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 ...

Phytochelatins (PCs) are a family of thiol-rich peptides, with the general structure (gamma-Glu-Cys)(n)()-Gly, with n = 2-11, induced in plants upon exposure to excessive amounts of heavy metals and some metalloids, such as arsenic. Two types of PC analyses are currently used, i.e., acid extraction and separation on HPLC with either precolumn derivatization (pH 8.2) with monobromobimane (mBBr) or postcolumn derivatization (pH 7.8) with Ellman's reagent [5, 5'-dithiobis(2-nitrobenzoic acid), DTNB]. Although both methods were satisfactory for analysis of Cd-induced PCs, formation of (RS)(3)-As complexes during extraction of As-induced PCs rendered the DTNB method useless. This paper shows that precolumn derivatization with mBBr, during which the (RS)(3)-As complexes are disrupted, provides a qualitative and quantitative analysis of both Cd- and As-induced PCs. In addition, derivatization efficiencies of both methods for the oligomers with n = 2-4 (PC(2)(-)(4)) are compared. Derivatization efficiency decreased from 71.8% and 81.4% for mBBr and DTNB derivatization, respectively, for PC(2) to 27.4% and 50.2% for PC(4). This decrease is most likely due to steric hindrance. Correction of measured thiol concentration is therefore advised for better quantification of PC concentrations in plant material.

Summary The mechanisms of enhanced root to shoot metal transport in heavy metal hyperaccumulators are incompletely understood. Here, we compared the distribution of nickel (Ni) over root segments and tissues in the hyperaccumulator Thlaspi caerulescens and the nonhyperaccumulator Thlaspi arvense, and investigated the role of free histidine in Ni xylem loading and Ni transport across the tonoplast. Nickel accumulation in mature cortical root cells was apparent in T. arvense and in a high‐Ni‐accumulating T. caerulescens accession, but not in a low‐accumulating T. caerulescens accession. Compared with T. arvense, the concentration of free histidine in T. caerulescens was 10‐fold enhanced in roots, but was only slightly higher in leaves, regardless of Ni exposure. Nickel uptake in MgATP‐energized root‐ and shoot‐derived tonoplast vesicles was almost completely blocked in T. caerulescens when Ni was supplied as a 1 : 1 Ni–histidine complex, but was uninhibited in T. arvense. Exogenous histidine supply enhanced Ni xylem loading in T. caerulescens but not in T. arvense. The high rate of root to shoot translocation of Ni in T. caerulescens compared with T. arvense seems to depend on the combination of two distinct characters, that is, a greatly enhanced root histidine concentration and a strongly decreased ability to accumulate histidine‐bound Ni in root cell vacuoles.

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In response to a range of Cd concentrations, the root tips of Cdtolerant plants of Silene