Ultraviolet radiation: a double-edged sword in old forest Lobaria lichens-reducing growth while enhancing acclimation

This study examines the effects of three light regimes-1) photosynthetic light (PAR) only, 2) PAR + UV-A, and 3) PAR + UV-A + UV-B radiation-on area and biomass growth in two old forest lichens: the widely distributed Lobaria pulmonaria, noted for rapid growth, and the rarer L. virens, with previously undocumented growth rates but known susceptibility to high light. To overcome the constraints of artificial light sources, we utilized solar filters in sun-exposed field conditions to assess how UV impacts growth and influences mycobiont traits (melanic pigmentation, thallus thickness) and photobiont responses (Chl content, Chl fluorescence, CO₂ uptake). While UV exposure significantly reduced growth rates, it did not impact photobiont processes. Lobaria pulmonaria demonstrated robust melanin synthesis under UV-B, preventing photoinhibition and safeguarding chlorophylls, whereas L. virens, with lower melanin production, showed reduced Chl content and increased vulnerability to solar radiation. Enhanced specific thallus mass, a proxy of water holding capacity, correlated with increased melanic pigmentation, suggesting that UV-B also aids in acclimation of drought tolerance. Despite reduced growth, UV exposure promotes acclimation to environmental stressors, revealing a trade-off between growth and acclimation. These findings challenge previous assumptions regarding UV-B susceptibility in shade-adapted old forest lichens and underscore the intricate interplay between lichen growth and acclimation.