THE RELATIONSHIP BETWEEN CADMIUM UPTAKE AND HEAVY-METAL TOLERANCE IN THE LICHEN GENUS PELTIGERA

Intra- and extra-cellular Cd uptake were investigated in members of the lichen genus Peltigera sampled from sites with contrasting metal status. P. membranacea (Ach.) Nyl. from a disused Zn mine, previously shown to be tolerant to Zn-induced inhibitions of photosynthesis, displayed a reduced rate of Cd uptake into the cell compared with Peltigera spp. from unpolluted sites, suggesting metal exclusion was a tolerance mechanism. Although some differences in extracellular Cd uptake were found, these appeared to be a reflection of inter-specific variation rather than environmentally-induced alterations in Cd-binding capacity. Exposure to a mixture of Cd and Mg showed that intracellular Cd uptake was less inhibited by Mg when lichens originated from the Zn mine, also implying that mine-site plants had modified uptake systems. Although intracellular Cd uptake is normally light-stimulated in Peltigera, lichens from the Zn mine lacked this response, suggesting that algal uptake was reduced more than fungal uptake. Photosynthetically Zn-tolerant P. hymenina (Ahc.) Delise from a disused Pb mine, more moderately polluted than the Zn mine, had intracellular Cd uptake rates comparable to those found in lichens from background localities. In addition, treatment of lichens for 1 wk with low concentrations of Zn, previously shown to induce photosynthetic Zn and Cd tolerance, had no effect on intracellular Cd uptake. Tolerance in these instances must have been achieved by some other means. A severe metal stress may be required before metal tolerance by exclusion occurs.