Abstract
To monitor over-reduction in stroma induced by photo-oxidative stress, we employed a simple non-destructive method to monitor redox state around photosystem I before and after high-light illumination based on the level of P700 oxidation by far-red light (FR). Oxidation level of P700 induced by FR illumination decreased during daytime and recovered at night. The recovery in the dark of once decreased oxidation level of P700 proceeded slowly with a half recovery time of about 5 hours. This depression of was probably due to cyclic electron donation to P700 or charge recombination within PSI. In either case, stromal over-reduction induced by sunlight was the direct cause, since the depression of P700 oxidation under FR was reproduced by illumination with saturating white light to dark-adapted leaves. Possible effects on photosynthetic activity of over-reduction induced by high-light and their physiological relevance is discussed.
