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Plant and Cell Physiology Advance Access originally published online on March 23, 2005
Plant and Cell Physiology 2005 46(5):775-781; doi:10.1093/pcp/pci084
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JSPP © 2005

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

Tsuyoshi Endo1, Daiju Kawase and Fumihiko Sato

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

1 Corresponding author: E-mail, tuendo{at}kais.kyoto-u.ac.jp; Fax, +81-75-753-6398.

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 of 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 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 {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.

(Received October 27, 2004; Accepted March 6, 2005 )
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