Nitrogen tolerance in the lichen Xanthoria parietina: the sensitive side of a resistant species.

This study investigated if N tolerance in the N-tolerant lichen Xanthoria parietina is influenced by N level of its native environment. Physiological response of thalli of X. parietina grown in a N-poor environment (nitrogen emissions of ca. 2 kg/ha/y) was compared with the response of thalli of X. parietina grown in a N-rich environment (nitrogen emissions of ca. 52 kg/ha/y) and of the N-sensitive species Evernia prunastri and Usnea sp. Lichens were treated with ammonium solutions (concentrations: 0, 25 mM, 50 mM and 250 mM) for 5 hours per day during three days in a week. After each treatment, the maximal PSII efficiency was measured, while localization of ammonium ions and concentrations of K+ and of Mg2+ were measured only after the third treatment. Buffering capacity of lichen extracts was determined. The results showed a more similar behavior between X. parietina from the N-poor environment and the sensitive species E. prunastri and Usnea sp. than between X. parietina from the N-poor environment and the same species from the N-rich environment, suggesting that lacking the original stimulus of high N atmospheric concentration, defense mechanisms are not induced in the resistant species.