Sn. Whiting et al., Positive responses to Zn and Cd by roots of the Zn and Cd hyperaccumulatorThlaspi caerulescens, NEW PHYTOL, 145(2), 2000, pp. 199-210
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.