The influence of air pollution on the concentration of airborne elements and on the production of stress-ethylene in the lichen Usnea hirta (L.) Weber em. Mot. transplanted in urban sites in Oulu, N. Finland

The purpose of the present study was to examine the response, as indicated by the production of stress-ethylene, of the lichen Usnea hirta, to a buffered acidic solution and to a solution containing bisulfite ion, both at pH 4.0. In addition, the study investigated the accumulation capacity of U. hirta transplanted either to the vicinity of two streets with slow traffic, or to the vicinity of a highway in the city of Oulu, N. Finland for a short period (45 d), and to analyze the comparative production of stress- ethylene and the amounts of airborne elements accumulated in the thallus. Thalli of U. hirta manifested a high accumulation capacity for K, Ca, Na, Mg, Fe, Zn, and Mn in samples exposed in one of the two streets with slow traffic in Oulu, in comparison with thalli left at the control site in the forest outside of the city. Two of the observed elements, Pb and Ca, exhibited a positive correlation with the concentration of ethylene detected consecutive to the treatment of lichen thalli with H₂O (pH 6.8). The concentrations of stress-ethylene in lichen thalli retrieved from the streets with slow traffic and less cars (14,000 per day), soaked in solutions of NaHSO₃ (pH 4.0), were higher than those produced by thalli retrieved from a site near the highway (with 25,000 cars per day) and by thalli left in the forest. It is suggested that the streets with slow traffic and less cars are more polluted than the highway as a result of bad ventilation conditions and a slow rate of dispersion of airborne pollutants.