Monitoring photosynthetic activity of crustose lichens using a PAM-2000 fluorescence system

CO₂ exchange and fluorescence yield of the crustose lichen Buellia frigida were measured in situ by means of a CO₂ porometer and a PAM-2000, a newly developed portable fluorescence system. The pulse amplitude modulation system of the PAM-2000 allows measurements in the field under ambient light, temperature and moisture conditions without dark adaptation of the sample. CO₂ exchange and fluorescence measurements were well correlated when measured under natural conditions in continental Antarctica during a drying cycle of melt-water-soaked lichen thalli. It was shown that the fluorescence parameter ΔF/Fm' is a measure of the photosynthetic activity of the lichen. It proved possible, using the PAM-2000, to differentiate the physiological performance of the thallus centre and the marginal lobes. The distribution of water in the thallus during a drying cycle was shown to be inhomogeneous. The photosynthetic rates of B. frigida calculated on an area basis are comparatively high and indicate that this lichen is well adapted to its habitat conditions in this part of continental Antarctica.