The effects of adjuvents upon the pesticide uptake and penetration of foliage
The foliar uptake by wheat of the wild oat herbicide flampropmethyl has been studied following application of ul droplets of an aqueous solution. The results show that over the time required for the drop to dry the compound does not partition into the surface waxes and crystallises as an external deposit. This deposit is stable to volatilization and photochemical and biochemical degradation, and is depleted by foliar penetration. Adsorbtion by the leaf surface wax is initially rapid and thereafter occurs at a relatively constant rate until the almost complete exhaustion of the surface deposit. Penetration of the epicuticular wax layer generally proceeds at a slower rate than does adsorption of the surface deposit and the herbicide is accumulated in the wax layer. The compound is lost from the epicuticular waxes only slowly to the leaf tissues. Movement within the leaf tissues away from the area of uptake is predominantly acropetal to the leaf tip and it is presumed that the compound moves with the transpiration stream. A similar fate befalls the compound when aqueous solutions of flamprop-methyl are applied as ul droplets to barley or wild oat, no evidence of selectivity between wheat, barley and wild oat as a consequence of preferential uptake was found. Foliar uptake of flamprop-methyl is shown to depend on the amount of compound applied to un1 t area of leaf surface and to increase as the amount applied is increased or as the area to which application is made is increased. These trends are discussed in terms of compartmental model of foliar uptake. Foliar uptake of flamprop-methyl is also shown to be slightly influenced by environmental factors; in an environment in which maximum daily temperatures exceeded 35 o C penetration was especially rapid and tended to completion within 24 hours. The surface properties of aqueous solutions of selected polyoxyethylene non-ionic surfactants have been studied prior to investigating the effect of these compounds on the foliar uptake by cereals of flamprop-methyl. Two types of non-ionic surfactant were included in this study, the alkylphenol ethoxylates as exemplified by the Triton X products (ex Rohn and Haas) and alcohol ethoxylates as exemplified by the Brij products (ex Atlas Chemical Industries). Products from both sources were analysed using instrumental and separational methods; the results of these analyses supported the manufacturers description of the compounds. Surface and interfacial tensions of aqueous solutions of the surfactants were determined at various concentrations. Maximum"reduction of surface tension in the alkylphenol ethoxylate solutions was found with Triton X-35 and Triton X-45. Maximum reduction of surface tension in the alcohol ethoxylate solutions was found with the tetraoxyethylene dodecyl ether (Brij 30) and was reduced with increasing oxyethylene content with anyone hydrophobe. At constant oxyethylene content it was shown that surface activ1 ty was dependent on the nature of the hydrophobe. Critical micelle concentrations were determined from plots of surface tension vs concentration for alkylphenol and alcohol ethoxylates. The spreading and wetting properties of aqueous solutions of alkylphenol ethoxylates were investigated by the Draves test and in terms of calculated spreading coefficients. The foliar uptake by wheat of ethoxy1ated non-ionic surface active agents has been studied following topical application of aqueous solutions. The permeability of the cereal leaf to these compounds has been demonstrated. the rate of uptake was shown to depend on the mean oxyethy1ene content of the surfactant, decreasing as the oxyethy1ene content was increased within the Triton X series of compounds. In quali tati ve terms the trend in uptake was paralleled by changes in both partition coefficient and (estimated) diffusion coefficient. The distribution of oligomers within anyone surfactant was apparently retained during transcuticular movement. Movement of the surfactant across the epicuticular waxes resulted in an accumulation of the penetrant in the tissues underlying the site of application. Movement of the surfactant in treated leaves was acropetal. The foliar uptake of f1amprop-methy1 by wheat has been studied following topical application of the herbicide formulated in aqueous surfactant solutions. Non-ionic ethoxylated surfactants have been found to markedly enhance transcuticular movement of the herbicide but do not promote transport of the compound wi thin treated leaves. The extent to which uptake was enhanced was largely influenced by the concentration of the formulation with respect to the surfactant, was much less dependant on either hydrophobe or hydrophile structure and was apparently independant, within experimental limits, of the herbicide concentration. Optimum surfactant concentrat1ons were between 100 and 1000 ppm. Correlation between penetration enhancement and the surface properties of the formulations was non-existent. The effect of the surfactant on penetration is discussed in terms of a surfactant/lipid interaction which facilitated diffusion of the herbicide. At high surfactant concentrations a variety of responses were identified ranging from varying degrees of enhancement through to an inhibition of transcuticular movement: these responses were shown to correlate with the resistance to uptake afforded by a persistent surfactant residue on the leaf surface.