Equilibrium or disequilibrium? A dual-wavelength investigation of photosystem I donors
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Oxidation of photosystem I (PSI) donors under far-red light (FRL), slow re-reduction by stromal reductants and fast re-reduction in the dark subsequent to illumination by white light (WL) were recorded in leaves of several C3 plants at 810 and 950 nm. During the re-reduction from stromal reductants the mutual interdependence of the two signals followed the theoretical relationship calculated assuming redox equilibrium between plastocyanin (PC) and P700, with the equilibrium constant of 40 ± 10 (ΔE m = 86–99 mV) in most of the measured 24 leaves of nine plant species. The presence of non-oxidizable PC of up to 13% of the whole pool, indicating partial control of electron transport by PC diffusion, was transiently detected during a saturation pulse of white light superimposed on FRL or on low WL. Nevertheless, non-oxidizable PC was absent in the steady state during fast light-saturated photosynthesis. It is concluded that in leaves during steady state photosynthesis the electron transport rate is not critically limited by PC diffusion, but the high-potential electron carriers PC and P700 remain close to the redox equilibrium.
KeywordsPhotosystem I Plastocyanin Redox equilibrium
- Cyt b6f
Cytochrome b6f complex
Electron transport rate
Midpoint redox potential
Equilibrium constant between PC and P700
Photon flux density
PSI donor pigment
This study was supported by Targeted Financing Theme SF0180045s08 from Estonian Ministry of Education and Science and Grants 6607 and 6611 from Estonian Science Foundation. We appreciate the contribution by R. B. Peterson (The Connecticut Agricultural Experiment Station) during writing the manuscript.
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