Preferential distribution of excitation energy into photosystem I of desiccated samples of the lichen Cladonia impexa and the isolated lichen-alga Trebouxia pyriformis

Abstract The effect of desiccation on distribution of excitation energy between the two photosystems has been studied in the lichen Cladonia impexa Harm., in the green alga Trebouxia pyriformis Archibald, isolated from Cladonia squamosa; and in the non-lichen green alga Scenedesmus obliquus, strain D₃. The method used was to compare the low temperature fluorescence emission of samples equilibrated with air with different humidity prior to freezing in liquid nitrogen. Desiccation of Cladonia and Trebouxia caused a pronounced increase of the height of the far red fluorescence emission band, F 715, over the short wave bands, F685 and F697; the ratio between the two short wave bands remained essentially constant. Upon rewetting, these species regained normal fluorescence emission properties, indicating that they are desiccation-tolerant. Scenedesmus, which was used as a desiccation intolerant species, also showed an increase of the far red fluorescence band over the two short wave bands upon desiccation, but the original fluorescence spectrum was not restored upon rewetting. These results are interpreted as showing that desiccation of tolerant species such as Cladonia and Trebouxia causes a preferential energy distribution into photosystem I. We tentatively believe that desiccation induces conformational changes within the chloroplast thylakoids, thereby controlling distribution of energy between the two photosystems. Furthermore, this change in energy distribution may be of ecological significance as the mechanism by which desiccated lichens or algae avoid photo-dynamic destruction of the photosynthetic apparatus when photosynthesis is inhibited under dry conditions. By a preferential distribution of absorbed energy into photosystem I, the organisms avoid the formation of strong, harmful oxidants in photosystem II when photosynthesis is inhibited. It is suggested that β-carotene associated with the far red-absorbing chlorophyll a fraction of the reaction center antenna of photosystem I is the final sink for excess excitation energy in dry, desiccation-tolerant lichens and algae.