SIMULATION OF THE FOLIAR UPTAKE OF SEMI-VOLATILE FUNGICIDES
M RIEDERERl, A DAISS2, N GILBERT2 and H KOHLE3
lLehrstuhl fur Botanik 11, Universität Würzburg, D-97082 Würzburg, Germany; 2BASF AG, Technische Entwicklung, D-67056 Ludwigshafen, Germany; 3BASF AG, Landwirtschaftliche Versuchsstation, D-67114 Limburgerhof, Germany
Background and objectives
Most systemic fungicides are applied to the above-ground surfaces of crops. The primary site of contact with the plant is the cuticle which represents the interface between the deposit of formulation and the internal plant tissues. Non-volatile active ingredients of the formulation can enter the plant exclusively via the cuticular pathway . However, active ingredients having a finite vapour-pressure at environmental temperatures may, in addition to cuticular penetration, also be taken up through open stomata . A prerequisite for this mechanism of uptake is the presence of active ingredients in the vapour phase of the boundary layer of the leaf. The objective of the present study was to investigate this mechanism of foliar uptake by a numerical simulation of the diffusion and convection processes involved.
Results and conclusions
Three model systems with increasing complexity and closeness to the real situation at the leaf surface were studied. Diffusive and convective transport in the leaflatmosphere interface was simulated using a finite elements approach. The three-dimensional model systems were translated into appropriate grid structures and numerical simulations performed. The simulations were based on the physical-chemical properties of fenpropimorphe, kresoxim-methyle and epoxiconazole which were taken as reference compounds. Diffusion coefficients of 1x10-10 M 2 S-1, 1 X, 0-11 M2 S-1, 1 X, 0-1 1 M 2 S-1 and 1x10-5 M 2 S-1 were assumed for the cell wall, cuticular polymer matrix, cuticular wax and atmosphere, respectively.
The simulations demonstrate that the rates of vapour-phase uptake of active ingredients into the interior of a leaf strongly depend on the physical-chemical properties of the chemical. Low vapour pressures and high solubilities in cuticular wax as well as high vapour pressures are unfavourable for the build-up of sufficiently high concentrations of the active ingredient in the boundary layer of the leaf. Active ingredients meeting the delicate balance between vapour pressure and wax solubility may effectively be transiocated in and taken up from the unstirred layers of air covering leaf or fruit surfaces. Active ingredients meeting the delicate balance between vapour pressure and wax solubility may effectively be transiocated into the undisturbed layers of air covering the leaf or fruit surfaces, and as a consequence a larger part of them can be taken up into the plant via the stomata.
< b> References
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2. Riederer M, 1995. In: Plant contamination: Modeling and simulation of organic chemical processes (Trapp S, McFariane C, eds), Boca Raton: Lewis Publishers, pp. 153-190.