Oecologia (1992) 92:457 462 Oecologia 9 Springer-Verlag 1992 Original papers Monitoring photosynthetic activity of crustose lichens using a PAM-2000 fluorescence system B. Schroeter 1, T.G.A. Green 2, R.D. Seppelt 3, and L. Kappen 1 Botanisches Institut und Institut ffir Polar6kologie, Universit/it Kiel, Olshausenstrasse 40, W-2300 Kiel, Federal Republic of Germany 2 Biological Sciences, University of Waikato, Hamilton, New Zealand 3 Australian Antarctic Division, Kingston, Tasmania, Australia Received 15 June 1992 / Accepted in revised form 30 July 1992 Summary. CO2 exchange and fluorescence yield of the crustose lichen Buell& frigida were measured in situ by means of a CO2 porometer and a PAM-2000, a newly developed portable fluorescence system. The pulse am- plitude modulation system of the PAM-2000 allows measurements in the field under ambient light, tem- perature and moisture conditions without dark adapta- tion of the sample. CO2 exchange and fluorescence mea- surements 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 AF/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 inhomoge- neous. 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. Key words: Antarctica - Buellia frigida - Fluorescence yield - Net photosynthesis - Water content Recent years have seen a considerable number of publi- cations on field measurements of the CO2 exchange of lichens. Many ecosystems have been covered from the polar regions to rain forests (reviewed by Kershaw 1985; Kappen 1988; see also Green and Lange 1991; Lange et Dedicated to Prof. Dr. W. Nultsch, Marburg, on the event of his 65th birthday Abbreviations. Fro': maximum yield of fluorescenceunder ambient light; Fs: yield of fluorescence at steady state; AF. difference in yield of fluorescence at maximal Fm' and steady state Fs under ambient light; NP: net photosynthesis; PAR: photosynthetically active radiation; PS II: photosystem II Correspondence to: B. Schroeter al. 1991; Schroeter et al. 1991a). Most of this research has been on foliose or fruticose lichens. The few excep- tions mainly concern Antarctic and desert lichens and include laboratory studies on crustose (Lange and Kap- pen 1972) and endolithic species (Kappen and Fried- mann 1983), and field studies ofcrustose, epilithic species by Lange et al. (1970) and Kappen et al. (1990). Crustose lichens pose obvious technical problems when studying gas exchange, including low rate of assimilation and the inability to separate the substrate. Schroeter et al. (1991b) have demonstrated that a specially designed fluorescence system can be used to monitor the photosynthetic activity of a fruticose lichen giving both an indication of rates and period of activity. The system used (FL-1, BBE, Germany), based on a modulated fluorescence system, is designed for unattended, long- term monitoring of lichen activity and thus is limited in the fluorescence parameters that can be measured. Until recently, portable fluorescence systems have always been based on non-modulated systems and were only able to measure in the dark, i.e. dark-adapted samples. Mod- ulated systems such as the PAM-101 (Walz, Germany) as principally described by Schreiber (1986) or the MFMS MK.II (Hansatech, UK) which measure fluores- cence yield under ambient light conditions were essential- ly designed for laboratory studies and are not easily portable (Bolhfir-Nordenkampf et al. 1989). A newly developed modulated fluorescence system (PAM-2000, Walz, Germany), combines the advantages of the Walz PAM-101 system with easy portability. The fluorescence system weighs only 2.8 kg including bat- teries, fibre optics and all required light sources such as modulated red, actinic red 0~=650 nm) or white (L<710nm), saturation pulse (s up to 12000 l.tmol m -2 s- 1 photosynthetically active radiation, PAR) and far-red (peak wavelength L = 735 nm) light as well as a computerised control and data capture system. The fluorescence parameters Fo, Fm, Fv/Fm, Fo', Fro', AF/Fm' = (Fro'-Fs)/Fm'("yield"), Qp and Qn are dis- played or calculated online (for nomenclature see van Kooten and Snel 1990).