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

180 6 Diffusion of Non-Electrolytes
6.3.2 Unilateral Desorption from the Outer Surface
Solute mobility in the limiting skin can be estimated from unilateral desorption
from the outer surface. This method eliminates solute loss through the inner surface
because no desorption medium is in contact with it. Some changes in apparatus were
made to simplify handling and to adopt it to neutral solutes. The cuticles are inserted
in an apparatus shown in Fig. 6.17. The morphological inner surface is exposed in
the orifice of the lid. 14 C-labelled test compounds are dissolved in water or buffer
and applied as 200µL droplet to the centre of the inner surface of the CM. During
evaporation of water the lipophilic solutes quantitatively penetrate into the sorption
compartment (soco). As seen in Fig. 6.15, desorption is a very rapid process as half
of the 2,4-D was desorbed in about a minute. Sorption is the reverse process but it
proceeds with the same velocity. As soon as the water had evaporated, the opening
of the lid was closed with transparent sticky tape (Tesafilm). This ensures 100%
humidity in the air over the inner surface of the CM, and it prevents a radioactive
spill in case a membrane breaks.
Desorption from the outer surface is initiated by pipetting the desorption medium
into the chamber. The chambers are placed with the cuticle facing down into wells
of a thermostated aluminium block which is rocked slightly to ensure mixing. As a
desorption medium, a buffer in which the solutes are ionised can be used with weak
acids or bases. With neutral solutes, a phospholipid suspension (10g l −1 ) is suitable.
It is prepared by sonicating soybean lecithin in hot water (60 ◦ C). Small vesicles are
Fig. 6.17 Apparatus used for UDOS (unilateral desorption from the outer surface) experiments