Photoinhibition in lichens depends on cortical characteristics and hydration

Two shade-adapted (Lobaria pulmonaria and Cetraia islandica) and two sun-adapted lichen populations (Xanthoria parietina and Cetraria nivalis) were exposed to three irradiance regimes (1: photosynthetic radiation-PAR, 2: PAR + UV-A, 3: PAR + UV-A + UV-B) and two hydration regimes (1: no hydration, 2: daily hydration) in a growth cabinet for three weeks. Shade-adapted thalli had transparent upper cortices without coloured pigments, whereas sun-adapted thalli had coloured UV-B absorbing cortical pigments masking the photobiont. Manipulation of pigment concentration was the third factor used in the factorial design (1: pigments intact, 2: pigments non-destructively extracted from air-dry living thalli by acetone). Inhibition of the photobiont due to PAR alone was severe in the two shade-adapted populations, but no applied UV wavelength bands caused additional aggravation of photoinhibition. Shade-adapted thalli of the ubiquitous C. islandica were more PAR-susceptible than of the rare old forest lichen L. pulmonaria, suggesting that screening by the mycobiont rather than photobiont characteristics, account for their different success in sun-exposed localities. Hydration of shade-adapted species during exposure reduced their photoinhibition substantially, probably because of moisture-activated repair mechanisms. On the contrary, the sun-adapted X. parietina was most phototolerant in the desiccated state, whereas hydration caused increased photoinhibition. When removing the orange cortical pigment parietin, the photoinhibition in moist thalli was aggravated, confirming a PAR-protective function of parietin. No effects of irradiance treatment, pigment extraction (usnic acid), or hydration level were observed in C. nivalis.