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Oscillations in photosynthesis are initiated and supported by imbalances in the supply of ATP and NADPH to the Calvin cycle
Authors:Agu Laisk  Katharina Siebke  Ulvi Gerst  Hillar Eichelmann  Vello Oja  Ulrich Heber
Affiliation:(1) Astrofüüsika ja Atmosfäärifüüsika Instituut, Eesti Teaduste Akadeemia, Toravere Observatoorium, 202444 Tartu, Eesti;(2) Julius-von-Sachs-Institut für Biowissenschaften der Universität Würzburg, Lehrstuhl Botanik I, Mittlerer Dallenbergweg 64, W-8700 Würzburg, Germany
Abstract:Oscillations in the rate of photosynthesis of sunflower (Helianthus annuus L.) leaves were induced by subjecting leaves, whose photosynthetic apparatus had been activated, to a sudden transition from darkness or low light to high-intensity illumination, or by transfering them in the light from air to an atmosphere containing saturating CO2. It was found that at the first maximum, light-and CO2-saturated photosynthesis can be much faster than steady-state photosynthesis. Both QA in the reaction center of PS II and P700 in the reaction center of PS I of the chloroplast electron-transport chain were more oxidized during the maxima of photosynthesis than during the minima. Maxima of P700 oxidation slightly preceded maxima in photosynthesis. During a transition from low to high irradiance, the assimilatory force FA, which was calculated from ratios of dihydroxyacetone phosphate to phosphoglycerate under the assumption that the reactions catalyzed by NADP-dependent glyceraldehydephosphate dehydrogenase, phosphoglycerate kinase and triosephosphate isomerase are close to equilibrium, oscillated in parallel with photosynthesis. However, only one of its components, the calculated phosphorylation potential (ATP)/(ADP)(Pi), paralleled photosynthesis, whereas calculated NADPH/NADP ratios exhibited antiparallel behaviour. When photosynthetic oscillations were initiated by a transition from low to high CO2, the assimilatory force FA declined, was very low at the first minimum of photosynthesis and increased as photosynthesis rose to its second maximum. The observations indicate that the minima in photosynthesis are caused by lack of ATP. This leads to overreduction of the electron-transport chain which is indicated by the reduction of P700. During photosynthetic oscillations the chloroplast thylakoid system is unable to adjust the supply of ATP and NADPH rapidly to demand at the stoichiometric relationship required by the carbonreduction cycle.Abbreviations PGA 3-phosphoglycerate - DHAP dihydroxyacetone phosphate - P700 electron-donor pigment in the reaction enter of PS I - QA quinone acceptor in the reaction center of PS IIThis work received support from the Estonian Academy of Sciences, the Bavarian Ministry of Science and Art and the Sonderforschungsbereich 251 of the University of Würzburg. We are grateful for criticism by D.A. Walker, Robert Hill Institute, University of Sheffield, U.K. and by Mark Stitt, Institute of Botany, University of Heidelberg, FRG.
Keywords:Assimilatory force  Calvin cycle  Electron flow (cyclic)  Helianthus (photosynthesis)  Oxygen reduction  Phosphorylation potential  Photosynthesis (oscillations)
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