Chlorophyll fluorescence characteristics of the cyanobacterial lichen Peltigera rufescens under field conditions: I. Seasonal patterns of photochemical activity and the occurrence of photosystem II inhibition

Abstract

Photosystem (PS) II fluorescence of the cyanobacterial lichen Peltigera rufescens, together with microclimate parameters (light, temperature, relative humidity, and rainfall), was recorded over a complete year (September 1992 - August 1993). Measurements were made on thalli at two quasi-natural growing sites in a xerothermic steppe formation in the Botanical Garden, Würzburg. The sites spanned the natural habitat range for the species, one being partly shaded whilst we increased exposure at the other by removing the steppe canopy. Chlorophyll fluorescence parameters were automatically determined at 20 minute intervals using a PAM-2000 fluorometer. From the fluorescence data, metabolically active phases of the poikilohydrous lichen could clearly be distinguished from dormancy. In the majority of cases, dormancy could be attributed to desiccation. During winter, frost inhibited activity completely at temperatures below −5°C. Metabolic activity of the lichen occurred over a wide range of temperatures and light conditions including periods of very high light when photoinhibitory damage might have been expected. However, values of the optimal quantum efficiency of PS II (determined under low light at dawn) showed no depression (photoinhibition) except when metabolic activity of the lichen had been severely curtailed by frost and/or extended drought. The inhibition was reversed after even brief periods of normal metabolic activity. Peltigera rufescens, therefore, seemed to be well adapted to its natural environment and showed little photoinhibition as long as it frequently hydrated and became metabolically active.