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Introduction: Chlorophyll fluorescence induction (FI) measured with photosynthetic samples (higher plants, algae, cyanobacteria) within the first seconds of intensive light excitation (thousands of mmol photons m-2 s -1) reveals characteristic O-J-I-P transient. Parameters calculated from the transient are often used for detection of plant stress on the photosystem (PS) level (e.g. Strasser et al., 2000). We investigated the FI in lichen thalli and isolated lichen photobionts. Material and Methods: Thalli of foliose lichen species Lasallia pustulata and Umbilicaria hirsuta were collected in the field. The lichen photobiont strain, Trebouxia erici Ahmadjian, as a representative of green alga genus presented in both lichen species, was cultured in BBM3N medium at 20 C at the regime 16h light/8h dark. FI was measured using Plant Efficiency Analyser (Hansatech, Norfolk, UK). Intensity of red exciting light was about 6000 ľmol photons m-2 s-1. Results and Conclusions: FI of predarkened lichen species revealed the P step splitted into two distinct steps appearing at 200 ms and 1-2 s. Similar steps in FI (called H and G, respectively) have been reported for some foraminifers (Tsimilli-Michael et al., 1999). In lichens, the G step was found markedly and reversibly reduced after photoinhibitory treatment (1500 mmol photons m-2 s -1, PAR; 50 min; 5 C) while relative heights of the other steps in FI were retained. The FI measured with the photobiont suspension revealed again the O-J-I-H-G transient. Whereas the H and G steps remained discriminated in FI in the presence of methylviologen, an electron acceptor behind PSI, they were merged in the presence of artificial PSII electron acceptors. References: Strasser RJ, Srivastava A, Tsimilli-Michael M. 2000. The fluorescence transient as a tool to characterize and screen photosynthetic samples. In: Yunus M, Pathre U, Mohanty P, eds. Probing photosynthesis: mechanisms, regulation and adaptation. London UK: Taylor & Francis, 445-483. Tsimilli-Michael M, Pecheux M, Strasser RJ. 1999. Light and heat stress adaptation of the symbionts of temperate and coral reef foraminifers probed in hospite by the chlorophyll a fluorescence kinetics. Z. Naturforsch. 54c: 671-680. Acknowledgements: Supported by GA ČR, grant No. 522/03/0754).
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