Use of labeled carbon dioxide for separation of CO2evolution from true CO2uptake by photosynthesizingAmaranthusand sunflower leaves

摘要

After 90ensp;min of steady-state photosynthesis in a flow through gas circuit, illuminatedAmaranthusand sunflower leaves were offered14CO2in 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.CO2evolution is not apparent in concentrations of low oxygen and the rates of14CO2uptake by photosynthesizingAmaranthusand sunflower leaves in 600ensp;p.p.m. CO2and 1 O2, which were constant during the short time14C fixation, showed that the volume of the closed system was not rate limiting.In 600ensp;p.p.m. CO2and 50 O2Amaranthusleaves did not evolve carbon dioxide from recently fixed carbon-14. Under the same conditions the14C 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 1ensp;min after the introduction of the14CO2. The calculated minimum rate of CO2evolution in the light was at least 5.5ensp;mg CO2/h.dm2. The true values and kinetics can be approached if14CO2is fed for shorter periods of time.