Use of labeled carbon dioxide for separation of CO2 evolution from true CO2 uptake by photosynthesizing Amaranthus and sunflower leaves

After 90 min of steady-state photosynthesis in a flow through gas circuit, illuminated Amaranthus and sunflower leaves were offered 14CO2 in a closed system and subsequently killed in liquid nitrogen. The lyophilized tissue was combusted by a modified in-vial microcombustion method. The radioactivity of the released carbon dioxide trapped in monoethanolamine was measured in a liquid scintillation spectrometer.CO2 evolution is not apparent in concentrations of low oxygen and the rates of 14CO2 uptake by photosynthesizing Amaranthus and sunflower leaves in 600 p.p.m. CO2 and 1% O2, which were constant during the short time 14C fixation, showed that the volume of the closed system was not rate limiting.In 600 p.p.m. CO2 and 50% O2Amaranthus leaves did not evolve carbon dioxide from recently fixed carbon-14. Under the same conditions the 14C content versus time relationship by sunflower leaves rapidly became curvilinear, indicating that increasing amounts of labeled carbon dioxide were being evolved from the tissue. Radioactive carbon was determined in the released carbon dioxide less than 1 min after the introduction of the 14CO2. The calculated minimum rate of CO2 evolution in the light was at least 5.5 mg CO2/h.dm2. The true values and kinetics can be approached if 14CO2 is fed for shorter periods of time.