Metal hyperaccumulation and distribution in Anthyllis vulneraria from Cave del Predil: zinc or thallium?

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Abstract Background and aims Anthyllis vulneraria , a common and widespread legume in Europe, spontaneously colonizes metalliferous habitats highly enriched in thallium and zinc, with hyperaccumulation of these elements reported in its subspecies from some populations. Methods Metallicolous and non-metallicolous accessions of Anthyllis vulneraria subsp. polyphylla from northeast Italy were cultivated hydroponically and exposed to different concentrations of zinc and thallium to assess thallium and zinc accumulation in the plants and to analyze the effects on growth and photosynthetic efficiency. Synchrotron X-ray fluorescence microscopy was used to elucidate elemental distribution at the cellular level in plant organs in field-collected plants. Results Synchrotron X-ray fluorescence microscopy revealed distinct localization of zinc and thallium in the epidermis of the root and petiole, as well as in the vasculature of the leaves. In the zinc treatment in hydroponics, root concentrations were significantly higher than in the shoots for both accessions, reaching the highest value when dosed with 2000 µM zinc, with 4000 µg g –1 in the shoot and 11,000 µg g –1 in the roots of Anthyllis vulneraria subsp. polyphylla . In contrast, in both thallium treatments, the shoot:root ratio was greater than one. Photosynthetic activity and plant growth were reduced in the non-metallicolous population, whereas in the metallicolous population it remained largely stable across the treatments. Conclusion The spatial distribution of zinc and thallium in field-collected plants has features typical of metal hyperaccumulators, such as preferential accumulation in the epidermal cells. Under controlled conditions genuine hyperaccumulation was shown only for thallium, while exclusion mechanisms combined with high internal tolerance were found for zinc.

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