Study of Root Uptake and Xylem Translocation of Cinmethylin and Related Compounds in Detopped Soybean Roots Using a Pressure Chamber Technique

Abstract

A pressure chamber technique was used to study the root uptake and xylem translocation of nonradiolabeled cinmethylin and its analogs in detopped soybean (Glycine max) roots. Quantifications of compounds were achieved by gas chromatography analysis using a mass spectrometry detector under selected ion monitoring. The compounds tested, with octanol-water partition coefficients (log Kow values) ranging from 0.96 to 5.3, were all nonionizable under the experimental conditions. Root efflux curves of all compounds exhibited a steady-state kinetic profile. The time required to achieve the steady state efflux concentration in the xylem sap correlated with log Kow values in a manner very similar to the root binding profile reported previously by GG Briggs et al. ([1982] Pestic Sci 13: 495-504). After reaching the steady state efflux, the concentration ratio of each compound in the xylem sap to the final concentration in the pressure chamber was taken as the transpiration stream concentration factor (TSCF). A nonlinear relationship was observed between TSCF and log Kow values. The highest TSCF value was between 0.6 to 0.8 for compounds with log Kow between 2.5 to 3.5. The range of optimal log Kow values was slightly higher than that reported earlier by Briggs et al. ([1982] Pestic Sci 13: 495-504). After taking into account the binding of the compound to soil, the apparent optimal Kow value for best root-to-shoot translocation is lowered to around 1. The relationship of root-to-shoot and phloem translocation was also discussed to promote a better understanding at the whole plant level of the uptake and translocation of a soil-applied xenobiotic.