Effect of CO2 on Photophosphorylation in vivo as Revealed by the Light-dependent Cl⊖ Uptake in Elodea densa

The light-dependent Cl⊖ uptake in Elodea densa exhibits dual DCMU inhibition curves dependent on the presence and absence of CO2. This suggests that cyclic photophosphorylation (energy surce in the absence of CO2) is also inhibited by higher concentrations of DCMU in vivo. Further, at low DCMU concentrations the Cl⊖ uptake in the presence of CO2 is much smaller than would be expected if cyclic photophosphorylation were operative. This points to a parallel inhibition of non-cyclic and cyclic photophosphorylations by low concentrations of DCMU in the presence of CO2. The apparent inhibition of cyclic photophosphorylation at low concentrations of DCMU can be reversed by withdrawal of CO2. It is concluded that CO2 may be important in the regulation of cyclic and non-cyclic photophosphorylations in vivo.