The role of photophosphorylation in SO2 and SO 3 (2-) inhibition of photosynthesis in isolated chloroplasts.

Sulphur dioxide inhibits noncyclic photophosphorylation in isolated envelope-free chloroplasts. This inhibition was shown to be reversible and competitive with phosphate, with an inhibitor constant of Ki=0.8mM. The same inhibition characteristics were observed when phosphoglycerate (PGA)- or ribulose-1,5-bisphosphate (RuBP)-dependent oxygen evolution was examined in a reconstituted chloroplast system in the presence of SO 3 (2-) . Using an ATP-regenerating system (phosphocreatine-creatine kinase), it was demonstrated that the inhibition of PGA-dependent oxygen evolution is solely the result of inhibited photophosphorylation. It is concluded that at low SO2 and SO 3 (2-) concentrations the inhibition of photophosphorylation is responsible for the inhibition of photosynthetic oxygen evolution.