Effects of Ca2+ and other divalent cations on uptake of Ni2+ by excised barley roots

Excised roots of 7‐day‐old, dark‐grown barley (Hordeum vulgare L. cvs Gunilla and Møyar) were used to characterize effects of Ca2+ and other divalent cations on uptake of Ni2+ (63Ni2+). The uptake of Ni2+ in the roots (free space uptake and influx) and the Ni2+ remaining in the roots after desorption for 15 min in 1.0 mM EDTA (mainly in‐flux) were both independent of the Ca2+ concentration in the pretreatment solution up to about 0.1 mM and then increased to a maximum at 1 mM Ca2+. Without pretreatment, the higher the Ca2+ concentration in the uptake solution, the less Ni2+ was taken up. Ni2+ influx in the roots was reduced less by Ca2+ than was loose binding of Ni2+ in the free space of the roots (the amount of Ni2+ removed by the desorption procedure). The ratio between Ni2+ influx and Ni2+ uptake had a maximum around 1 mM Ca2+ and was higher the lower the external Ni2+ concentration. With Mg2+ in the uptake solution, Ni2+ influx was more reduced than loose binding of Ni2+, in contrast to the results with Ca2+. Both Ca2+ and Mg2+ were non‐competitive inhibitors of Ni2+ influx. Mn2+ did not influence Ni2+ influx, whereas Zn2+ and Cu2+ inhibited Ni2+ influx strongly and competitively and Co2+ weakly and competitively. The heavy metals Cd2+ and Pb2+ appeared to inhibit Ni2+ influx non‐competitively.