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

6.2 Steady State Penetration 171
a relatively high surface tension. With leaves of Zebrina it was shown that infiltration
of stomata will not occur if surface tension is 35mN m −1 or higher (Schönherr
and Bukovac 1972a). This even permits using low concentrations of surfactants to
improve wetting of leaf surfaces. There is no need to worry that infiltration of stomata
occurs but is not noticed. Infiltration can be detected with the bare eye, because
dark spots will be seen in incident light which look bright in transmitted light. This
phenomenon is due to a local change in refractive index when intercellular air spaces
are filled with water.
Diffusion of solutes into the wound caused by cutting off the leaf at the petiole
is no problem, because it can be quantified and corrected for easily (Fig. 6.4). The
donor solution must be agitated to ensure mixing. If donor solutions are at ambient
pressure (vessels open), there is no need to worry about pressure forcing liquid into
open stomata. When working with barley leaves and conifer needles this was not a
problem, even though test tubes were tightly closed.
So far the methods have been used only with lipophilic solutes. There is no reason
why it should not work with polar non-electrolytes or with ions. With polar solutes
the sizes of CPT1 and CPT2 are probably very small, and the y-intercept is close to
zero. Permeance can be calculated from the slope of the penetration or the desorption
graphs which are superimposed if sorption in wax and cuticles is insignificant
(cf. Fig. 6.13). With leaves that are easily wetted, a surface film of donor can be
estimated by desorption, and all donor solution will be washed off with the first
change of desorption medium. This offers the possibility to measure permeability
of delicate leaves such as Arabidopsis.
The method works well as long the leaf surface is not densely populated by
microorganisms, as was observed when working with older conifer needles sampled
from forest trees (Schreiber and Schönherr 1992c). Plants grown in growth chambers
or greenhouses usually have clean surfaces. In any case, it is good practice to
check for surface contaminations.
6.2.3 Steady State Penetration into Leaf Disks Using the Well
Technique
Broadleaved plants can have very large leaves which are not suitable for the submersion
technique. In these cases, penetration can be measured using a droplet
method. Small droplets of donor solutions are placed on the upper or lower surfaces
of leaves attached to or dissected from plants. This approach is much closer
to the situation after spray application to the foliage, and this is often considered
an advantage. However, experiments of this kind are beset with severe problems,
as penetration proceeds under uncontrolled conditions. For instance, concentrations
and pH of donor solutions change during droplet drying, temperature of the donor
differs from leaf and surrounding air, contact area between donor and leaf is difficult
to estimate precisely and may vary with time, and the solutes may solidify or
crystallise. These problems are almost as bad in growth chambers than in the field.