THE ROLE OF METAL-ION BINDING IN MODIFYING THE TOXIC EFFECTS OF SULPHUR DIOXIDE ON THE LICHEN UMBILICARIA...

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.