Radiationless transitions as a protection mechanism against photoinhibition in higher plants and a red alga.

Exposure of the red alga Porphyra perforata or leaves of Phytolacca americana and Echinodorus sp. to white light equivalent to full sunlight for short periods induced large decreases of variable fluorescence measured at 695 nm at 77K. This change was not produced by photoinhibition but rather appeared to result from an inorease in the rate constant of radiationless transition in the reaction centers of photosystem II. It is proposed that this increase is related to the formation of the high energy state which serves as a photoprotective mechanism in plants.