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

To examine whether root exudates of the Zn/Cd hyperaccumulator Thlaspi caerulescens play a role in metal hyperaccumulation, we compared the metal mobilization capacity of root exudates collected from two ecotypes of T. caerulescens, 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 than 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 little 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 treatment, elicited large increases in the metal mobilization capacity of the root exudates from wheat. We conclude that root exudates from T. caerulescens do not significantly enhance mobilization of Zn and Cd, and therefore are not involved in Zn and Cd hyperaccumulation. (C) New Phytologist (2001).