Cadmium reduces zinc uptake but enhances its translocation in the cadmium-accumulator, Carpobrotus rossii, without affecting speciation

Interactions between Cd and Zn occur in soils and plants but are inconsistent. This study examined how Cd/Zn interactions influence the growth of Carpobrotus rossii (Haw.) and the accumulation of Cd and Zn in plants. Plants were grown in nutrient solutions containing 5-100 mu M Zn and 0, 5 or 15 mu M Cd. Plant growth and tissue concentrations were measured, and the speciation of Zn within the plant tissues determined using synchrotron-based X-ray absorption spectroscopy. There was an additive negative interaction between Cd and Zn on root growth. Only the highest level of Zn (100 mu M) decreased Cd concentrations in root and shoot tissues (by 40-64%), whilst 100 mu M Zn enhanced Cd translocation at 5 mu M Cd but decreased it at 15 mu M Cd. In contrast, both 5 and 15 mu M Cd decreased Zn concentrations in root and shoot tissues but increased Zn translocation by 30-90%. This interaction was not associated with changes in Zn speciation within the plants, with most Zn associated with oxalate (48-87%). The presence of Zn and Cd resulted in an additive negative effect on root growth, but an antagonistic pattern in their accumulation in shoots of C. rossii.