Journal Article

Cyclic Flow of Electrons within PSII in Thylakoid Membranes

Chikahiro Miyake and Akiho Yokota

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 42, issue 5, pages 508-515
Published in print May 2001 | ISSN: 0032-0781
Published online May 2001 | e-ISSN: 1471-9053 | DOI: http://dx.doi.org/10.1093/pcp/pce063
Cyclic Flow of Electrons within PSII in Thylakoid
Membranes

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In photosynthesis, the electrons released from PSII are considered to be shared mainly by carbon metabolism and the water–water cycle. We demonstrated previously that some electrons are utilized in a CO2- and O2-independent manner in leaves of wild watermelon [Miyake and Yokota (2000) Plant Cell Physiol. 41: 335]. In the present study, we examined the mechanism of this alternative flow of electrons in thylakoid membranes, isolated from fresh spinach leaves, by simultaneously measuring the quantum yield of PSII and the flux of the linear flow of electrons. In the presence of the protonophore nigericin, which eliminates the pH gradient across thylakoid membranes, the quantum yield and the flux of the linear flow of electrons were directly proportional to one another. The quantum yield at a given linear flux of electrons was much higher in the absence of nigericin than in its presence, indicating that an additional or alternative flow of electrons can occur independently of the linear flow in the absence of nigericin. In the presence of nigericin, the alternative flux of electrons increased with decreasing pH and with increasing reduction of the plastoquinone pool. Cyclic flow of electrons in PSII appears to be the most plausible candidate for the alternative flow of electrons. The flux reached 280 µmol e (mg Chl)–1 h–1 and was similar to that of the CO2- and O2-independent alternative flow of electrons that we found in leaves of wild watermelon. The cyclic, alternative flow of electrons in PSII provides a possible explanation for the alternative flow of electrons observed in vivo.

Keywords: Key words: Alternative flow of electrons — Cyclic flow of electrons — Photoinhibition — PSII — Thylakoid — Water–water cycle.; Abbreviations: α, the molar ratio of PSII to PSI in thylakoid membranes; Ci, the intercellular partial pressure of CO2; ΔpH, a gradient in pH across thylakoid membranes; Φ(PSII), the quantum yield of electron transport in PSII at a steady state, defined as (F′m – Fs)/F′m or (ΔF/F′m); Fm, maximal yield of Chl fluorescence after dark adaptation; Fm′, Fm after illumination; Fo, minimal Chl fluorescence after dark adaptation; Fo′, minimum yield of Chl fluorescence at the light-adapted state; Fs, yield of steady-state Chl fluorescence; Fd, ferredoxin; Ja, alternative flux of electrons; Ja(PSII), Ja in PSII; Ja(O2-depend), Ja that depends on pO2; Ja(O2-independ), Ja that does not depend on pO2; Je(PSII), the electron flux in PSII; Je(PCR), the electron flux to the PCR cycle; Je(PCR + PCO), the electron fluxes to both the PCR and the PCO cycles; LHCII, the light-harvesting chlorophyll-protein complex of PSII; MV, methyl viologen; NPQ, the non-photochemical quenching coefficient of Chl fluorescence; PCO cycle, photorespiratory carbon-oxidation cycle; PCR cycle, photosynthetic carbon-reduction cycle; PFD, photosynthetically active photon flux density; pO2, ambient partial pressure of O2; PQ, plastoquinone; PQH2, plastoquinol; qp, the photochemical quenching coefficient of Chl fluorescence; V(O2), the rate of the uptake of O2.

Journal Article.  5451 words.  Illustrated.

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

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