Journal Article

Stromal Over-reduction by High-light Stress as Measured by Decreases in P700 Oxidation by Far-red Light and its Physiological Relevance

Tsuyoshi Endo, Daiju Kawase and Fumihiko Sato

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 46, issue 5, pages 775-781
Published in print May 2005 | ISSN: 0032-0781
Published online May 2005 | e-ISSN: 1471-9053 | DOI: http://dx.doi.org/10.1093/pcp/pci084
Stromal Over-reduction by High-light Stress as Measured by Decreases in P700 Oxidation by Far-red Light and its Physiological Relevance

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The oxidation level of P700 induced by far-red light (ΔAFR) in briefly dark-treated leaves of some sun plants decreased during the daytime and recovered at night. The dark recovery of decreased ΔAFR 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 ΔAFR. The depression of ΔAFR found during the daytime was reproduced by controlled illumination with saturating light of fully dark-treated leaves. Simultaneous measurement of P700 redox and chlorophyll fluorescence showed that the depression of ΔAFR was associated with dark reduction of the plastoquinone pool, which represented cyclic electron transport activity. The decrease of ΔAFR 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 ΔAFR. 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.

Keywords: Charge recombination; Chlorophyll fluorescence; Cyclic electron transport; Over-reduction; P700; Photoprotection; ΔAFR, P700 oxidation level induced by far-red light; ΔAMAX, maximum P700 oxidation level induced by a xenon flash under background far-red light; DBMIB, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone; F, steady-state fluorescence yield under actinic illumination; Fm, maximum yield of fluorescence at closed PSII centers in dark-treated leaves; Fm′, Fm during the measurement; Fo, minimum fluorescence yield at open PSII centers; Fo′, Fo during the measurement; Fv, Fm–Fo; FR, far-red light; MV, methyl viologen; NDH, NAD(P)H dehydrogenase; P700, reaction center chlorophyll in PSI; QA, the primary quinone acceptor in PSII; qP, photochemical quenching.

Journal Article.  4663 words.  Illustrated.

Subjects: Biochemistry ; Molecular and Cell Biology ; Plant Sciences and Forestry

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