The effect of metal diffusion and supply limitations on conditional stability constants determined for durum wheat roots

Conditional stability constants (log K), and binding site densities (Γ_max) for dissolved metals and biota are important input parameters for the Biotic Ligand Model. However, determination of these binding parameters is likely to be influenced by solution kinetics because roots have a large metal-binding capacity and can accumulate metals rapidly. The aim of this study was to determine if the rate of free metal ion diffusion to the root surface, and amount of metal in the bulk solution, is sufficient to accommodate the maximum root–metal accumulation capacity. The extent to which these kinetic limitations affect the magnitude of log K and Γ_max values was also assessed. Seven day old hydroponically grown durum wheat (Triticum turgidum L. var durum, cv ‘Arcola’) were exposed to solutions with p{Cu²⁺}s ranging from 10.54 to 2.26 (~20 °C, pH = 6.0, ionic strength = 0.03 M). Exposure solutions were prepared with and without the metal buffer nitrilotriacetic acid (NTA) so that the total amount of metal in the exposure solution, and net flux of metal to the root, could be varied. The results demonstrate that NTA enhances Cu accumulation at exposure p{Cu²⁺}s between 10 and 6. Comparison of the diffusive flux to the root with the metal flux into the root, for (−NTA) and (+NTA) Cu exposures, showed that the flux of the un-buffered free metal ion to the root was not large enough to accommodate the maximum Cu binding capacity between 10 and 6 p{Cu²⁺} in solution. The total amount of Cu in solution may have limited uptake for exposure p{Cu²⁺}s of 10.01 and 9.01, but the background concentrations of Cu in the control plants prevented definitive conclusions from being made within this exposure range. Similar results were found for Mn and Ni. For Cd, which had lower background concentrations in the roots, the amount of metal in solution did not limit uptake until a p{Cd²⁺} of 10.01. Limiting the supply of Cu²⁺ to the root (i.e. low {Cu²⁺}s with no NTA) caused only a moderate bias in Γ_max values, but suppressed the log K value by 3.44 log units. The log K values for Cd, Mn and Ni, in the absence of NTA, were more similar than expected, which suggests that the kinetics of free ion re-supply to the root surface limited metal uptake, as it did for Cu. Section Editor: T. B. Kinraide