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Plant and Cell Physiology Advance Access published online on March 23, 2005
Plant and Cell Physiology, doi:10.1093/pcp/pci084
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Plant and Cell Physiology 2005 © The Japanese Society of Plant Physiologists (JSPP); all rights reserved.
Received October 27, 2004
Accepted March 6, 2005

Regular Papar

Stromal Over-Reduction by High-Light Stress as Measured by Decreases in P700 Oxidation by Far-Red Light and Its Physiological Relevance

Tsuyoshi Endo 1*, Daiju Kawase 1, and Fumihiko Sato 1

1 Graduate School of Biostudies, Kyoto University, Sakyo, Kyoto, 606-8502 Japan

* To whom correspondence should be addressed.
Tsuyoshi Endo, E-mail: tuendo{at}kais.kyoto-u.ac.jp


  Abstract

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

The decrease of {Delta}AFR in the light-stressed chloroplasts was partly reversed by the treatment with 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, an inhibitor of electron transport at cytochrome b6/f complex, and the subsequent addition of methyl viologen, an efficient electron acceptor from 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 {Delta}AFR. 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: Chlorophyll fluorescence; Charge recombination; Cyclic electron transport; Over-reduction; Photoprotection; P700.
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