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Sunflower ( Helianthus annuus L.) leaf discs were exposed to CO or CO followed by CO in an open gas-exchange system with incoming gas of approximately 400 ppm CO and either 21% or 1% O. The CO and CO 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 CO efflux by the leaf discs was ca. 20% of the net photosynthetic rate at 21% O but no CO efflux could be detected at 1% O. At both O concentrations the specific activity of 3-phosphoglyceric acid (3-PGA) increased and decreased rapidly for the first 5 min, and then more slowly during CO feeding and CO flushing. At 21% O, glycine, serine and alanine changed more slowly in specific activity than 3-PGA and at 1% O their specific activities were much lower than at 21% O. The results at both O concentrations indicated that the glycolate pathway compounds were not derived solely from Calvin-cycle intermediates. At 1% O 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% O. 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. [ABSTRACT FROM AUTHOR]

Sunflower ( Helianthus annuus L.) leaf discs were exposed to CO or CO followed by CO at 21% O and three different CO concentrations. After intervals of up to 15 min, the specific activity of some photosynthetic intermediates was determined. At all CO concentrations, the specific activity of 3-phosphoglyceric acid (3-PGA) increased most rapidly and after 15 min of CO feeding was 92% (967 ppm CO), 87% (400 ppm CO) and 53% (115 ppm CO) of CO supplied to the assimilation chamber. The specific activity of glycine, serine and the photorespiratory CO was similar at all CO 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 CO 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 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 CO and 400 ppm CO but inhibited at 967 ppm CO. [ABSTRACT FROM AUTHOR]

The CO- and CO-exchange of illuminated corn leaf discs were measured at normal (21%) and low (1%) oxygen. After periods of exposure to CO or to CO followed by CO, the discs were killed and the specific activities of some metabolites were determined. At both O concentrations the specific activity of 3-PGA increased and decreased rapidly during the first 5 min of CO-feeding or CO-flushing but did not equilibrate with that of the CO in the assimilation chamber even after 15 min. The specific activity of aspartic acid also showed bimodal kinetics during both feeding and flushing. The specific activities of 3-phosphoglyceric acid (3-PGA), aspartic acid and alanine were higher at 1% O than at 21% O, but glycine and serine were lower in specific activity at 1% O. The results are in agreement with the proposed initial fixation of CO into C-dicarboxylic acids and subsequent transfer of this carbon to 3-PGA. Indirect evidence supports the idea that at 21% O, CO was produced by the corn leaf discs in the light and was refixed into C-dicarboxylic acids. At 1% O, the photorespiratory process could also have been active although the flux of carbon through the glycolate pathway was probably smaller than at 21% O. [ABSTRACT FROM AUTHOR]