Fast cyclic electron transport around photosystem I in leaves under far-red light: a proton-uncoupled pathway?
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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.
KeywordsCyclic electron transport P700 Plastocyanin Cytochrome b6f 810 nm transmittance Far-red light
Cyclic electron transport
Midpoint redox potential
Electron transport rate
Proton conductivity of ATPase
Linear electron transport
Photon absorption flux density
Incident photon flux density
- PS II
- PS I
Donor pigment of PS I
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.
- Golbeck JH, Bryant DA (1991) Photosystem I. Curr Top Bioenerg 16:83–177Google Scholar
- Havaux M (1992) Photoacoustic measurements of cyclic electron flow around photosystem I in leaves adapted to light-states 1 and 2. Plant Cell Physiol 33(6):799–803Google Scholar
- Heber U, Bukhov NG, Neimanis S, Kobayashi Y (1995a) Maximum H+/v PSI stoichiometry of proton transport during cyclic electron flow in intact chloroplasts is at least two, but probably higher than two. Plant Cell Physiol 36:1639–1647Google Scholar
- Hind G, Crowther D, Shahak Y, Slovacek RE (1981) The function and mechanism of cyclic electron transport. In: Akoyunoglou G (ed) Photosynthesis II. Electron transport and photophosphorylation. Balaban International Science Services, Philadelphia, PA, pp 87–97Google Scholar
- Laisk A, Oja V (1998) Dynamic gas exchange of leaf photosynthesis. Measurement and interpretation. CSIRO Publishing, Collingwood, AustraliaGoogle Scholar
- Miyake C, Miyata M, Shinzaki Y, Tomizawa K (2005b) CO2 response of cyclic electron flow around PSI (CEF-PSI) in tobacco leaves—relative electron fluxes through PSI and PSII determine the magnitude of non-photochemical quenching (NPQ) of Chl fluorescence. Plant Cell Physiol 46:629–637CrossRefPubMedGoogle Scholar
- Oja V, Eichelmann H, Laisk A (2008) Equilibrium or disequilibrium? A dual-wavelength investigation of photosystem I donors. In: Allen JF, Gantt E, Golbeck JH, Osmond B (eds) Photosynthesis. energy from the Sun: 14th international congress on photosynthesis. Springer, Berlin, pp 687–690Google Scholar
- Siggel U (1974) The control of electron transport by two pH-sensitive sites. In: Avron M (ed) Proceedings of the 3rd international congress on photosynthesis. Elsevier, Amsterdam, pp 645–654Google Scholar
- Yamamoto H, Kato H, Shinazaki Y, Horiguchi S, Shikanai T, Hase T, Endo T, Nishioka M, Makino A, Tomizawa K, Miyake C (2006) Ferredoxin limits cyclic electron flow around PSI (CEF-PSI) in higher plants—stimulation of CEF-PSI enhances non-photochemical quenching of Chl fluorescence in transplastomic tobacco. Plant Cell Physiol 47:1355–1371CrossRefPubMedGoogle Scholar