Changes in specific radioactivity of sunflower leaf metabolites during photosynthesis in14CO2and12CO2at three concentrations of CO2

✍ Scribed by J. D. Mahon; H. Fock; D. T. Canvin

• Publisher: Springer-Verlag
• Year: 1974
• Tongue: English
• Weight: 567 KB
• Volume: 120
• Category: Article
• ISSN: 0032-0935
• DOI: 10.1007/bf00390292

✦ Synopsis

Sunflower (Helianthus annuus L.) leaf discs were exposed to 14C02 or 14C02 followed by 12C02 at 21% 02 and three different CO 2 concentrations. After intervals of up to 15 min, the specific activity of some photosynthetic intermediates was determined. At all COs concentrations, the specific activity of 3-phosphoglyceric acid (3-PGA) increased most rapidly and after 15 min of 1~C02 feeding was 92% (967 ppm C02), 87% (400 ppm C02) and 53% (115 ppm COs) of C02 supplied to the assimilation chamber. The specific activity of glycine, serine and the photorespiratory CO 2 was similar at all CO s concentrations, in agreement with their proposed close metabolic relationship in the glycolate pathway. However, the kinetics of serine and glycine labeling suggested that serine was not totally derived from glycine. Because the specific activity of these glycolate-pathway intermediates was very different from that of 3-PGA at all CO 2 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 CO 2 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 COs and 400 ppm CO s but inhibited at 967 ppm CO 2.