Lichen photobionts play a pivotal role in a lichen as they provide the carbohydrates that sustain the symbiosis. Hence, the relationship between thallus water content and photosynthetic efficiency of photobionts is of paramount importance. In a recent study on Flavoparmelia caperata and its isolated photobiont, Trebouxia gelatinosa, it was shown that the latter loses cell turgor and experiences a subsequent decrease of the maximum PSII efficiency (measured as Fv/Fm) at water potentials (?) less negative than in the intact lichen thallus. To verify if this phenomenon occurs in other species, a comparative analysis of water relation parameters and chlorophyll a fluorescence (ChlaF) was carried out during dehydration on the lichens F. caperata, Lobaria pulmonaria and Xanthoria parietina and their isolated photobionts, i.e. T. gelatinosa, Symbiochloris reticulata and T. decolorans, respectively. ? isotherms and ChlaF were analysed to obtain the osmotic potential at full turgor, ? at turgor loss point (?tlp), the bulk modulus of elasticity, and the Fv/Fm trend upon ? decrease. All water relation parameters differed between lichens and isolated photobionts. Notably, ?tlp was less negative in isolated photobionts than in lichens. ? values corresponding to 20% (?20) or 50% (?50) decrease of Fv/Fm were proposed as novel proxies relating the lichen water status to the photosynthetic efficiency of photobionts. Both ?20 and ?50 did not differ between F. caperata and T. gelatinosa, whereas ?20 differed of-5.5 MPa and -3.44 MPa in L. pulmonaria and in X. parietina, respectively, in respect to their isolated photobionts. The shift of ?20 towards more negative ? values in lichens vs their isolated photobionts suggests that Fv/Fm starts to decrease at more negative ? in lichen thalli, supporting the recently formulated hypothesis that during dehydration the photobiont layer of a lichen has a different water status than the surrounding thallus.
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