Table III. Estimated binding constants of metal and competitive ions for metal uptake by spinach.

Binding constants were derived by fitting the model, which combines a diffusion equation with a competitive Michaelis-Menten equation (Eq. 7), to the observed data. sd of parameter estimates (in parentheses) was calculated from the correlation matrix, using the SolverAid macro from de Levie (1999). The model R2 (calculated as 1 − SSE/SSTO, with SSE the error sum of squares and SSTO the total sum of squares of log-transformed uptake fluxes) was 0.90 for Cd, 0.94 for Zn, and 0.97 for Ni. n = Number of data points.

Binding Constant	Cd	Zn	Ni
	n = 78	n = 39	n = 43
logKM	8.7a	8.7 (0.1)	4.5 (<0.1)
logKH	5.8 (< 0.1)	5.8 (0.3)
logKMg			2.5 (0.6)

^aThe covariation between logK_Cd and logK_H was too large to fit them together. The binding constant for Cd was calculated as the reciprocal of the K_m at pH 7 and high degree of buffering (complex [Cd-NTA] to free ion concentration ratio of 10⁵).