Light screening in lichen cortices can be quantified by chlorophyll fluorescence techniques for both reflecting and absorbing pigments |
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Authors: | Knut Asbjørn Solhaug Per Larsson Yngvar Gauslaa |
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Institution: | 1. Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, P.O. Box 5003, 1432, ?s, Norway
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Abstract: | Lichens, representing mutualistic symbioses between photobionts and mycobionts, often accumulate high concentrations of secondary
compounds synthesized by the fungal partner. Light screening is one function for cortical compounds being deposited as crystals
outside fungal hyphae. These compounds can non-destructively be extracted by 100% acetone from air-dry living thalli. Extraction
of atranorin from Physcia aipolia changed the lichen colour from pale grey to green in the hydrated state, whereas acetone-rinsed and control thalli were all
pale grey when dry. Removal of parietin from Xanthoria parietina changed the colour of desiccated thalli from orange to grey. Colour changes were quantified by reflectance measurements.
By a new chlorophyll fluorescence method, screening was assessed as the decrease in incident irradiance (PAR) necessary to
reach identical effective quantum yields of PSII (ΦPSII) in acetone-rinsed and control thalli. Thereby, we estimated a screening efficiency due to cortical atranorin crystals at
61, 38, and 40% of blue, green and red light, respectively, whereas parietin screened 81, 27 and 1% of these wavelength ranges.
Removal of atranorin caused similar levels of increased photoinhibition for P. aipolia in blue, green and red light, whereas parietin-deficient thalli of X. parietina exhibited increased photoinhibition with decreasing wavelengths. Atranorin possibly prevents water from entering the spaces
between the hyphae in the cortex. The air-filled cavities with white atranorin crystals reflect excess light, whereas the
yellow compound parietin absorbs excess light. Thereby, both atranorin and parietin play significant photoprotective roles
for symbiotic green algae, but with compound-specific screening mechanisms. |
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