THE ROLE OF METAL‐ION BINDING IN MODIFYING THE TOXIC EFFECTS OF SULPHUR DIOXIDE ON THE LICHEN UMBILICARIA MUHLENBERGII

K⁺ efflux from Umbilicaria muhlenbergii was not affected following the uptake of known amounts (< 20 μmol g^-1 ) of Ca²⁺ , Mg²⁺ , Sr²⁺ and Zn²⁺ , but was increased in the cases of Cu²⁺ and Pb²⁺ . In a number of samples that had taken up increasing amounts of Cu²⁺ , a discontinuity in K⁺ release during metal incubation correlated with a decrease in subsequent ¹⁴ C fixation. Some of the binding sites associated with the first phase of Cu²⁺ uptake (up to 12 μmol g^-1 ) are interpreted to occur near, on or within the algal cells. The larger K⁺ loss associated with higher Cu²⁺ uptake levels indicated that a component of the second Cu²⁺ uptake phase involved binding at or penetration of the fungal membranes. The uptake of Sr²⁺ , Ni²⁺ and Zn²⁺ conferred some protection to samples subsequently exposed to 75 p.p.m. aqueous SO₂ for 1 h, while Mg²⁺ , and Ca²⁺ had no effect. The combined effects of Cu²⁺ uptake and SO_S exposure (and also Pb²⁺ and SO₈ ) were approximately cumulative. Finally, the results are explained by reference to a classification that separates metal ions into three chemically and biologically significant categories. It is concluded that class A metal ions (those with a preference for ionic interactions) and borderline metal ions with class A character tend to protect lichens against SO₂ damage, while borderline metal ions with class B character (a preference for covalent interactions) have the opposite effect.