Growth responses, metal accumulation and phytoremoval capability in Amaranthus plants exposed to nickel under hydroponics

The characterisation of plant responses to metal exposure represents a basic step to select a plant species for phytoremediation. In the present work, 3- week-old Amaranthus paniculatus L. plants were subjected to nickel chloride concentrations of 0 (control), 25, 50, 100 and 150 ?M in hydroponic solution for 1 week to evaluate morphophysiological responses, such as biomass production and partitioning, nickel accumulation in plants and nickel removal ability from the polluted solutions. The results showed a progressive decrease in plant organ dry mass with the enhancement of nickel (Ni) concentration in the solution, suggesting a good metal tolerance at 25 ?M Ni and a marked sensitivity at 150 ?M Ni. The modification of biomass partitioning was particularly appreciated in leaves, analysing the organ mass ratio, the total leaf area and the specific leaf area. Amaranthus plants accumulated a significant amount of Ni in roots exposed to the highest Ni concentrations, while lower metal contents were observed in the aerial organs. The Ni uptake ratio was progressively reduced in plants exposed to increased Ni concentrations. The metal translocation from root to shoots, appreciated by the Ni translocation index, showed a far lower value in Ni-exposed plants than in controls. Moreover, by measuring the daily Ni content of the solutions, a lower Ni removal ability was found in Amaranthus plants at increasing Ni concentrations. Remarkably, plants exposed to 25 ?M Ni succeeded in removing almost 60 % of the initial Ni content of the solution showing no stress symptoms. The potential of A. paniculatus for phytoremediation was discussed.