Changes in in vivo chlorophyll fluorescence quenching in lichen thalli as a function of water content and suggestion of zeaxanthin-associated photoprotection

The function of photosystem II (PSII) during desiccation was investigated via analysis of Chl a fluorescence emission in thalli from Parmelia quercina (Willd.) Vainio, Parmelia acetabulum (Necker) Duby, Ramalina farinacea (L.) Ach., Pseudevernia furfuracea (L.) Zopf., and Evernia prunastri (L.) Ach. Water loss followed the same exponential pattern in all these species, the half time being dependent on species. Desiccation affected the fluorescence parameters. Dark-adapted maximum fluorescence (F_m), instantaneous fluorescence (F_o) and the ratio of variable (F_m–F_o) to F_m were dependent on water content and decreased in two distinct phases: a slow and apparently linear phase, followed by a more steep decline at low water content. Actual PSII photochemical yield (φ_PSII), non-photochemical quenching (NPQ), efficiency of photon capture (φ_exc), and photochemical quenching (q_p) remained nearly constant until 30% relative water content (RWC), decreasing rapidly thereafter. In contrast, increased NPQ appeared to occur only at water content values lower than 20%. Treatment of thalli with dithiothreitol (DTT) effectively reduced NPQ during desiccation and increased susceptibility to photoinhibition caused by exposure to high light as measured by dark recovery of the F_vF_m ratio. HPLC analysis showed that the level of the de-epoxidized xanthophyll cycle pigments antheraxanthin (Anth) and zeaxanthin (Zea) increased during lichen desiccation. The results point towards the existence of a photoprotective mechanism with the involvement of Zea and Anth in non-radiative dissipation of the desiccation-induced excess of energy.