Natural variation in cadmium tolerance and its relationship to metal hyperaccumulation for seven populations of Thlaspi caerulescens from western Europe

Thlaspi caerulescens J. & C. Presl is a distinctive metallophyte of central and western Europe that almost invariably hyperaccumulates Zn to > 1.0% of shoot dry biomass in its natural habitats, and can hyperaccumulate Ni to > 0.1% when growing on serpentine soils. Populations from the Ganges region of southern France also have a remarkable ability to accumulate Cd in their shoots to concentrations well in excess of 0.01% without apparent toxicity symptoms. Because hyperaccumulation of Cd appears to be highly variable in this species, the relationship between Cd tolerance and metal accumulation was investigated for seven contrasting populations of T. caerulescens grown under controlled conditions in solution culture. The populations varied considerably in average plant biomass (3.1fold), shoot : root ratio (2.2-fold), Cd hyperaccumulation (3.5-fold), shoot : root Cd-concentration ratio (3.1-fold), and shoot Cd : Zn ratio (2.6-fold), but the degree of hyperaccumulation of Cd and Zn were strongly correlated. Two populations from the Ganges region were distinct in exhibiting high degrees of both Cd tolerance and hyperaccumulation (one requiring 3 m M Cd for optimal growth), whereas across the other five populations there was an inverse relationship between Cd tolerance and hyperaccumulation, as has been noted previously for Zn. Metal hyperaccumulation was negatively correlated with shoot : root ratio, which could account quantitatively for the differences between populations in shoot Zn (but not Cd) concentrations. On exposure to 30 m M Cd, the two Ganges populations showed marked reductions in shoot Zn and Fe concentrations, although Cd accumulation was not inhibited by elevated Zn; in the other five populations, 30 m M Cd had little or no effect on Zn or Fe accumulation but markedly reduced shoot Ca concentration. These results support a proposal that Cd is taken up predominantly via a high-affinity uptake system for Fe in the Ganges populations, but via a loweraffinity pathway for Ca in other populations. Total shoot Cd accumulated per plant was much more closely related to population Cd tolerance than Cd hyperaccumulation, indicating that metal tolerance may be the more important selection criterion in developing lines with greatest phytoremediation potential. Key-words : Thlaspi caerulescens ; cadmium; cation selectivity; metal hyperaccumulation; metal tolerance; shoot : root ratio; zinc.