CONCENTRATION-DEPENDENT MOBILITY OF CHLORFENVINPHOS IN ISOLATED PLANTCUTICLES

The mobility of chlorfenvinphos in isolated pear (Pyrus communis cv. B artlett) leaf cuticular membranes (CM) was studied as a function of co ncentration of chlorfenvinphos sorbed in the cuticle. Mobilities of ch lorfenvinphos increased approximately 9-fold when the amount sorbed in creased from 1 to 100 mu g cm(-2) pear leaf cuticle. From the amounts per area, average volume fractions of chlorfenvinphos in the cuticle w ere calculated ranging from 2 x 10(-3) to 5.1 x 10(-2). The increase i n mobilities was steepest at the lower and levelled off at higher volu me fractions. This correlation could be described for the whole range of volume fractions investigated by an equation which assumes homogene ously dispersed chlorfenvinphos. Temperature dependence of mobilities was studied at 17, 25 and 35 degrees C and chlorfenvinphos volume frac tions of 5.5 x 10(-3) and 0.12, respectively. Arrhenius graphs were li near for both volume fractions, showing that cuticles did not undergo a phase transition due to the high amount of sorbed chlorfenvinphos. H owever, at a volume fraction of 0.12, the activation energy of diffusi on, E(D), was significantly lower (83.6 kJ mol(-1)) than at 5.5 x 10(- 3) (135 kJ mol(-1)) we interpret these findings as evidence for a plas ticising effect on cuticular waxes by chlorfenvinphos. So far, such an effect had been demonstrated only for certain adjuvants (ethoxylated alcohols) but not for active ingredients. Chlorfenvinphos not only inc reased its own mobility in pear leaf cuticles, but also that of 2,4-D in Citrus leaf cuticles. This would be expected if plasticising of wax es was the sole mechanism responsible for increased mobilities. From t hese data we predict that permeabilities of cuticles to chlorfenvinpho s are not constant. Depending on temperature as well as types and amou nts of adjuvants, rates of foliar penetration of chlorfenvinphos can b e higher if its concentration in the spray liquid is increased.