Physiological evidence for a high-affinity cadmium transporter highly expressed in a Thlaspi caerulescens ecotype

Uptake kinetics and translocation characteristics of cadmium and zinc are p resented for two contrasting ecotypes of the Cd/Zn hyperaccumulator Thlaspi caerulescens, Ganges (southern France) and Prayon (Belgium). Experiments using radioactive isotopes were designed to investigate the phy siology of Cd and Zn uptake, and a pressure-chamber system was employed to collect xylem sap. In contrast to similar Zn uptake and translocation, measurements of concent ration-dependent influx of Cd revealed marked differences between ecotypes. Ganges alone showed a clear saturable component in the low Cd concentratio n range; maximum influx V-max for Cd was fivefold higher in Ganges; and the re was a fivefold difference in the Cd concentration in xylem sap. Addition of Zn to the uptake solution at equimolar concentration to Cd did not decr ease Cd uptake by Ganges, but caused a 35% decrease in Prayon. There is strong physiological evidence for a high-affinity, highly expresse d Cd transporter in the root cell plasma membranes of the Ganges ecotype of T. caerulescens. This raises evolutionary questions about specific transpo rters for non-essential metals. The results also show the considerable scop e for selecting hyperaccumulator ecotypes to achieve higher phytoextraction efficiencies.