Unpigmented lichen substances protect lichens against photoinhibition of photosystem II in both the hydrated and desiccated states

Lichen secondary metabolites have been suggested to play a great diversity of roles in lichen biology by acting as UV screens, antimicrobials, herbivore deterrents or allelopathic compounds. However, evidence is also beginning to accumulate that in addition to these roles, even faintly pigmented or unpigmented lichen substances can reduce photoinhibition caused by high levels of photosynthetically active radiation (PAR). Here we used an acetone rinsing technique to remove lichen substances from four common epiphytic Afromontane macro lichens, Parmelia perlata , Ramalina celastri , Usnea dasaea , and Heterodermia leucomela . Results showed that lichen substances can increase the tolerance of lichens to the photoinhibition of PSII of their photobionts when hydrated, apparently by increasing reflectance. However, in the field, lichens may suffer more photoinhibition when fully or partially desiccated. Here we show for the first time that, except for the atranorin-containing Heterodermia , lichen substances can also protect desiccated lichens. Interestingly, removal of substances has no effect on reflectance when lichens are dry, suggesting that lichen substances protect photobionts in other ways.