Abstract
In order to clarify the role of symbiotic association in desiccation tolerance of photosynthetic partners in lichens, responses to dehydration in a chlorolichen (Ramalina yasudae) and its photobiont (Trebouxia sp.) was studied. Responses to dehydration in the isolated Trebouxia sp. were different from R. yasudae. That is, (1) the photosystem II (PSII) reaction was totally inhibited in R. yasudae when photosynthesis was completely inhibited by desiccation, but it remained active in Trebouxia sp. (2) Dehydration-induced quenching of PSII fluorescence was smaller in the Trebouxia sp. compared to that in R. yasudae. (3) The isolated Trebouxia sp. showed a higher sensitivity to photoinhibition than R. yasudae in the dehydrated condition. We analyzed about the water extract of R. yasudae and found out that arabitol which contained large amount in water extract accelerated quenching of PSII fluorescence of Trebouxia in the dehydrated state. The phenomenon was confirmed by the analysis of picosecond time-resolved fluorescence spectra. In other words, it indicates that accumulated arabitol in lichen body play a important role against dehydrated tolerance of Trebouxia.
