ROOT UPTAKE AND XYLEM TRANSLOCATION OF PESTICIDES FROM DIFFERENT CHEMICAL CLASSES

A pressure-chamber technique was used to study the root uptake and xyl em translocation of some fungicides, herbicides and an insecticide fro m different chemical classes in detopped soybean roots. Physiological parameters such as K+ leakage from roots, K+ concentrations in the xyl em sap, and protein and ATP levels in the root cells were measured so as to evaluate any potential damage of this technique to the root syst em. HPLC was used to quantify the compounds in the xylem sap. The pres sure-chamber technique has proved useful to study the root uptake and translocation of pesticides, does not damage the root system, and allo ws one to obtain appreciable volumes of xylem sap that can be analysed directly by HPLC, thus avoiding dependence on the availability of rad io-labelled compounds. The concentration of each pesticide in the xyle m sap showed a steady-state kinetic profile. Non-linear regression ana lysis was used to calculate the steady-state concentration and the tim e required to achieve 50% of the steady-state concentration (TSSC50). TSSC50 was well correlated with log K-ow; the more lipophilic the comp ound the more time was required to reach the steady-state concentratio n. The efficiency of translocation was assessed by the transpiration s tream concentration factor (TSCF) and a non-linear relationship betwee n TSCF and log K-ow was observed. The highest TSCF values were measure d for those compounds with log K-ow values around 3, a lipophilicity v alue similar to that reported earlier in an analogous experiment with detopped soybean plants but slightly higher than that reported in earl ier experiments with intact barley plants. Lower TSCF values were obta ined with chemicals with log K-ow values below as well as above 3.