Positive responses to Zn and Cd by roots of the Zn and Cd hyperaccumulator Thlaspi caerulescens

The effects of localized zinc (ZnO) and cadmium (CdS) enrichment on the allocation of root biomass, root length and partitioning of current assimilate within root systems of the Zn hyperaccumulator Thlaspi caerulescens were investigated using a rhizobox system. The rhizoboxes contained either homogeneous soil or juxtaposed control and metal-enriched soil. In the heterogeneous 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 assimilate ((14)C) allocated to the roots was localized in the Zn-enriched soil of the 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 shoot biomass of the plants. The positive responses to the localized metal treatments were therefore characterized by a concomitant reduction in root allocation into the control soil. In contrast to T. caerulescens, when grown with juxtaposed control and Zn-enriched soil the non-accumulator Thlaspi arvense showed reduced root allocation into a patch enriched with 500 mg kg(-1) Zn. In a further experiment, two populations of T. caerulescens that differ 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-enriched soil. Plants from the population that did not accumulate Cd showed no such increase. The possibility that T. caerulescens forages for metals, and the precision of its root allocation with respect to localized metal enrichment is highlighted. The significance of these findings for the selection of hyperaccumulator plants for use in the phytoextraction of Zn and Cd from mine spoils (phytomining) and the phytoremediation of heterogeneous contaminated soils are discussed.