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

The absorption of Ni²⁺ 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 μm), 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 Ni²⁺ by the intact plant and its transfer from root to shoot were inhibited by the presence of Cu²⁺, Zn²⁺, Fe²⁺, and Co²⁺. Competition kinetic studies showed Cu²⁺ and Zn²⁺ to inhibit Ni²⁺ absorption competitively, suggesting that Ni²⁺, Cu²⁺, and Zn²⁺ are absorbed using the same carrier site. Calculated K_m and K_i constants for Ni²⁺ in the presence and absence of Cu²⁺ were 6.1 and 9.2 μm, respectively, whereas K_m and K_i constants were calculated to be 6.7 and 24.4 μm, respectively, for Ni²⁺ in the presence and absence of Zn²⁺. The mechanism of inhibition of Ni²⁺ in the presence of Fe²⁺ and Co²⁺ was not resolved by classical kinetic relationships.