Assimilatory Force and Regulation of Photosynthetic Carbon Reduction in Leaves

  • K.-J. Dietz
  • U. Heber
Part of the NATO ASI Series book series (NSSA, volume 168)


The assimilatory force Fa is the product of the phosphorylation potential ATP/(ADP Pi) and the redox ratio NADPH/NADP. The light-dependent change in the free energy of Fa is the driving force of carbon assimilation. It is small compared to energy turnover in assimilation and not directly related to carbon flux. Deviations from proportionality between force and flux is explained by changes in the resistance offered to carbon flux. Enzyme modulation by the chloroplast thioredoxin system and by effectors and changes in substrate or metabolite concentrations and in activation energy modify resistance to carbon flux. As a result, low phosphorylation potentials and low ratios of NADPH to NADP simultaneously satisfy substrate requirements of thylakoid reactions and energy requirements of carbon assimilation over a wide range of fluxes. In illuminated leaves, high Fa values are indicative of flux limitations. However, even when stomata are closed under water stress so that access of CO2 is retricted, Fa values are not excessive in the presence of air levels of oxygen. Under these conditions, control of photosystem II activity and NADPH and ATP consumption via photorespiratory CO2 turnover prevent over-reduction of the electron transport chain, a condition leading to photoinactivation. Burdens of flux control are shared by different photosynthetic reactions. The extent of control exerted by a particular reaction changes as flux conditions change. The complex relationship between control coefficient, AG and activation energy of a reaction is discussed.


Carbon Flux Free Energy Change Calvin Cycle Pyridine Nucleotide Carbon Reduction 
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Copyright information

© Plenum Press, New York 1989

Authors and Affiliations

  • K.-J. Dietz
    • 1
  • U. Heber
    • 1
  1. 1.Institute of Botany and Pharmaceutical BiologyUniversity of WürzburgWürzburgGermany

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