Carbon dioxide fixation in the light and in the dark by isolated spinach chloroplasts.

Factors affecting CO(2) fixation in the spinach (Spinacia oleracea) chloroplast were investigated. Free magnesium ions are shown to be highly inhibitory for photosynthetic CO(2) fixation in isolated intact spinach chloroplasts. The pH optimum for CO(2) fixation is about 8.5 but is dependent upon the reaction medium. Conditions are defined under which chloroplasts illuminated in the absence of CO(2) accumulate ribulose 1,5-diphosphate, and fix CO(2) in a subsequent dark period when high magnesium ion concentrations are provided. The regulation of photosynthetic CO(2) assimilation by these factors is discussed.