DEVELOPMENT OF A PREDICTIVE UPTAKE MODEL TO RATIONALIZE SELECTION OF POLYOXYETHYLENE SURFACTANT ADJUVANTS FOR FOLIAGE-APPLIED AGROCHEMICALS

Composition-concentration relationships between a series of C-13/C-14 polyoxyethylene primary alcohol (AE) surfactants and the foliar uptake enhancement of five model neutral organic compounds were examined in factorially designed experiments on wheat (Triticum aestivum L.) and f ield bean (Vicia faba L.) plants grown under controlled environment co nditions. Model compounds were applied to leaves as c.0.2-mul droplets of 0.5 g litre-1 solutions in aqueous acetone in the absence or prese nce of surfactants at 0.2, 1 and 5 g litre-1. Uptake of the highly wat er-soluble compound, methylglucose (log octanol-water partition coeffi cient (P) = -3.0) was best enhanced by surfactants with high E (ethyle ne oxide) contents (AE15, AE20), whereas those of the lipophilic compo unds, WL110547 (log P = 3.5) and permethrin (log P = 6.5), were increa sed more by surfactants of lower E contents, especially AE6. However, there was little difference between AE6, AE11, AE15 and AE20 in their ability to promote uptake of the two model compounds of intermediate p olarity, phenylurea (log P = 0.8) and cyanazine (log P = 2.1). Absolut e amounts of compound uptake were also influenced strongly by both sur factant concentration and plant species. Greatest amounts of uptake en hancement were often observed at high surfactant concentration (5 g li tre-1) and on the waxy wheat leaves compared with the less waxy field bean leaves. The latter needed higher surfactant thresholds to produce significant improvements in uptake. Data from our experiments were us ed to construct a simple response surface model relating uptake enhanc ement to the E content of the surfactant added and to the physicochemi cal properties of the compound to be taken up. Qualitative predictions from this model might be useful in rationalising the design of agroch emical formulations.