Cadmium accumulation by durum wheat roots in ligand-buffered hydroponic culture: uptake of Cd-ligand complexes or enhanced diffusion?

The objectives of this study were to test the free ion model of Cd phytoavailability in solutions where the dissolved free ion was buffered by SO4 or EDTA and to examine if Cd2+ diffusion to roots might be limiting to its accumulation in roots. At similar Cd2+ activities, solutions that contained EDTA or a higher nominal SO4 concentration (with SO4 supplied as K2SO4, and more total Cd) resulted in greater accumulation of Cd by roots than predicted by the Cd2+ activity. These solutions were predicted using MINEQL(+) (chemical speciation software) to contain greater concentrations of Cd complexes (CdEDTA(2-) or CdSO40 ((aq))) in solution; when SO4 was supplied by MgSO4, the increase in Mg2+ suppressed enhanced accumulation of Cd. Estimates of diffusion of Cd to the roots through the boundary layer and flux of Cd across the root surface, for this study, were similar in magnitude across the range of solution Cd concentrations studied, particularly when Cd uptake was assumed to occur at the root tips only. We conclude that CdEDTA(2-) and CdSO4(aq)0 could have alleviated this diffusion limitation by buffering Cd2+ at the root surface and (or) been taken up directly as a complex.