## Abstract

Selenate and selenite transport through tomato root systems were followed for periods up to 4 h after removal of the plant tops, using 75Se as a tracer.

With selenate, 75Se concentrations in the xylem exudate were 6 to 13 times higher than in the external solution, and chromatographic analysis showed that the selenium was transported as inorganic selenate ($SeO42-$ ).

With selenite, 75Se concentrations in the exudate were always lower than in the external solution. Analyses of exudate samples showed that negligible amounts of selenium were transported as inorganic selenite ($HSeO3−$ except at very high external selenite concentrations (500 µM), when up to 7 per cent was transported as selenite. Most of the selenium transport in selenite-fed plants was as selenate or as an unknown selenium compound, the relative proportions of these two forms varying both with time and with external selenite concentration. Addition of a 5-fold excess of sulphate over selenite had no detectable effect on the concentrations of selenate in the exudate, but caused substantial decreases in the maximum concentrations of both total selenium (c. 47 per cent decrease) and the unknown selenium compound (c. 69 per cent decrease). Addition of a 5-fold excess of sulphite decreased the concentration of the unknown (c. 39 per cent) but caused a large (2.7-fold) increase in the maximum total selenium concentration in the exudate and a 7.9-fold increase in the maximum concentration of selenate. The results suggest metabolic involvement in the uptake and long distance transport of solenium supplied as selenite, despite lower 75Se concentrations in the xylem exudate than in the external solution. An attempt is made to incorporate the new and existing information into a selenium transport model.