Photosynthetic Oxygen Evolution by Isolated Chloroplasts in the Presence of Carbon Cycle Intermediates

Abstract

RECENTLY developed techniques have led to the isolation of intact chloroplasts which will assimilate carbon dioxide at rates approaching those achieved by the intact plant1–6. This has made it possible to measure the associated oxygen evolution, polarographically, in aerobic conditions7. Normally, oxygen evolution parallels carbon dioxide fixation7,8. On illumination, both start slowly and accelerate gradually until a maximum rate is reached after several minutes. This induction phase is thought to reflect corresponding changes in the concentration of carbon cycle intermediates as these increase autocatalytically to a steady state level2–4,7–11. If 3-phosphoglycerate (PGA) is provided as a substrate the initial lag is virtually eliminated7 and the kinetics then approximate to those observed in the Hill reaction12 in which an artificial hydrogen acceptor reacts more directly with the photochemical system. This was held7 to be consistent with the view13 that the 3-phosphoglycerate is the immediate precursor of the hydrogen acceptor (1,3-diphosphoglycerate) in the carbon cycle. If this is correct, it would follow that the oxygen evolution observed when PGA is added as a substrate7 should not be dependent on carbon dioxide-bicarbonate (Fig. 1). Conversely, any stimulation of oxygen evolution by triose phosphate should cease in the absence of carbon dioxide. Experiments which substantiate these conclusions are reported here.