Maize and radish sequester excess cadmium and zinc in different ways

The aims were to compare the reactions of radish and maize to excess cadmium (Cd), zinc (Zn) and Cd + Zn in terms of metal extractability and the amounts of metal in roots and shoots bound through phytochelatins (PCs). Seedlings were grown in hydroponics and exposed to 1 muM Cd, 50 muM Zn and their combination for 10 and 15 days. Tissues were extracted with buffer followed by acid. Buffer soluble metal was resolved by small-scale gel filtration. Six buffer extracts removed 87-95% of the Cd from roots of radish and maize, 43-57% of the Zn in their roots, yet 71-86% of the Cd and Zn in shoots. Acid removed most of the remaining metal. Buffer soluble Cd yielded three peaks including a prominent high molecular weight (HMW) PC-based complex and the low molecular weight (LMW) counterpart. In radish these complexes accounted for 69% of the Cd in the roots and 42% of the Cd in shoots, for maize 88% in the roots and 27% in the shoots. Most buffer soluble Zn (83-87%) from radish and maize roots and shoots eluted close to the total volume of the gel filtration column. A small Zn peak (4-7%) corresponding to the HMW Cd-binding complex was found in maize roots, not in radish. PCs were not prominent ligands for Zn in maize and not at all in radish. The ligands for most of the Zn remain unidentified. (C) 2003 Elsevier Ireland Ltd. All rights reserved.