Equilibrium or disequilibrium? A dual-wavelength investigation of photosystem I donors.
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 C(3) 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 (Delta E (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.
Tartu Ulikooli Molekulaar-ja Rakubioloogia Instituut, Riia tn. 23, Tartu, 51010, Estonia.
Vello
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Oja
Hillar
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Eichelmann
Agu
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Anijalg
Heikko
H
Rämma
Agu
A
Laisk
D002245:Carbon Dioxide / Q000378:metabolism; D004579:Electron Transport / Q000528:radiation effects; D004583:Electrons; D005288:Ferredoxins / Q000378:metabolism; D006368:Helianthus / Q000502:physiology* / Q000528:radiation effects*; D008027:Light; D010084:Oxidation-Reduction / Q000528:radiation effects; D010788:Photosynthesis / Q000528:radiation effects; D045331:Photosystem I Protein Complex / Q000378:metabolism*; D018515:Plant Leaves / Q000378:metabolism / Q000528:radiation effects; D010970:Plastocyanin / Q000378:metabolism; D013057:Spectrum Analysis
D005288:Ferredoxins; D045331:Photosystem I Protein Complex; D002245:Carbon Dioxide; D010970:Plastocyanin