Abstract
To study the effect of O(2) on the photosynthetic and glycolate pathways, maize leaves were exposed to (14)CO(2) during steady-state photosynthesis in 21 or 1% O(2). At the two O(2) concentrations after a (14)CO(2) pulse (4 seconds) followed by a (12)CO(2) chase, there was a slight difference in CO(2) uptake and in the total amount of (14)C fixed, but there were marked changes in (14)C distribution especially in phosphoglycerate, ribulose bisphosphate, glycine, and serine. The kinetics of (14)C incorporation into glycine and serine indicated that the glycolate pathway is inhibited at low O(2) concentrations. In 1% O(2), labeling of glycine was reduced by 90% and that of serine was reduced by 70%, relative to the control in 21% O(2). A similar effect has been observed in C(3) plants, except that, in maize leaves, only 5 to 6% of the total (14)C fixed under 21% O(2) was found in glycolate pathway intermediates after 60 seconds chase. This figure is 20% in C(3) plants. Isonicotinyl hydrazide did not completely block the conversion of glycine to serine in 21% O(2), and the first carbon atom of serine was preferentially labeled during the first seconds of the chase. These results supported the hypothesis that the labeled serine not only derives from glycine but also could be formed from phosphoglycerate, labeled in the first carbon atom during the first seconds of photosynthesis.Another noticeable O(2) effect concerned differential labeling of phosphoglycerate and ribulose bisphosphate. Phosphoglycerate is more labeled than ribulose bisphosphate in air; the reverse is observed in 1% O(2). Changes in ribulose bisphosphate and phosphoglycerate pools exhibit similar trends. To understand the effect of O(2) on the distribution of (14)C in these two intermediates, it was postulated that, in air, there remains an oxygenase function which produces additional phosphoglycerate at the expense of ribulose bisphosphate.