THE ROLE OF SURFACE-TENSION OF SPREADING DROPLETS IN ABSORPTION OF A HERBICIDE FORMULATION VIA LEAF STOMATA

The foliar absorption by field bean (Vicia faba L.) of triclopyrtriethylamine from solutions containing an organosilicone surfactant, 'Silwet L-77' at 0, 1.0, 2.5 and 5.0 ml litre-1, was investigated using fluorescence and radiotracer studies. Physical characteristics such as formulation surface tension and contact angles of droplet formed on the field bean leaf were determined and combined to obtain a critical surface tension value (gamma(critical)) of 19.5-22.9 mN m-1 for the adaxial leaf surface. The apparent surface tensions of the three formulations containing L-77 were equivalent and low, an average of 22 mN m-1, which would be sufficiently low to permit stomatal infiltration. The presence of L-77 at 2.5 or 5.0 ml litre-1 significantly enhanced the uptake of triclopyr-triethylamine into the leaf, particularly via stomatal pore regions. However, stomatal uptake from solutions containing 0 or 1.0 ml litre-1 L-77 was limited or non-existent. A phenomenon termed 'adsorptive dilution' is proposed to explain the absence of stomatal infiltration with the 1.0 ml litre-1 formulation. This involves a migration of surfactant from the bulk solution to the expanding droplet surfaces as the droplet spreads. This dilutes the surfactant concentration within the bulk solution, which can in turn increase surface tension.