Secondary metabolites of the lichen Hypogymnia physodes (L.) Nyl. and their presence in spruce (Picea abies (L.) H. Karst.) bark

Abstract

Lichen species typically have a characteristic profile of secondary metabolites. Dense populations of Hypogymnia physodes growing frequently as epiphytes on tree branches have harmful effects on the host, likely due to their secondary compounds, which were undetected in tree tissues until now. The aim of the present study was to re-characterise the suite of secondary metabolites of H. physodes thalli and to estimate their translocation into spruce (Picea abies) bark. Thallus and bark extracts were compared using ultra-performance liquid chromatography–tandem mass spectrometry (UPLC–MS/MS). The compounds were identified based on their UV, MS and MS/MS spectra as well as retention factors of their TLC analysis. In addition to the previously described secondary metabolites (protocetraric, physodalic, 3-hydroxyphysodic, physodic, and 2′-O-methylphysodic acids, atranorin and chloroatranorin) of H. physodes, further three were identified in its thalli: conphysodalic, 4-O-methylphysodic and α-alectoronic acids. Fragmentation patterns from the negative ionisation of each compound were proposed, some of which were described for the first time. Among all of the detected lichen substances, a few, e.g., physodalic, 3-hydroxyphysodic, physodic acids and atranorin, were present in the bark of spruce branches that were abundantly colonised by lichen. The newly identified compounds of H. physodes thalli may belong to its constant or accessory secondary metabolites. These compounds may be useful in the chemotaxonomic classification of this species. The presence of some lichen substances in spruce bark confirmed their ability to penetrate host tissues. These data suggest that H. physodes compounds may cause long-term effects on spruces in nature.