Effect of Cl on Cd uptake by Swiss chard in nutrient solutions

Swiss chard (Beta vulgaris L., cv. Fordhook Giant) was grown in nutrient solution with C1 concentrations varying between 0.01 mM and 120 mM. Solution Na concentration and ionic strength were maintained in all treatments by compensating with NaNO3. All solutions contained Cd (50 nM, spiked with Cd-109). Three different Cd2+ buffering systems were used. In one experiment, Cd2+ activity was unbuffered; its activity decreased with increased C1 concentration as a result of the formation of CdCln2-n species. In the other experiments, Cd2+ activity was buffered by the chelator nitrilotriacetate (NTA, 50 mu M) and ethylene-bis-(oxyethylenenitrilo)-tetraacetate (EGTA, 50 mu M) at about 10(-9) M and 10(-11) M respectively. Plant growth was generally unaffected by increasing C1 concentrations in the three experiments. In unbuffered solutions, Cd concentrations in plant tissue decreased significantly (p<0.01) (approximately 2.4-fold) as solution C1 concentration increased from 0.01 mM to 120 mM. However, this decrease was smaller in magnitude than the 4.7-fold decrease in Cd2+ activity as calculated by the GEOCHEM-PC program for the same range of C1 concentrations. In solutions where Cd2+ activity was buffered by NTA, Cd concentrations in plant tissue increased approximately 1.4-fold with increasing C1 concentration in solution, while the Cd2+ activity was calculated to decrease 1.3-fold. In solutions where Cd2+ activity was buffered by EGTA, Cd concentrations in the roots increased 1.3-fold with increasing C1 concentration in solution but there was no effect of C1 on shoot Cd concentrations. The data suggest that either CdCln2-n species can be taken up by plant roots or that CI enhances uptake of Cd2+ through enhanced diffusion of the uncomplexed metal to uptake sites.