MECHANISMS OF CADMIUM MOBILITY AND ACCUMULATION IN INDIAN MUSTARD

Indian mustard (Brassica juncea L.), a high biomass crop plant, accumu lated substantial amounts of cadmium, with bioaccumulation coefficient s (concentration of Cd in dry plant tissue/concentration in solution) of up to 1100 in shoots and 6700 in roots at nonphytotoxic concentrati ons of Cd (0.1 mu g/mL) in solution. This was associated with a rapid accumulation of phytochelatins in the root, where the majority of the Cd was coordinated with sulfur ligands, probably as a Cd-S-4 complex, as demonstrated by x-ray absorption spectroscopy. In contrast, Cd movi ng in the xylem sap was coordinated predominantly with oxygen or nitro gen ligands. Cd concentrations in the xylem sap and the rate of Cd acc umulation in the leaves displayed similar saturation kinetics, suggest ing that the process of Cd transport from solution through the root an d into the xylem is mediated by a saturable transport system(s). Howev er, Cd translocation to the shoot appeared to be driven by transpirati on, since ABA dramatically reduced Cd accumulation in leaves. Within l eaves, Cd was preferentially accumulated in trichomes on the leaf surf ace, and this may be a possible detoxification mechanism.