THE ROLE OF EDTA IN LEAD TRANSPORT AND ACCUMULATION BY INDIAN MUSTARD

Indian mustard (Brassica juncea) plants exposed to Pb and EDTA in hydr oponic solution were able to accumulate up to 55 mmol kg(-1) ph in dry shoot tissue (1.1% [w/w]). This represents a 75-fold concentration of Pb in shoot tissue over that in solution. A threshold concentration o f EDTA (0.25 mM) was found to be required to stimulate this dramatic a ccumulation of both Pb and EDTA in shoots. Below this threshold concen tration, EDTA also accumulated in shoots but at a reduced rate. Direct measurement of a complex of Pb and EDTA (Pb-EDTA) in xylem exudate of Indian mustard confirmed that the majority of Pb in these plants is t ransported in coordination with EDTA. The accumulation of EDTA in shoo t tissue was also observed to be directly correlated with the accumula tion of Pb. Exposure of Indian mustard to high concentrations of ph an d EDTA caused reductions in both the transpiration rate and the shoot water content. The onset of these symptoms was correlated with the pre sence of free protonated EDTA (H-EDTA) in the hydroponic solution, sug gesting that free H-EDTA is more phytotoxic than Pb-EDTA. These studie s clearly demonstrate that coordination of Pb transport by EDTA enhanc es the mobility within the plants of this otherwise insoluble metal io n, allowing plants to accumulate high concentrations of Pb in shoots. The finding that both H-EDTA and Pb-EDTA are mobile within plants also has important implications for the use of metal chelates in plant nut ritional research.