Positive responses to Zn and Cd by roots of the Zn and Cd hyperaccumulatorThlaspi caerulescens

The effects of localized zinc (ZnO) and cadmium (CdS) enrichment on the all ocation of root biomass, root length and partitioning of current assimilate within root systems of the Zn hyperaccumulator Thlaspi caerulescens were i nvestigated using a rhizobox system. The rhizoboxes contained either homoge neous soil or juxtaposed control and metal-enriched soil. In the heterogene ous treatments the Zn-enriched soil contained 250, 500 or 1000 mg Zn kg(-1) . The plants consistently allocated c. 70% of their total root biomass and length into the metal-enriched soil. Moreover, 70% of the current assimilat e (C-14) allocated to the roots was localized in the Zn-enriched soil of th e heterogeneous treatments. The concentration of Zn (250-1000 mg kg(-1)) in the enriched soil had no effect on these patterns of root allocation, nor were there significant effects of the Zn treatments on the total root or sh oot biomass of the plants. The positive responses to the localized metal tr eatments were therefore characterized by a concomitant reduction in root al location into the control soil. In contrast to T. caerulescens, when grown with juxtaposed control and Zn-enriched soil the non-accumulator Thlaspi ar vense showed reduced root allocation into a patch enriched with 500 mg kg(- 1) Zn. In a further experiment, two populations of T. caerulescens that dif fer in their abilities to accumulate Cd were grown with juxtaposed control and Cd-enriched soil. The plants from a population that accumulated Cd also showed increased root biomass and root length allocation into the Cd-enric hed soil. Plants from the population that did not accumulate Cd showed no s uch increase. The possibility that T. caerulescens forages for metals, and the precision of its root allocation with respect to localized metal enrich ment is highlighted. The significance of these findings for the selection o f hyperaccumulator plants for use in the phytoextraction of Zn and Cd from mine spoils (phytomining) and the phytoremediation of heterogeneous contami nated soils are discussed.