Journal Article

Cyclic Electron Flow within PSII Functions in Intact Chloroplasts from Spinach Leaves

Chikahiro Miyake, Kuniaki Yonekura, Yoshichika Kobayashi and Akiho Yokota

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 43, issue 8, pages 951-957
Published in print August 2002 | ISSN: 0032-0781
Published online August 2002 | e-ISSN: 1471-9053 | DOI: http://dx.doi.org/10.1093/pcp/pcf113
Cyclic Electron Flow within PSII Functions in Intact Chloroplasts from Spinach Leaves

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Using thylakoid membranes, we previously demonstrated that accumulated electrons in the photosynthetic electron transport system induces the electron flow from the acceptor side of PSII to its donor side only in the presence of a pH gradient (ΔpH) across the thylakoid membranes. This electron flow has been referred to as cyclic electron flow within PSII (CEF-PSII) [Miyake and Yokota (2001) Plant Cell Physiol. 42: 508]. In the present study, we examined whether CEF-PSII operates in isolated intact chloroplasts from spinach leaves, by correlating the quantum yield of PSII [Φ(PSII)] with the activity of the linear electron flow [V(O2)]. The addition of the protonophore nigericin to the intact chloroplasts decreased Φ(PSII), but increased V(O2), and relative electron flux in PSII [Φ(PSII) × PFD] and V(O2) were proportional to one another. Φ(PSII) × PFD at a given V(O2) was much higher in the presence of ΔpH than that in its absence. These effects of nigericin on the relationship between Φ(PSII) × PFD and V(O2) are consistent with those previously observed in thylakoid membranes, indicating the occurrence of CEF-PSII also in intact chloroplasts. In the presence of ΔpH, CEF-PSII accounted for the excess electron flux in PSII that could not be attributed to photosynthetic linear electron flow. The activity of CEF-PSII increased with increased light intensity and almost corresponded to that of the water–water cycle (WWC), implying that CEF-PSII can dissipate excess photon energy in cooperation with WWC to protect PSII from photoinhibition under limited photosynthesis conditions.

Keywords: Keywords: Alternative electron flow — Chloroplasts — Cyclic electron flow — Photoinhibition — PSII — Water–water cycle.; Abbreviations: AEF, alternative electron flow; α, the molar ratio of PSII to PSI in thylakoid membranes; APX, Asc peroxidase; Asc, ascorbate; CEF-PSII, cyclic electron flow within PSII; ΔpH, a pH gradient across thylakoid membranes; Φ(PSII), the quantum yield of electron transport in PSII at steady state, defined as (F′m–Fs)/F′m; Fm, maximal yield of Chl fluorescence after dark adaptation; Fm′, Fm after illumination; Fo, minimal Chl fluorescence after dark adaptation; Fs, yield of steady-state Chl fluorescence; Je(PSII), the electron flux in PSII; MDA, monodehydroascorbate radical; NPQ, non-photochemical quenching coefficient of Chl fluorescence, defined as Fm/Fm′–1; PCO cycle, photorespiratory carbon-oxidation cycle; PCR cycle, photosynthetic carbon-reduction cycle; PFD, photosynthetically active photon flux density; PQ, plastoquinone; PQH2, plastoquinol; SOD, superoxide dismutase; V(O2), the rate of O2 uptake; WWC, the water–water cycle.

Journal Article.  5151 words.  Illustrated.

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

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