increased on droplet drying is a matter
of conjecture. With 2,4-D, pH and degree of ionisation probably decreased during
droplet drying, while with tebuconazole the factor was only 2.2 and here an increase
in droplet area during drying could have been responsible. Change in droplet area
[cf. (6.14)] is of course also a factor to be considered with urea and 2,4-D.
These speculations show that simulation of foliar penetration produces very
complex data, and it is difficult to explain differences among chemicals and plant
species. This should not distract from the fact that SOFP can provide us with quantitative
data about rates of penetration as affected by experimental conditions, which
can be close to the situation in the field. The method has been used extensively to
study the effects of humidity, adjuvants, plasticisers, wetting agents and temperature
on rates of penetration of ionic species (Chap. 5). No other method discussed
in Chap. 6 is as versatile as SOFP.
6.5 Diffusion in Reconstituted Isolated Cuticular Waxes
Simultaneous bilateral desorption (Sect. 6.3.1) of lipophilic molecules from CM
(Fig. 6.15) is excellent evidence for the pronounced asymmetry of cuticles. Furthermore,
it shows that the transport-limiting barrier must be located at the outer
side of the cuticle. The effect of wax extraction, resulting in an increase of permeability
to water (Table 4.7) and lipophilic molecules (Table 6.4) by factors of
10–10,000, convincingly shows that the transport-limiting barrier of the CM for
lipophilic molecules is due to cuticular waxes. Sorption and diffusion of molecules
in the waxy barrier of the CM was investigated to better understand the nature of
this waxy barrier. Reconstituted cuticular waxes were used in these studies. Isolated