Response of two wetland plant species to Cd exposure at low and neutral pH

Emerged and submerged plants are used in treating various metal-containing wastewaters, such as stormwater (neutral pH) and acid mine drainage (low pH). The aim was to investigate the appearance of a set of possible mechanisms to detoxify Cd in plants and whether their appearance differ due to the surrounding pH and/or plant type. One emergent (Carex rostrata) and one submerged (Elodea canadensis) macrophyte were exposed to 0, 0.05, and 0.5 μM Cd for 3 d in hydroponic solutions at pH 3.5 and 6.9. We analysed cadmium accumulation, thiol-rich peptide concentrations and cell wall-bound Cd in plants, organic acid content and pH change in surrounding water cation exchange capacity (CEC) of plant tissue. Both plant species accumulated Cd in their tissues, and thus did not exclude it and C. rostrata decreased the relative Cd distribution to its shoots with increasing Cd addition. In both species, Cd was immobilized through cell wall binding, and thiol-rich peptides synthesized in the presence of Cd that may participate in Cd binding. In addition, E. canadensis increased its CEC by synthesizing new metal-binding sites in the cell walls. Organic acid composition in surrounding water did not change with Cd addition and had no effect on Cd detoxification. However, E. canadensis increased the surrounding pH from pH 3.5 in the presence of Cd; the surrounding pH did not, however, influence the detoxification mechanisms.