ACTION OF CO2 ON LIGHT-DEPENDENT CL--UPTAKE BY ELODEA DENSA - REGULATION BETWEEN NONCYCLIC AND CYCLIC PHOTOPHOSPHORYLATION

The light curve of Cl--uptake (uptake vs. light intensity) by Elodea densa in pure N2 shows that saturation is reached at a very low light intensity. In N2+3% CO2, on the other hand, there is considerably less Cl- uptake. Under these conditions, the saturation attained at low light intensity is only temporary, and the Cl--uptake increases steadily with a further rise in light intensity. It is suggested that the reason for the low intensity of light saturation may be the necessity for an intracellular transport of ATP from the site of its formation to the site of Cl--uptake. CO2 exerts a strong inhibitory influence on the Cl--uptake, especially at low light intensities. At higher intensities the inhibition diminishes and it is nearly absent at high intensities of white light. The inhibition by CO2 is also a function of the wavelength of the light; it is greatest in the region below 683 nm, where photosynthesis occurs with high efficiency, but it is still present at wavelengths beyond 700 nm. CO2 also inhibits the Cl--uptake at high light intensities of white light when small concentration of DCMU (5×10-7 M) is present at the same time. The inhibitory action of CO2 is partly interpreted as a consequence of a competition for ATP between CO2-assimilation (espectially below 683 nm) and the light-dependent Cl--uptake. In addition, however, it is suggested that at low light intensities the presence of CO2 effects a regulation between noncyclic and cyclic electron transports and photophosphorylation which is supposed to be a consequence of a change in the redox potential of ferredoxin or another cofactor acting in noncyclic and cyclic electron transports. Especially the inhibition of the Cl--uptake by CO2 in far-red light (λ>700 nm) and in the presence of DCMU is taken to be an indication of this intracellular regulation. © 1969 Springer-Verlag.