Chlorophyll fluorescence characteristics of the cyanobacterial lichen Peltigera rufescens under field conditions II. Diel and annual distribution of metabolic activity and possible mechanisms to avoid photoinhibition

Photosystem IT fluorescence of the terrestrial cyanobacterial lichen Peltig era rufescens, together with microclimate parameters, was recorded continuo usly over a complete year at two quasi-natural growing sites in a xerotherm ic steppe formation in the Botanical Garden Wurzburg. From the fluorescence data, metabolically active phases of the poikilohydrous lichen could easil y be distinguished from desiccation or frost-induced inactivity. According to the fluorescence signal patterns, the daily time courses of lichen perfo rmance could be grouped in four response types: (1) metabolically active (i .e., measurable quantum yield) throughout the day due to favourable hydrati on: (2) lichen active at dawn due to nightly rain or dew condensation with subsequent drying; (3) activation during the daylight hours through rain or thawing; (4) no photosystem II activity recognisable all day. The seasonal distribution of these types is analysed. Overall, the lichen remained inac tivated for 46.5% of the total year, it was photosynthetically active in th e light for 25.6% and hydrated during the nights for 27.9% of the time. Earlier laboratory experiments had shown that dry thalli of P. rufescens we re not affected by high Light, but, in the hydrated state, became drastical ly photoinhibited when suddenly exposed to high light stress. However, no, or only minor, photoinhibition was recognisable during the field measuremen ts. We calculated light receipt by the lichen in nature under both dry and hydrated conditions. For the longest periods of light stress, the lichen wa s well protected through desiccation. Nevertheless, substantial periods of time occurred when the hydrated thalli were exposed to high light and some form of photoprotection must have taken place. There was a clear correlatio n between light receipt and thallus content of certain carotenoids (especia lly the ratio between concentrations of canthaxanthin and beta-carotene) an d the hypothesis is supported that canthaxanthin formation is involved in p hotoprotection. Experiments under controlled conditions with P. rufescens revealed a comple x relationship between gross photosynthetic CO2 uptake and relative electro n transport as obtained from fluorescence measurements. In particular, quan tum yield overestimated CO2 assimilation when high light occurred in combin ation with CO2 diffusion being inhibited by suprasaturation of the thallus with water. Thus, we could not use the fluorescence data to predict carbon gain of the lichen under field conditions, and possible reasons are discuss ed.