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

Chapter 7
Accelerators Increase Solute Permeability
of Cuticles
Many agrochemicals are sprayed on leaves of weeds and crop plants. Foliar
application minimises contamination of the soil and inactivation or degradation of
active ingredients by soil microorganisms. Binding to constituents of the soil is also
avoided. Systemic active ingredients must penetrate cuticles before they reach their
sites of action in the leaves or in other organs of the plants following translocation
(Kirkwood 1999). Pesticides are always formulated to improve wetting, deposition,
rain fastness and rates of cuticular penetration. Formulations are mixtures of active
ingredient, solvents, carriers, emulsifiers, wetting agents, etc. Compounds added to
active ingredients are called adjuvants. Most adjuvants are biologically inert, but
they improve biological performance (Kirkwood 1993). Surface active agents (surfactants)
are typical adjuvants, and among other things they improve adhesion and
spreading of spray droplets on the leaf surface. Surfactants may also act as emulsifiers
for active ingredients having a low water solubility, and after droplet drying
they can maintain the spray deposits on the cuticle in the liquid state by solvent and
by hygroscopic action (Baur 1999; Baur et al. 1997b, 1999; Tadros 1987).
Some adjuvants increase diffusion coefficients of solutes in wax and cutin. This
is accomplished by increasing fluidity of waxes and cutin polymer chains. As a
consequence, permeability of cuticles to solutes is increased (Schönherr and Bauer
1994). In the technical polymer literature, compounds which render brittle polymers
more flexible by increasing fluidity of polymer chains are called plasticisers.
They intercalate between polymer chains, and they are added to the polymer melt
during production (Gächter and Müller 1990). Plasticisers useful for increasing diffusion
coefficients of solutes in cuticles have been termed accelerators (Schönherr
1993a, b). Some surfactants are very effective plasticisers, but surface activity is not
a prerequisite for the plasticising activity. Accelerators must be added to the formulation,
and they are applied to the cuticle at the same time as the active ingredient.
Hence, it is a prerequisite for their usefulness that they penetrate faster into the waxy
limiting barrier of the cuticle than the active ingredient, and they should remain
sorbed in the wax until most of the active ingredient has penetrated the cuticle.
In this chapter we deal with the mechanistic aspects of sorption and diffusion
of plasticisers in wax and in cuticles, and their effect on permeability. All other
L. Schreiber and J. Schönherr, Water and Solute Permeability of Plant Cuticles.
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