PHYSIOLOGICAL RESPONSE OF LICHENS TO LONG-TERM AND SHORT-TERM NITROGEN TREATMENTS UNDER CONTROLLED CONDITIONS

Lichens are among the most sensitive organisms to nitrogen (N) pollution at the ecosystem level. They respond differently depending on their functional traits, N form and dose, and exposure time. Several authors have considered N tolerance in lichens, however many questions on the topic are still unsolved . The Whim Bog manipulation site with wet and dry deposition to ombrotrophic bog vegetation allows long- and short-term experiments on in situ and transplanted lichens in a controlled environment. 1) Long-term treated samples of C. portentosa exposed to wet deposition were collected and their physiological parameters (pH, gas exchange, photosynthetic parameters, vitality index) as well as algal ultrastructural characteristics were analyzed. The role of potassium (K) and phosphorous (P) in alleviating N toxicity symptoms was considered. Algal cells showed changes in ultrastructure in response to all treatments. Samples receiving P and K showed increased activities of the algal partner. Thallus pH was shown to be influenced by different forms and concentrations of N. 2) Thalli of an N-sensitive lichen (Evernia prunastri) and an N-tolerant one (Xanthoria parietina) were collected from clear areas and transplanted along the NH3 transect. Their physiological response (Fv/Fm) to short-term NH3 exposure was compared with their frequency of occurrence along an NH3 field gradient. Both frequency and Fv/Fm of E. prunastri decreased abruptly above 3 mg m-3 NH3, suggesting direct adverse effects of NH3 on its photosynthetic performance. In contrast, the frequency of X. parietina increased with NH3, despite the decreased capacity of its photosystem II above 50 mg m-3 NH3, suggesting that the ecological success of X. parietina at ammonia-rich sites might be related to indirect effects of increased N (NH3) availability. These results link physiological and morphological effects to the ecological consequences of excess N, allowing more integrated responses to pollution, and offering important clues for future Research.