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

5.3 Cuticular Penetration of Electrolytes 135
three (Fig. 5.7a). POD of calcium chloride and calcium nitrate are 32% and 50%,
respectively (Appendix B). Rate constants with the organic calcium salts were lower,
and with acetate and lactate very little penetrated at 70% or 80% humidity. POD of
these salts are much higher, and amounted to 95% (propionate), 95% (lactate) and
100% (acetate) respectively. At 80% humidity and below, all organic salt residues
on the CM had a whitish appearance, indicating that they had crystallised. All these
three salts should not have penetrated at 90% humidity and below, but significant
rates were in fact measured. This is due to stagnant air layers directly over the
CM. Since water constantly penetrates the CM from the receiver, this leads to thin
water films between salt and CM. The POD of potassium nitrate (KNO3) is 94%
(Appendix B). As a consequence, KNO3 penetration across the cuticle ceases at
humidities below the 94% because the salt crystallises. However, potassium carbonate
[K2(CO3)2] having a POD of 45% penetrated at all humidities investigated
(Fig. 5.7b).
Humidity has a dual function in cuticular penetration of salts. Humidity must be
higher than POD for the salt to deliquesce. However, even above the POD penetration
rates of the salts increased with increasing humidity (Figs. 5.5 and 5.7). The
driving force for cuticular penetration is the donor concentration (Chap. 2). Inspection
of residues shows that salt solutions are present on CM if humidity is above
POD, and the salt is crystalline when humidity is below POD. However, the salt concentration
is not known in these experiments, and for this reason −ln (1 − Mt/M0)
was plotted. Linearity shows that the salt fraction left on the CM decreased exponentially
with time, and we accept this as evidence that the salt concentration also
decreased exponentially with time. At high humidity the salt concentration is probably
lower than at low humidity, and this implies that driving force is lower at high
humidity. Nevertheless, rate constants increase with humidity, indicating permeability
of CM did not depend on concentration. Concentrations decreased exponentially
with time, and this did not depend on the magnitude of the initial amount or concentration
(M0 or C0). Schönherr (2000) studied the effect of initial CaCl2 in the donor
droplets. At concentrations of 2, 4, 6 and 10gl −1 and 90% humidity, penetration
plots were linear and rate constants were in fact independent of initial donor concentration
and amounts of CaCl2 deposited initially. At a concentration of 1gl −1 ,
the rate constant decreased when more than 60% of the salt had penetrated. This
was attributed to an inhomogeneous distribution of the salt residue on the cuticle.
The effect of humidity on rate constants indicates a change in permeability of CM.
In Sects. 4.5 and 4.6 it was shown that water permeability and water content of
CM increased with increasing humidity. This leads to an increase of the number of
aqueous pores available for cuticular penetration of salts.