Distribution and Redistribution of 109Cd and 65Zn in the Heavy Metal Hyperaccumulator Solanum nigrum L.: Influence of Cadmium and Zinc Concentrations in the Root Medium

Heavy metal redistribution is relevant for the quality of edible crops and the suitability of hyperaccumulators for bioremediation. Root-to-shoot transfer via the xylem and redistribution in the aerial parts via the phloem differ between various heavy metals. In general, cadmium is more slowly released to the shoot than zinc (e.g., in wheat, bean, and lupin). However, rapid cadmium transport to the shoot was detected in the hyperaccumulator Solanum nigrum L. This is a key aspect in this article and might be important for bioremediation. The radionuclides Cd-109 and Zn-65 were used to investigate the respective influence of elevated cadmium or zinc in the root medium on the dynamics of the two heavy metals in S. nigrum. Although transport via the xylem to the leaves was similar for Cd-109 and Zn-65, the further redistribution from older leaves to younger leaves, flowers, and fruits via the phloem was far less efficient for Cd-109 than for Zn-65. Furthermore, the redistribution of Cd-109 within the shoot was negatively influenced by increased cadmium (but not by increased zinc) concentrations in the nutrient medium. The redistribution of Zn-65 in the shoot was selectively decreased by increased zinc concentrations (but generally not by cadmium).