Effects of surfactants on foliar uptake of herbicides - a complex scenario.

Surfactants are almost always present in herbicide treatment solutions with the aim to improve spray droplet retention on and penetration of active ingredients (a.i.s) into plant foliage. The effects of surfactants on the foliar uptake of herbicides are complex and only partially understood. The influence of a range of non-ionic polyoxyethylene surfactants on the uptake of two herbicides, glyphosate and 2,4-dichlorophenoxyacetic acid (2,4-D), was compared using three plant species, wheat, broad bean and common lambsquarters. Surfactants of higher ethylene oxide (EO) content provided greater uptake enhancement for glyphosate, whereas those of lower EO content were more beneficial for 2,4-D uptake. Among surfactants of the same EO content, those containing a C(13)/C(15) linear alkane hydrophobe moiety appeared to be more efficient for promoting the uptake of both herbicides. When a suitable surfactant was used, glyphosate uptake into both bean and wheat foliage increased steadily with increasing surfactant concentration and reached a maximum at 0.5%. In the presence of a constant surfactant, higher percentage uptake of herbicide was obtained with higher a.i. concentrations for glyphosate, but with lower a.i. concentrations in the case of 2,4-D. In the presence of an organosilicone surfactant, the stomatal uptake of glyphosate varied with both surfactant concentration and plant species. The effect of non-silicone surfactants on the cuticular uptake of glyphosate also varied with plant species. It can be concluded that the effects of surfactants on foliar uptake of herbicides depend not only on their chemical structures (hydrophobe and hydrophile moieties) and concentration, but also on the physicochemical properties and concentration of a.i.s and the leaf surface character of the plant species.