Stromal Over-reduction by High-light Stress as Measured by Decreases in P700 Oxidation by Far-red Light and its Physiological Relevance
Plant and Cell Physiology
The oxidation level of P700 induced by far-red light (∆A FR ) in briefly dark-treated leaves of some sun plants decreased during the daytime and recovered at night. The dark recovery of decreased ∆A FR proceeded slowly, with a half-time of about 5 h. We propose that stromal over-reduction induced by sunlight was the direct cause of the depression of ∆A FR . The depression of ∆A FR found during the daytime was reproduced by controlled illumination with saturating light of fully dark-treated
... s. Simultaneous measurement of P700 redox and chlorophyll fluorescence showed that the depression of ∆A FR was associated with dark reduction of the plastoquinone pool, which represented cyclic electron transport activity. The decrease of ∆A FR in the light-stressed chloroplasts was partly reversed by treatment with 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, an inhibitor of electron transport at the cytochrome b 6 /f complex, and the subsequent addition of methyl viologen, an efficient electron acceptor from photosystem I (PSI), stimulated further recovery, showing that both cyclic electron flow around PSI and the charge recombination within PSI were responsible for the light-induced depression of ∆A FR . The dark level of blue-green fluorescence, an indicator of NAD(P)H concentration, from intact chloroplasts was increased by high-light stress, suggesting that NADPH accumulated in stroma as a result of the high-light treatment. Possible effects on photosynthetic activity of over-reduction and its physiological relevance are discussed.