Accumulation of cadmium by durum wheat roots: Bases for citrate-mediated exceptions to the free ion model

The accumulation of Cd in durum wheat (Triticum turgidum) roots from hydroponic solutions, with the proportion of total Cd (8.9-445 nM Cd) as Cd2+ varied by the addition of citrate, was determined to test the free-ion model (FIM) of metal bioavailability for higher plants. Calcium, Mg, and K were also varied. Citrate enhanced root-Cd accumulation at higher Cd2+ concentrations but not lower relative to the same Cd2+ concentrations in solutions containing 0 mM citrate. Elevating Ca2+ and Mg2+ concentrations in the citrate solution to the same as those in control solutions alleviated some of the citrate-mediated enhancement but not all. Solutions containing 66% less Ca or Mg than control but the same Cd2+ concentration and no citrate also resulted in increased root Cd. Elevated K+ did not influence Cd accumulation. Regression relationships between root-Cd accumulation and total Cd in solution were similar for the control and pooled amended solutions, whereas they were different for root-Cd accumulation and solution Cd2+. These results contribute to the growing body of evidence that the FIM alone is likely insufficient to predict plant accumulation of metals from soils, although it may be a useful probe for the mechanistic bases of metal bioavailability.