PREDICTABILITY OF THE COMBINED EFFECTS OF SULFUR-DIOXIDE AND NITRATE ON THE GREEN-ALGAL LICHEN RAMALINA-FARINACEA

The interactive effects of SO2 and NO3- on the green-algal lichen Rama lina farinacen (L.) Ach. were investigated using the degree of chlorop hyll phaeophytinization and ultrastructural damage to the Trebouxia ph otobiont. Thalli were submitted to several factorial design experiment s in which they were exposed to mean SO2 concentrations ranging from 1 18 to 140 nmol.mol(-1) plus or minus aqueous solutions of NO3- at eith er 50 or 1000 mu mol.L-1, for 6-14 days. Interactive effects of SO2 an d NO3- were synergistic when the effects of each individual pollutant were slight. However, this pattern reverted to additive effects, and s equentially to antagonistic effects as the detrimental impact of SO, a lone became more severe. Despite this transition, neutralization of th e damaging impact of SO2 by simultaneous treatment with NO3- was not o bserved. The results show that interactions between SO2 and NO3- were variable, but predictable on the basis of the expected effects followi ng exposure to each pollutant alone. The coincidence of this empirical pattern with those observed in previous studies suggests involvement of unspecific causes. The wider ecological significance of interaction s between SO2 and NO3- is discussed in the light of other findings wit h respect to the effects of air pollution on lichens in the field.