Differential responses of certain lichen species to sulfur-containing solutions under acidic conditions as expressed by the production of stress-ethylene.

To determine whether fluctuations in the concentration of ethylene produced by lichens exposed to sulfur-containing solutions at a low pH correlate with the tolerance/sensitivity of these lichens to air pollution, we measured the amount of ethylene produced by thalli soaked in H2SO4 and NaHSO3. The exposure of Hypogymnia physodes, Cladina stellaris, and Bryoria fuscescens to H2SO4 at a pH ranging between 4.0 and 2.0 did not produce changes in the concentration of ethylene in comparison with samples wetted with H2O at pH 6.8. The exposure of two pendulous lichens, Usnea hirta and Alectoria sarmentosa, to 1.0 and 5.0 mM H2SO4 at pH 2.7 and 2.0, respectively, stimulated only a slight increase of ethylene production, whereas another pendulous lichen, Bryoria fremontii, exposed to H2SO4 at pH 4.0-2.0 decreased its production of ethylene. The soaking of H. physodes, U. hirta, C. stellaris, and A. sarmentosa thalli in NaHSO3 at pH 4.0 gradually increased the production of ethylene. The exposure of B. fremontii and B. fuscescens to low NaHSO3 concentrations depressed the production of ethylene in these lichens. The indifference of H. physodes to H2SO4 under strong acidic conditions correlated with its resistance to SO2 in the air. In accordance with a model by D.M. Reid (In "Effects of Atmospheric Pollutants on Forests, Wetlands and Agricultural Ecosystems" (T.C. Hutchinson and K.M. Meema, Eds.), Vol G 16, pp. 241-245. NATO ASI Series, Springer-Verlag, Berlin and Heidelberg, 1987) referring to higher plants, it is suggested that sulfur-containing solutions under acidic conditions increase the solubility of particles containing heavy metals entrapped among the mycobiont hyphae in lichens. This may lead to an increase of the production of endogenous ethylene in lichens as they are exposed to sulfur-containing chemicals, to acidic rain, or to heavy metal-polluted air.