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THE ROLE OF METAL-ION BINDING IN MODIFYING THE TOXIC EFFECTS OF SULPHUR DIOXIDE ON THE LICHEN UMBILICARIA MUHLENBERGII: II. 14 C-FIXATION STUDIES.

Total 14 C fixation in Umbilicaria muhlenbergii was not significantly affected at levels near the capacity of the first binding site (c. 12 μmol g-1 ) of class A metal ions or borderline metal ions with class A characteristics: i.e. Sr2+ , Mg2+ , Ca2+ , Ni2+ , and Zn2+ . Although both Cu2+ and Pb2+ are borderline ions with significant class B character, only Cu2+ greatly reduced fixation. However Ni2+ and Pb2+ , as well as Cu2+ , induced increased photosynthate release from lichen samples. Protection against damage due to SO2 exposure was rendered by the uptake of class A ions and even borderline ions except those normally accumulated intracellularly (Mg2+ , Zn2+ ) or possessing the capacity to penetrate cells readily (Cu2+ ). Of the borderline ions tested, Pb2+ , Cu2+ and Ni2+ , as well as SO2 , induced changes resulting in the incorporation of less 14 C into ribitol and more into sucrose and other sugars during photosynthesis by the lichen samples. The presence of the class A ion Ca2+ caused a shift in the opposite direction. The presence of Ca2+ , Ni2+ , Pb2+ , but not Cu2+ , on the lichen tended to inhibit the large SO2 -induced shift away from ribitol production. The threshold levels of various metals reported from field studies to result in visible damage are discussed in terms of the observed photosynthetic perturbations, K2+ leakage and factors determining metal uptake by lichens. Finally, the ecological significance of the ability of Ca2+ to protect lichens against the damaging effects of air pollution is discussed.