Magnesium Activation of Photosynthetic Co₂‐fixation in a Reconstituted Chloroplast System

S ummary Measurements of O 2 ‐evolution by a reconstituted chloroplast system showed that photosynthetic electron transport and 3‐phosphoglycerate reduction proceed at rapid rates in the presence of 0.5‐1 ran MgCl 2 . However, to obtain maximal rates of CO 2 ‐dependent O 2 ‐evolution under these conditions, it was necessary to increase the Mg concentration to 5 mM (70‐90% of the maximum rate was observed at 2.5 mM Mg). Rapid rates of CO 2 ‐dependent O 2 evolution were also recorded at 1 mM Mg when the bicarbonate concentration was increased from 1 to 40 mM. Simultaneous measurements of 14 CO 2 fixation and O 2 evolution in the presence of ribulose‐1, 5‐diphosphate confirmed that the additional Mg was required for the carboxylation step and showed that the Mg activation was a more rapid process than the kinetics of CO 2 ‐dependent O 2 evolution might suggest.