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

Chapter 5
Penetration of Ionic Solutes
In Chap. 4, we characterised the pathways for water in cuticles. Cutin is the major
constituent of the polymer matrix. It is lipophilic and constitutes the lipophilic pathway.
The polymer matrix contains polar polymers which sorb water and swell. This
hydration water is continuous, and gives rise to aqueous pores which traverse the
cuticle (Sect. 4.5). Waxes occur both sorbed in cutin and as epicuticular wax on the
surface of the polymer matrix. Waxes associated with cutin greatly reduce permeability
of the lipophilic pathway, and for this reason we have also used the term
“waxy” pathway. Permeance of the waxy pathway is proportional to the partition
coefficient that is to solubility of solutes in cutin and waxes. Polar solutes have very
low partition coefficients, and for this reason permeance of the waxy pathway is
very low but finite (Sect. 4.6).
With ionic solutes the situation differs, because under physiological conditions
ionic groups are surrounded by water molecules. This hydration water is bound
very strongly by ion–dipole interactions which renders them essentially insoluble
in oils, fats, cutin and waxes. For this reason, hydrated ions cannot access the waxy
pathway. Penetration of inorganic and organic ions is limited to the aqueous pathway
(Schönherr 2006). Negative and positive ions must penetrate in equal numbers to
maintain electroneutrality. Each cation must be accompanied by equivalent amounts
of anions. For instance, Ca 2+ must be accompanied by two Cl − ions. This is true
as long there is no drop of electric potential across the cuticle, which is always
the case under natural conditions. Thus, the appropriate term is salt or electrolyte
permeability, not ion permeability.
Whenever salt or electrolyte penetration is observed in the field or in the laboratory,
this is definitive evidence for the presence of aqueous pores in the cuticles of
leaves and stems investigated. Strugger (1939) was one of the first to demonstrate
presence of aqueous pores in plant cuticles. Agriculturalists and horticulturalists
interested in foliar nutrition have studied salt permeation into leaves or isolated
CM (cf. Yamada et al. 1964; McFairlane and Berry 1974; Tuckey 1970; Schönherr
2000, 2001; Schlegel and Schönherr 2002; Schreiber 2005). Many agricultural
chemicals are ionic (bentazon, glyphosate, paraquat) and penetrate into the foliage
only when aqueous pores occur in cuticles of these leaves. Penetration of glyphosate
L. Schreiber and J. Schönherr, Water and Solute Permeability of Plant Cuticles.
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