Table II. Kinetic parameters for uptake of Cd, Zn, and Ni by spinach and tomato in unbuffered solutions or in solutions buffered with metal complexes.

The K_m was determined based on uptake in unstirred, unbuffered solutions (K_m*) or in solutions that were buffered with complexes and in which uptake was likely internalization limited (K_m). The permeability P corresponds to the slope of the uptake curve at low concentrations and is calculated as the ratio of F_max to K_m, with F_max expressed on a surface area basis (based on a specific root area of 200 cm² per g root fresh weight [RFW]). Data for solutions with 2 mm Ca(NO₃)₂ at pH 6.0.

Plant Species/Element	Fmax	Unbuffered		Buffereda
		Km*	P	Km	Pmb
	nmol (g RFW)−1 h−1	μm	cm s−1	μm	cm s−1
Spinach
Cd	99.4 (0.2)c	2.83 (0.01)	4.9 × 10−5	0.0021d	6.6 × 10−2
Zn	89.6 (0.1)	1.43 (0.02)	8.7 × 10−5	0.0029d	4.5 × 10−2
Ni	460 (1)	36.8 (0.1)	1.7 × 10−5	22–57	1.1–3.0 × 10−5
Tomato
Cd	83.4 (0.1)	1.49 (0.01)	7.8 × 10−5	0.0033d	3.5 × 10−2
Zn	190 (3)	4.72 (0.26)	5.6 × 10−5	0.0040d	6.5 × 10−2
Ni	270 (1)	19.8 (0.1)	1.9 × 10−5	11–36	1.1–3.3 × 10−5

^aFor Cd and Zn: buffered at a ratio of metal-NTA complex to free ion concentration of 10⁵; for Ni: range of K_m derived for solutions buffered at complex to Ni²⁺ concentration ranging between 30 and 100.  ^bAssumed to correspond to the membrane permeability P_m since uptake was likely internalization limited.  ^csds (between brackets) were calculated from the correlation matrix, using the SolverAid macro from de Levie (1999).  ^dsd estimated at less than 0.1 nm, but this does not take into account the uncertainty in the complexation constants used to calculate the solution speciation of the buffered solutions.