Possible functional roles of cortical depsides and medullary depsidones in the foliose lichen Hypogymnia physodes |
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Authors: | Knut Asbjørn Solhaug Marius Lind Line Nybakken Yngvar Gauslaa |
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Affiliation: | Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, P.O. Box 5003, NO-1432 Ås, Norway |
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Abstract: | Secondary compounds were quantified in 75 thalli of the foliose lichen Hypogymnia physodes collected in habitats along a natural shade-sun gradient from dark spruce forests to sun-exposed sea cliffs. The irradiance in all the 75 lichen sites was estimated from hemispherical photographs. The content of lichen compounds per thallus area correlated positively with irradiance level (r2=0.73), and the mean concentration increased from 6.7% in the spruce forest to 14.4% on sea cliffs. The medullary depsidones, physodic, physodalic and protocetraric acids, constituted >95% of the total pool of extractable secondary compounds, the cortical depsides, atranorin and chloratranorin, represented <5%. Both cortical compounds correlated well with direct and with diffuse radiation, whereas the three medullary compounds correlated better with diffuse than with direct radiation. Mentioned trends are consistent with a solar radiation screening hypothesis of both groups of these colourless compounds occuring as tiny crystals outside fungal hyphae. However, the UV-B protective hypothesis of the compounds was further tested in a lab experiment. Unnaturally high UV-B doses were required to significantly reduce the PSII efficiency (FV/FM) of symbiotic algae. Removal of the major pool of secondary compounds with acetone did not increase photobiont susceptibility to UV-B. Therefore, the main functional role of the UV-B absorbing secondary compounds in H. physodes is hardly UV-B screening. Other roles such as PAR-screening and defence against herbivores and pathogenic organisms are discussed. |
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