Among metals, Ni has been indicated as one of the most dangerous for the environment, and plants exposed to this metal are frequently reported to undergo a severe stress condition. In this work, the tolerance responses to different Ni concentrations at physiological and biochemical levels were evaluated in Amaranthus paniculatus L., a plant species previously characterised for their ability to phytoremove Ni from metal-spiked water. Results indicated a good metal tolerance of this plant species at environmentally relevant Ni concentrations, while clear symptoms of oxidative damages were detected at higher Ni concentrations, both in roots and leaves, by measuring lipid peroxide content. At the photosynthetic level, pigment content determination, chlorophyll fluorescence image analysis and gas-exchange parameter measurements revealed a progressive impairment of the photosynthetic machinery at increasing Ni concentrations in the solution. Regarding biochemical mechanisms involved in antioxidative defence and metal binding, antioxidative enzyme (ascorbate peroxidase, APX; catalase, CAT; guaiacol peroxidase, GPX; superoxide dismutase, SOD) activity, polyamine (PA) content, polyamine oxidase (PAO) activity and organic acid (OA) content were differently affected by Ni concentration in the growth solution. A role for GPX, SOD, PAs, and oxalic and citric acid in Ni detoxification is suggested. These results can contribute to elucidate the tolerance mechanisms carried out by plants when facing environmentally relevant Ni concentrations and to identify some traits characterising the physiological and biochemical responses of Amaranthus plants to the presence and bioaccumulation of Ni.

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Abstract.To evaluate a possible utilization in the phytoremediation of metal contaminated substrates, Amaranthus paniculatus L. plants were grown for one week in Ni-spiked growth solutions at 0, 25, 50, 100, 150 µM NiCl 2 in hydroponics under controlled climate conditions. Results showed a high tolerance to Ni in plants exposed to low Ni concentrations. Tolerance decreased as Ni concentration in the growth solutions enhanced. Ni concentrations in plant organs (root, stem and leaves) revealed a trend to increase in parallel with the enhancement of Ni content in the growth solution. The ability to accumulate Ni in plants was also evaluated by calculating the bioconcentration factor (BCF). An inverse relation between BCF and Ni concentrations in the growth solution was evidenced. Ni phytoremoval ability of A. paniculatus plants was particularly appreciable at 25 µM NiCl 2 , where more than 65% of the initial Ni amount was taken up by plants in one week of treatment. The capability of plants to translocate Ni from roots to shoots (stem+leaves) was evaluated by the translocation factor (Tf). Results revealed a low Tf in plants exposed to low Ni concentration, suggesting a tolerance mechanism to protect physiological processes occurring in leaves. Overall, A. paniculatus plants showed a valuable capability to phytodecontaminate Ni-polluted waters, particularly at low Ni concentrations.