Foliar uptake of organosilicone surfactant oligomers into bean leaf in the presence and absence of glyphosate

The nonionic organosilicone surfactant ‚Silwet’︁® L‐77TM (1, 2 and 5 g litre−1) and [14C]glyphosate‐monoisopropylammonium (10 g litre−1 a.e.), were applied, alone and in combination with each other, to the upper surfaces of leaves of Vicia faba L. plants growing in a controlled environment. Over 80% of the glyphosate was taken up within 10 min in the presence of 5 g litre−1 L‐77, with no further uptake over the remaining period. A similar pattern of uptake was observed with 2 g litre−1 L‐77, but only 30% was taken up initially. In the presence of 1 g litre−1 L‐77 initial glyphosate uptake was only 15%, but uptake continued, subsequently reaching c.25% after 360 min, this behaviour being essentially the same as that of glyphosate when applied without surfactant. The uptake of L‐77 was not significantly affected by the addition of glyphosate, being c.80% within 10 min at 5 g litre−1, but with a slow, further increase of c.12%. At 2 g litre−1 uptake was c.45% within 10 min, rising to c.70% by 360 min. The 1 g litre−1 solutions showed a similar pattern, with uptake increasing from c.35% at 10 min to over 60% after 360 min. In the absence of glyphosate, individual L‐77 oligomers penetrated at similar rates. In the presence of glyphosate, however, some discrimination was evident among the oligomers, in particular when the surfactant was added at 1 g litre−1. Literature on the foliar uptake of surfactants is reviewed and discussed in relation to the results of this study.