Influences of extracellular calcium and iron on membrane sensitivity to bisulphite in the mosses Pleurozium schreberi and Rhytidiadelphus triquetrus

Potassium leakage from shoots of the terricolous mosses Pleurozium schreberi (Brid.) Mitt. and Rhytidiadelphus triquetrus (Hedw.) Warnst. was studied under laboratory conditions using NaHSO3 solutions to simulate exposure to SO, pollution. To investigate the influence of extracellular cations on bisulphite-induced K+ leakage the moss samples were pretreated with solutions of CaCl2, FeCl3 or Na-2-EDTA. Pretreatment with Ca2+ ions significantly increased the exchangeable Ca2+ concentration of the tissues and led to a reduction of K leakage in the following incubation with bisulphite. Fe3+ pretreatment also increased the extracellular concentration of this element but accentuated bisulphite-induced K+ leakage from both mosses. When no metal pretreatment was given the bisulphite incubation caused greater K+ leakage from P. schreberi than from R. triquetrus, probably reflecting the higher natural iron content and lower natural calcium content of P. schreberi than R. triquetrus. Pretreatments with Na-2-EDTA or NiCl2 solution displaced some of the natural exchangeable Ca2+ and led to increased K+ leakage during bisulphite incubation. The protective effect of Ca2+ against bisulphite is interpreted as a stabilization of membrane structure by this cation, whereas Fe3+ may contribute to membrane instability, perhaps by stimulating peroxidation of its component lipids. Despite its negative effect on membrane integrity, pretreatment with Fe3+ also had a beneficial effect by stimulating loss of (probably through oxidation) bisulphite from the incubation solution. These results are in agreement with the field observation that calcareous soils form a refuge for R. triquetrus in SO2-polluted regions. In contrast, the strict calcifuge P. schreberi may derive some protection against SO2 by adsorbing modest quantities of extracellular Fe3+ from the underlying soil, which renders the pollutant harmless by oxidation in the cell wall free space.