ACCUMULATION OF CADMIUM BY DURUM WHEAT ROOTS: BASES FOR CITRATE-MEDIATED EXCEPTIONS TO THE FREE ION MODEL

摘要

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 Cd~(2+) 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 Cd~(2+) concentrations but not lower relative to the same Cd~(2+) concentrations in solutions containing 0 mM citrate. Elevating Ca~(2+) and Mg~(2+) 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 Cd~(2+) 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 Cd~(2+). 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.