The Effects of Desiccation on Excitation Energy Transfer at Physiological Temperatures Between the Two Photosystems of the Red Alga Porphyra perforata

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Summary Room temperature fluorescence emission and excitation spectra were measured for wet and desiccated thalli of the red alga Porphyra perforata. Porphyra that had been desiccated in the dark showed a reduced efficiency of photosystem II trap closure along with a decrease of the photosystem II fluorescence emission at 687 nm and a concomitant increase of the far red photosystem I fluorescence emission band. Porphyra responds to desiccation by producing a strong increase in the distribution of excitation energy from phycoerythrin of photosystem II to photosystem I that may be mediated by increases in α, the fraction of light that is transferred initially to photosystem I, and increases in spillover or both. No photosystem II trap closure was seen at room temperature in Porphyra that had been dried in strong light, indicating that the photosystem II traps were closed under these conditions. Light dried Porphyra showed a complete loss of the 687 nm fluorescence band compared to dark dried Porphyra . This loss of the 687 nm band was accompanied by a proportionate loss of the 737 nm band. In light dried Porphyra F o is not affected, only part of F v was eliminated. This observation suggests that reaction center quenching of fluorescence in photosystem II may account for these observations. Increased energy transfer to photosystem I in the desiccated state may protect the more photolabile reaction centers of photosystem II from photochemical damage when these plants are subjected to desiccation and high light intensities.