Do polyamines alter the sensitivity of lichens to nitrogen stress?

The sensitivity of lichens measuring photosynthetic efficiency and polyamines as modulator of nitrogen stress tolerance was investigated. Two lichen species with a markedly different tolerance to nitrogen compounds, namely Evernia prunastri (L.) Ach. and Xanthoria parietina (L.) Th.Fr., were incubated with deionized water (control) and solutions of KNO3, NH4NO3 and (NH4)2SO4 and then exposed to different light conditions. The Fv/Fm parameter (maximum quantum efficiency of photosystem II) was used as stress indicator. The results showed that Fv/Fm values, in the produced experimental conditions, were independent from the light gradient. Photosynthetic efficiency of E. prunastri was impaired by high ammonium concentrations, while nitrate had no effect; X. parietina was hardly influenced by nitrogen compounds. External supply of polyamines reduced the sensitivity of E. prunastri, while polyamine inhibitors reduced the tolerance of X. parietina to NH4+, suggesting that polyamines play an important role in modulating the sensitivity/tolerance to nitrogen stress.