In order to clarify the role of symbiotic association in drought tolerance of photosynthetic partners in lichens, responses to air-drying treatments in a green-algal lichen (Ramalina yasudae Rasänen) and its green algal photobiont (Trebouxia sp.) were studied. Responses to dehydration in the isolated Trebouxia sp. were different from those in the lichen. R. yasudae. Dehydration induced quenching of PS II fluorescence (d-NPQ) was less in the isolated Trebouxia sp. compared with that in R. yasudae, suggesting that a substance(s) or a mechanism(s) to dissipate absorbed light energy to heat was lost by the isolation of the photobiont, and the air-dried isolated Trebouxia sp. showed a higher sensitivity to photoinhibition than R. yasudae. We assayed the effects of various substances extracted from R. yasudae thalli on the fluorescence of Trebouxia sp. And we found the d-NPQ-type fluorescence quenching in the Trebouxia sp. was accelerated if they were dried with arabitol which was the main component of the water extract of R. yasudae. These results support the idea that arbitol triggered the photoprotection mechanism under drought conditions of the photobiont symbiotic with the mycobionts.