Hydrated thalli of the lichen Lobaria pulmonaria were either preconditioned to dim irradiance (DI, 5 µmol m -2 s -1 ) or medium irradiance (MI, 200 µmol m -2 s -1 ) for 6 h. After this 6 h period, the thalli were allowed to desiccate under the two respective irradiances. Thereafter, these dry lichens were exposed to high irradiance (HI, 1 000 µmol m -2 s -1 ) for 60 h. After this HI treatment, the maximal photochemical quantum yield (F V /F M ) and the de-epoxidation state of xanthophyll cycle pigments (DEPS) were highest in thalli preconditioned to MI. Hence irradiance in the last hydrated period before sampling is significant for the physiological state of lichens. A standardized irradiance pre-treatment before start of experiments is recommended.
Lichen thalli were exposed to 4 regimes differing in irradiance and duration of irradiation. Photosynthetic efficiency of thalli was monitored by chlorophyll fluorescence parameters and xanthophyll cycle analysis. Maximal quantum yield of photosystem 2 (F V /F M ) decreased gradually with time in long-term treatment. The effect of additional short-term high irradiance (HI) treatment applied each 24 h was not significant. Nevertheless, short-term HI applied repeatedly on thalli kept in the dark led to a significant decrease of F V /F M . Non-photochemical quenching recorded during the long-term treatment corresponded to the content of zeaxanthin (Z). In short-term treatment, however, proportion of Z (and antheraxanthin) to total amount of xanthophyll cycle pigments recovered to the initial values every 24 h after each repeated short-term HI event in thalli kept in dark. Thus duration of irradiation rather than irradiance and frequency of HI events is important for a decrease in primary photosynthetic processes in wet thalli of Lasallia pustulata. Rapidly responding photoprotective mechanisms, such as conversion of xanthophyll cycle pigments, are involved mainly in short-term irradiation events, even at HI.
This study investigated photosynthetic pigments chlorophyll a and b, carotenoids (including xanthophyll cycle pigments) and antioxidants glutathione and tocopherols contents in chloro- (Lobaria pulmonaria, Lasallia pustulata) and cyanolichens (Lobaria scrobiculata, Peltigera canina) from different habitats and of different geographical origin. Lichen thalli were treated with various levels of irradiance (from 100 to 1500 mmol m-2 s-1) for various time periods (from 20 min. to 5 h). The extent of increase of zeaxanthin content after different light treatments differed among chlorolichens, clearly distinguishing the species from light and shade habitats. Irrespective of light treatment, β-carotene content was always higher in cyanobacterial species than in green algal species. Highest canthaxanthin concentration, as well as β-carotene concentration, was found in L. scrobiculata from shade habitat. The content of total glutathione was the same in the same species from different collection sites. On the contrary, contents of a-tocopherol differed in the same species from different collection sites.
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