STUDIES ON OCTYLPHENOXY SURFACTANTS .11. EFFECT ON NAA DIFFUSION THROUGH THE ISOLATED TOMATO FRUIT CUTICULAR MEMBRANE

The effect of a homologous series of octylphenoxy surfactants [2-{alpha[4-(1,1,3,3-tetramethylbutyl)phenoxy]poly(2-ethoxy)}ethanol;'Triton-X'series], with five (TX-45),9-10 (TX-100), 16 (TX-165) and 40 (TX-405) units of ethylene oxide (EO) on penetration of NAA [2-(1-naphthyl)acetic acid] through enzymatically isolated tomato (Lycopersicon esculentum Mill.) fruit cuticles was investigated. Surfactant effects on NAA flux were related to EO chain length and to surfactant concentration, compared on a basis relative to the critical micelle concentration (CMC, 2-5 x CMC and CMC/2.5). At concentrations above the CMC, TX-45 and TX-100 increased NAA flux 2.5- and 1-5-fold, respectively, while TX-165 was without effect and TX-405 decreased NAA flux 0.7-fold. At concentrations below the CMC, all four surfactants increased NAA penetration by 1.1- to 1.3-fold. Time-course studies of NAA flux on addition of surfactant revealed that, where NAA flux always increased it did so slowly, but any decrease of flux occurred rapidly. Sorption of NAA by cuticular membranes decreased progressively with increased EO chain length at surfactant concentrations 2.5 x CMC. Below the CMC, surfactants had little or no effect on NAA sorption. Our data provide evidence that surfactants increase transcuticular penetration of NAA by increasing the diffusion coefficient and, hence, the mobility of NAA in the cuticle.