Summary• Here, Thlaspi caerulescens populations from contrasting soil types (serpentine, calamine and nonmetalliferous) were characterized with regard to tolerance, uptake and translocation of zinc (Zn), cadmium (Cd) and nickel (Ni) in hydroponic culture.• Results showed that high-level tolerances were apparently metal-specific and confined to the metals that were enriched at toxic levels in the soil at the population site.• With regard to metal accumulation, results suggested that, unlike Zn hyperaccumulation, Cd and Ni hyperaccumulation were not constitutive at the species level in T. caerulescens .• In general, the populations under study exhibited a pronounced uncorrelated and metal-specific variation in uptake, root to shoot translocation, and tolerance of Zn, Cd and Ni. The distinct intraspecific variation of these characters provides excellent opportunities for further genetic and physiological dissection of the hyperaccumulation trait.

The results presented in this paper describe the short-and long-term toxicity of arsenate in Silene vulgaris. Shortterm toxicity, measured as inhibition of root elongation, depended on phosphate nutrition, arsenate being much less toxic at high phosphate supply. At low phosphate levels more arsenic was taken up by the plants. Under chronic exposure, toxicity (measured as inhibition of biomass production) did not increase with time. In addition, the accumulation of phytochelatins (PCs) as a function of toxicity and duration of exposure was studied. Shortterm PC accumulation (over a 3 d period) was positively correlated with exposure. Isolation of peptide complexes from prolongedly exposed plants showed that PC # , PC $ and PC % were present, although the latter not until at least 3 d exposure. Arsenic co-eluted mainly with PC # and PC $ . Fractions containing PC % were devoid of As, probably due to dissociation of the complexes during extraction or elution. The breakdown of PCs after arresting As exposure was very slow. This could explain the continuous accumulation of PCs throughout longer periods of As exposure.Key words : Silene vulgaris, arsenic toxicity, arsenic detoxification, phytochelatins. Arsenic is taken up mainly by plant roots as arsenate (AsO % $−) (Macnair & Cumbes, 1987) through the phosphate-uptake system (Asher & Reay, 1979). Once the arsenate, As(V), is taken up it is reduced to arsenite, As(III), by glutathione (GSH) (Thompson, 1993). Only in phosphate-deficient conditions is arsenate subsequently methylated in plants (Nissen & Benson, 1982). Between the successive methylation steps, GSH serves to reduce the intermediate products (Scott et al., 1993 ;Thompson, 1993). Arsoniumphospholipids in freshwater plants (Nissen & Benson, 1982) and arsenic sugars in marine brown algae (Edmonds & Francesconi, 1981) have also been identified.Mostly there is little transport of As to the aboveground parts of the plants. Dicotyledonous plants appear to transport more As to the shoots than monocotyledonous plants (Otte, 1991). The form in *Author for correspondence (fax j31 20 444 7123 ; e-mail elsesnel!bio.vu.nl). which the As is transported is unknown. There is some indication that dimethylarsenic acid is transported to the shoots (Marin et al., 1993).Increased As levels may cause toxic symptoms in plants, such as a decrease in plant growth and fruit yield (Carbonell-Barrachina et al., 1995), root discoloration and root plasmolysis, wilting and necrosis of leaf tips and leaf margins (Machlis, 1941), and a decrease in photosynthetic capacity (Marin et al., 1993).Some authors have reported the accumulation of heavy metal-binding, thiol-rich phytochelatins (PCs) on exposure to As (Grill et al., 1986a(Grill et al., ,b, 1987Maitani et al., 1996). Phytochelatins have the structure (γ-glu-cys) n -gly, where n l 2-11 (Grill et al., 1985), and are produced in plants on exposure to a variety of heavy metals and metalloids (Gekeler et al., 1989). Phytochelatins are synthesized from GSH (Hayashi et ...

The genetic control of heavy metal tolerance in higher plants is poorly understood, possibly in part because of several inherent properties of tolerance tests and tolerance measures. In this study we compared different methods of testing for copper tolerance in Silene vulgaris. A new type of multiple concentration test has been used to analyse the genetic control of copper tolerance in this species. Provisional results indicate that the occurrence of any tolerance, relative to a non-tolerant reference population from uncontaminated soil, is governed by a single major gene. The level of tolerance, however, seems to be controlled by a number of modifiers, which are completely hypostatic to the major gene. This model agrees with that proposed for Mimulus guttatus by Macnair (1983).

Thlaspi caerulescens (Tc; 2n = 14) is a natural Zn, Cd and Ni hyperaccumulator species belonging to the Brassicaceae family. It shares 88% DNA identity in the coding regions with Arabidopsis thaliana (At) (Rigola et al. 2006). Although the physiology of heavy metal (hyper)accumulation has been intensively studied, the molecular genetics are still largely unexplored. We address this topic by constructing a genetic map based on AFLP ® markers and expressed sequence tags (ESTs). To establish a genetic map, an F 2 population of 129 individuals was generated from a cross between a plant from a Pb/Cd/Zn-contaminated site near La Calamine, Belgium, and a plant from a comparable site near Ganges (GA), France. These two accessions show diVerent degrees of Zn and, particularly, Cd accumulation. We analyzed 181 AFLP markers (of which 4 co-dominant) and 13 co-dominant EST sequences-based markers and mapped them to seven linkage groups (LGs), presumably corresponding to the seven chromosomes of T. caerulescens.

Summary• To analyse the relation between zinc (Zn) accumulation and Zn tolerance in the Zn hyperaccumulator, Thlaspi caerulescens , a cross was made between a plant from a nonmetallicolous population (LE: high accumulation, low tolerance) and one from a calamine population (LC: low accumulation, high tolerance).• More or less homogeneous F 3 lines with contrasting extreme accumulation phenotypes were selected and phenotyped for tolerance, using the threshold exposure level for chlorosis as a tolerance measure. Zn accumulation and tolerance segregated largely independently, although there was a significant degree of association between low accumulation and high tolerance.• Plants from an F 2 family were phenotyped for Zn tolerance and their Zn accumulation rates were compared. The plants with low Zn tolerance exhibited significantly higher Zn accumulation than did the more tolerant plants.• The results suggest that the superior Zn tolerance in LC plants compared with LE plants results from a superior plant-internal Zn sequestration capacity and, although to a lower degree, a reduced rate of Zn accumulation. It is argued that the relatively low Zn accumulation capacity levels found in LC and several other calamine T. caerulescens populations might represent an adaptive response to Zn-toxic soil.