Differential effects of iron starvation and iron excess on nickel uptake kinetics in two Iranian nickel hyperaccumulators, Odontarrhena bracteata and Odontarrhena inflata

Aims: To characterize Ni hyperaccumulation mechanisms in naturally Ni-hyperaccumulating Odontarrhena species, Ni uptake kinetics, and the effect of Fe starvation and Fe or Zn excess thereon, were investigated in the Iranian serpentine endemics O. bracteata (one population) and O. inflata (two populations). Methods: Plants were exposed to a series of Ni concentrations for 4 h, and Ni uptake rates were plotted against the Ni concentration in the solution. Kinetic parameters, Km and Vmax, were calculated from Lineweaver-Burke plots. Results: Ni uptake consistently showed Michaelis–Menten kinetics. Under normal Fe supply the Km was not significantly different between species or populations, however, the Vmax was 3-fold higher in O. bracteata and O. inflata from Baneh than in O. inflata from Marivan. The rate of Ni translocation to the shoot was similar in all of the species/populations. Fe starvation (1 wk) significantly increased the Vmax for Ni uptake in O. bracteata, but not in O. inflata, and a very similar increase in Fe uptake capacity in all the species/populations. Fe excess in the nutrient solution significantly increased the Km for Ni uptake in O. bracteata, but not in O. inflata. Zn excess in the nutrient solution (300 μM) marginally inhibited Ni uptake in O. inflata, but not in O. bracteata. Conclusions: Ni uptake seems to proceed via a Fe deficiency-inducible Fe transporter in O. bracteata, but not in O. inflata. Ni root-to-shoot translocation seems to be mediated by a Fe deficiency-inducible Fe translocation mechanism in all of the species/populations.