Oxygen Effect on Photosynthetic and Glycolate Pathways in Young Maize Leaves

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Abstract
To study the effect of O2 on the photosynthetic and glycolate pathways, maize [a C4 plant] leaves were exposed to 14CO2 during steady-state photosynthesis in 21 or 1% O2. At the two O2 concentrations after a 14CO2 pulse (4 s) followed by a 14CO2 chase, there was a slight difference in CO2 uptake and in the total amount of 14C fixed, but there were marked changes in 14C distribution especially in phosphoglycerate, ribulose bisphosphate, glycine and serine. The kinetics of 14C incorporation into glycine and serine indicated that the glycolate pathway is inhibited at low O2 concentrations. In 1% O2, labeling of glycine was reduced by 90% and that of serine was reduced by 70%, relative to the control in 21% O2. A similar effect was observed in C3 plants, except that, in maize leaves, only 5-6% of the total 14C fixed under 21% O2 was found in glycolate pathway intermediates after 60 s chase. This figure is 20% in C3 plants. Isonicotinyl hydrazide did not completely block the conversion of glycine to serine in 21% O2, and the first C atom of serine was preferentially labeled during the 1st seconds of the chase. Apparently, the labeled serine not only derives from glycine, but also could be formed from phosphoglycerate, labeled in the first C atom during the 1st seconds of photosynthesis. Another noticeable O2 effect concerned differential labeling of phosphoglycerate and ribulose bisphosphate. Phosphoglycerate is more labeled than ribulose bisphosphate in air; the reverse is observed in 1% O2. Changes in ribulose bisphosphate and phosphoglycerate pools exhibit similar trends. To understand the effect of O2 on the distribution of 14C in these 2 intermediates, it was postulated that, in air, there remains an oxygenase function which produces additional phosphoglycerate at the expense of ribulose bisphosphate.