Phytoremediation Potential of Amaranthus Hybrids: Antagonism between Nickel and Iron and Chelating Role of Polyamines

Three amaranth hybrids (Amaranthus paniculatus f. cruentus (Vishnevyi dzhem), A. paniculatus (Bronzovyi vek), and A. caudatus f. iridis (Izumrud) were grown in the climate-controlled chamber on Jonson nutrient medium supplemented with 2 mu M Fe3+-EDTA. When plants developed 5-6 true leaves (six-week-old plants), NiCl2 was added to medium to final concentrations of 0 (control), 50, 100, 150, 200, and 250 mu M. In 6 days, the increment in biomass of young and mature leaves, stems, and roots, and also the contents of Ni and Fe in them were measured. The red leaf amaranth hybrid Vishnevyi dzhem manifested the highest phytoremediation potential. i.e., the highest capacity for Ni accumulation in the shoots and the most pronounced symptoms of Fe deficit. In the presence of 150 and 250 mu M NiCl2 in medium, the shoots of these plants contained about 2 and 4 mg Ni/g dry wt, respectively. In experiments with Fe deficit in plants grown for a week in the presence of NiCl2 (0, 25, 50, 75, and 100 mu M), it was established that all tested nickel concentrations suppressed iron reduction in intact roots, which is catalyzed by ferric-chelate reductase, and this may underlie the antagonism between the two metals. In the presence of 50 mu M NiCl2 in medium and 2 mu M Fe3+ (Fe deficit) and especially 100 mu M Fe3+ (Fe excess), the content of MDA and proline in leaves increased and superoxide dismutase was activated; this indicates a development of oxidative stress. Leaf treatment with polyamines (putrescine or spermidine) with aminoguanidine (the inhibitor of H2O2 generation at polyamine oxidation) and with 1,3-diaminopropane led to the increase in nickel accumulation in leaves but did not result in the appearance of any signs of injury. This confirms our previous suggestion that polyamines manifest their protectory action as Ni chelators and detoxicants.