Measurement of xylem translocation of weak electrolytes with the pressure chamber technique

Xylem translocation and root uptake of weak electrolytes were investigated with the pressure chamber technique (PCT) using de-topped soybean plants. Two compounds were organic bases (fenpropimorph and imazalil) and four were organic acids (bentazone, primisulfuron-methyl, rimsulfuron and triasulfuron). The compounds covered a wide range of log KOW and pKa values. Concentrations in external solution and in xylem sap were measured by HPLC at pH values in external solution of 4.5, 6.5 and 8.5. For weak bases, translocation was higher at low pH and the transpiration stream concentration factors (TSCF) were in the range 0.31-0.95. At pH 8.5, the concentrations in leaking xylem sap were very low for fenpropimorph, and steady-state was probably not reached. For weak acids, TSCF values derived with external pH from 4.5 to 8.5 were in the range 0.55-1.50 for primisulfuron-methyl, 0.64-1.35 for rimsulfuron, 0.81-0.93 for triasulfuron and 0.69-0.92 for bentazone. The variation of TSCF of the weak electrolytes was much smaller in these PCT experiments than in recent experiments with intact plants. The likely reason is that de-topped soybean plants in the pressure chamber seemed to be unable to regulate their xylem sap pH, which was almost identical to the pH in external solution. Without pH differences, the ion-trap process, which is responsible for accumulation or exclusion of weak acids and bases in the xylem of living plants, does not take place. Model simulations carried out for intact and de-topped plants supported this hypothesis. By variation of the pH of the xylem sap, good agreement between measurements and simulations could be achieved.