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Articles

Oxygen Effect on Photosynthetic and Glycolate Pathways in Young Maize Leaves

Jack P. Farineau

Jean C. Huet

Laboratoire d'Etude Du Métabolisme intermédiaire et de Nutrition minérale, I.N.R.A., 78000 Versailles, France, Service de Biophysique, Département de Biologie, Centre d'Etudes Nucléaires de Saclay, B.P. No. 2, 91190 Gifsur-Yvette, France, Laboratoire d'Etude des Protéines, I.N.R.A., 78000 Versailles, France

To study the effect of O₂ on the photosynthetic and glycolate pathways, maize leaves were exposed to ¹⁴CO₂ during steady-state photosynthesis in 21 or 1% O₂. At the two O₂ concentrations after a ¹⁴CO₂ pulse (4 seconds) followed by a ¹²CO₂ 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 has been observed in C₃ plants, except that, in maize leaves, only 5 to 6% of the total ¹⁴C fixed under 21% O₂ was found in glycolate pathway intermediates after 60 seconds 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 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 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 two intermediates, it was postulated that, in air, there remains an oxygenase function which produces additional phosphoglycerate at the expense of ribulose bisphosphate.