Root exudates of the hyperaccumulator Thlaspi caerulescens do not enhance metal mobilization

To examine whether root exudates of the Zn/Cd hyperaccumulator Thlaspi caer ulescens play a role in metal hyperaccumulation, we compared the metal mobi lization capacity of root exudates collected from two ecotypes of T. caerul escens, and from the nonaccumulators wheat (Triticum aestivum) and canola ( Brassica napus). Plants were grown hydroponically and three treatments (control, -Fe and -Zn ) were later imposed for 2 wk before collection of root exudates. On a basis of root d. wt, the total soluble organic C in the root exudates of T. caerulescens was similar to that of wheat, and significantly higher t han that of canola. In all treatment, the root exudates of T. caerulescens and canola mobilized little Cu and Zn from Cu- or Zn-loaded resins, and lit tle Zn, Cd, Cu or Fe from a contaminated calcareous soil. By contrast, the root exudates of wheat generally mobilized more metals from both resin and soil. In particular, the -Fe treatment, and to a lesser extent the -Zn trea tment, elicited large increases in the metal mobilization capacity of the r oot exudates from wheat. We conclude that root exudates from T. caerulescens do not significantly en hance mobilization of Zn and Cd, and therefore are not involved in Zn and C d hyperaccumulation. (C) New Phytologist (2001).