Photosynthesis Research

, Volume 103, Issue 2, pp 79–95 | Cite as

Fast cyclic electron transport around photosystem I in leaves under far-red light: a proton-uncoupled pathway?

  • Agu LaiskEmail author
  • Eero Talts
  • Vello Oja
  • Hillar Eichelmann
  • Richard B. Peterson
Regular Paper


Fast cyclic electron transport (CET) around photosystem I (PS I) was observed in sunflower (Helianthus annuus L.) leaves under intense far-red light (FRL) of up to 200 μmol quanta m−2 s−1. The electron transport rate (ETR) through PS I was found from the FRL-dark transmittance change at 810 and 950 nm, which was deconvoluted into redox states and pool sizes of P700, plastocyanin (PC) and cytochrome f (Cyt f). PC and P700 were in redox equilibrium with K e = 35 (ΔE m = 90 mV). PS II ETR was based on O2 evolution. CET [(PS I ETR) − (PS II ETR)] increased to 50–70 μmol e m−2 s−1 when linear electron transport (LET) under FRL was limited to 5 μmol e m−2 s−1 in a gas phase containing 20–40 μmol CO2 mol−1 and 20 μmol O2 mol−1. Under these conditions, pulse-saturated fluorescence yield F m was non-photochemically quenched; however, F m was similarly quenched when LET was driven by low green or white light, which energetically precluded the possibility for active CET. We suggest that under FRL, CET is rather not coupled to transmembrane proton translocation than the CET-coupled protons are short-circuited via proton channels regulated to open at high ΔpH. A kinetic analysis of CET electron donors and acceptors suggests the CET pathway is that of the reversed Q-cycle: Fd → (FNR) → Cyt cn → Cyt bh → Cyt bl → Rieske FeS → Cyt f → PC → P700 →→ Fd. CET is activated when PQH2 oxidation is opposed by high ΔpH, and ferredoxin (Fd) is reduced due to low availability of e acceptors. The physiological significance of CET may be photoprotective, as CET may be regarded as a mechanism of energy dissipation under stress conditions.


Cyclic electron transport P700 Plastocyanin Cytochrome b6810 nm transmittance Far-red light 



Bisphosphoglyceric acid


Cyclic electron transport






Midpoint redox potential


Electron transport rate




Ferredoxin-NADP reductase


Far-red light


Proton conductivity of ATPase


Linear electron transport


Non-photochemical quenching




3-Phosphoglyceric acid


Photon absorption flux density


Incident photon flux density


Photosystem II


Photosystem I


Donor pigment of PS I








White light



This study was supported by Targeted Financing Theme SF 0180045s08 from Estonian Ministry of Education and Science, and by Grants 6607 and 6611 from Estonian Science Foundation.


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Copyright information

© Springer Science+Business Media B.V. 2009

Authors and Affiliations

  • Agu Laisk
    • 1
    Email author
  • Eero Talts
    • 1
  • Vello Oja
    • 1
  • Hillar Eichelmann
    • 1
  • Richard B. Peterson
    • 2
  1. 1.Institute of Molecular and Cell BiologyTartu UniversityTartuEstonia
  2. 2.Department of Biochemistry and GeneticsThe Connecticut Agricultural Experiment StationNew HavenUSA

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