Changes in specific radioactivity of sunflower leaf metabolites during photosynthesis in 14CO2 and 12CO2 at three concentrations of CO2

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Abstract
Sunflower (Helianthus annuus L.) leaf discs were exposed to 14CO2 or 14CO2 followed by 12CO2 at 21% O2 and three different CO2 concentrations. After intervals of up to 15 min, the specific activity of some photosynthetic intermediates was determined. At all CO2 concentrations, the specific activity of 3-phosphoglyceric acid (3-PGA) increased most rapidly and after 15 min of 14CO2 feeding was 92% (967 ppm CO2), 87% (400 ppm CO2) and 53% (115 ppm CO2) of CO2 supplied to the assimilation chamber. The specific activity of glycine, serine and the photorespiratory CO2 was similar at all CO2 concentrations, in aggreement with their proposed close metabolic relationship in the glycolate pathway. However, the kinetics of serine and glycine labelling suggested that serine was not totally derived from glycine. Because the specific activity of these glycolate-pathway intermediates was very differnet from that of 3-PGA at all CO2 concentrations, not all of the carbon traversing this pathway came directly from the Calvin cycle. The non-equilibration of the 3-PGA with the feeding gas reflects the recycling of C from the glycolate pathway into the photosynthetic reduction cycle. Measurements of the rates of CO2 evolution in the light and estimates of the C flux through the glycolate pathway suggest that the photorespiratory activity was high and similar at 115 ppm CO2 and 400 ppm CO2 but inhibited at 967 ppm CO2.