Changes in specific radioactivities of sunflower leaf metabolites during photosynthesis in 14CO2 and 12CO2 at normal and low oxygen

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Abstract
Sunflower (Helianthus annuus L.) leaf discs were exposed to 14CO2 or 14CO2 followed by 12CO2 in an open gas-exchange system with incoming gas of approximately 400 ppm CO2 and either 21% or 1% O2. The 14CO2 and 12CO2 gas-exchange of the leaf discs were measured, and the specific activities of several metabolites were determined after different lengths of time. The rate of CO2 efflux by the leaf discs was ca. 20% of the net photosynthetic rate at 21% O2 but no CO2 efflux could be detected at 1% O2. At both O2 concentrations the specific activity of 3-phosphoglyceric acid (3-PGA) increased and decreased rapidly for the first 5 min, and then more slowly during 14CO2 feeding and 12CO2 flushing. At 21% O2, glycine, serine and alanine changed more slowly in specific activity than 3-PGA and at 1% O2 their specific activities were much lower than at 21% O2. The results at both O2 concentrations indicated that the glycolate pathway compounds were not derived solely from Calvin-cycle intermediates. At 1% O2 the flux of carbon from the immediate fixation products was inhibited and serine was at least partially produced from a precursor of higher specific activity than glycine, although the glycolate pathway may have been active even at 1% O2. The difference between the specific activities of 3-PGA and the feeding gases could be explained by the recycling of C from the glycolate pathway.