Nickel in plants: I. Uptake kinetics using intact soybean seedlings.

The absorption of Ni(2+) by 21-day-old soybean plants (Glycine max cv. Williams) was investigated with respect to its concentration dependence, transport kinetics, and interactions with various nutrient cations. Nickel absorption, measured as a function of concentration (0.02 to 100 mum), demonstrated the presence of multiple absorption isotherms. Each of the three isotherms conforms to Michaelis-Menten kinetics; kinetic constants are reported for uptake by the intact plant and for transfer from root to shoot tissues. The absorption of Ni(2+) by the intact plant and its transfer from root to shoot were inhibited by the presence of Cu(2+), Zn(2+), Fe(2+), and Co(2+). Competition kinetic studies showed Cu(2+) and Zn(2+) to inhibit Ni(2+) absorption competitively, suggesting that Ni(2+), Cu(2+), and Zn(2+) are absorbed using the same carrier site. Calculated K(m) and K(i) constants for Ni(2+) in the presence and absence of Cu(2+) were 6.1 and 9.2 mum, respectively, whereas K(m) and K(i) constants were calculated to be 6.7 and 24.4 mum, respectively, for Ni(2+) in the presence and absence of Zn(2+). The mechanism of inhibition of Ni(2+) in the presence of Fe(2+) and Co(2+) was not resolved by classical kinetic relationships.