Osmotic and atmospheric dehydration effects in the lichens Hypogymnia physodes, Lobaria pulmonaria, and Peltigera aphthosa: an in vivo study of the chlorophyll fluorescence induction

Inactivation of photosynthesis during atmospheric and osmotic (highly conce ntrated NaCl or sucrose solutions) dehydration was monitored by measurement of chlorophyll fluorescence induction (OIP-phase, Kautsky-curves) in three lichen species. The induction curves were changed in a very similar way by all three treatments. all dehydration effects were rapidly reversible afte r rehydration. At relatively mild water stress, the rise time to the transi ent peak F-p was prolonged, and the variable part of fluorescence was dimin ished. In addition, at severe water stress, a considerable decline of the F -0 value was observed. For NaCl treatment this effect started at water pote ntials <-8.5 MPa in P. aphthosa, <-12 MPa in H. physodes, and <-21 MPa in L . pulmonaria. Above these water potentials, our observations are in agreeme nt with values from desiccation-tolerant algae, higher plants, and lichens, where an inactivation on the photosystem 2 (PS2) donor side has been postu lated. At very low water potentials, the decrease in F-0 probably monitors changes in the organization of the antenna apparatus of PS2.