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Dark Carbon Dioxide Fixation under Aerobic and Anaerobic Conditions in Maize Leaves after Preillumination in the Absence of Oxygen

Ribulose 1,5-Bisphosphate Can Serve as a Primary Acceptor of Carbon Dioxide

¹ Département de Biologie, Service de Radioagronomie, CEA, Centre d'Etudes Nucléaires de Cadarache, BP No. 1, 13115 Saint-Paul-Lez-Durance, France

When dark ¹⁴CO₂ fixation in maize leaves was carried out under anaerobic conditions after preillumination in the absence of O₂, the ¹⁴C incorporation in aspartic acid was transient; its maximum level was very low compared with that of malic acid. The addition of 5% O₂ during the dark fixation period increased the total uptake of ¹⁴CO₂ and the ¹⁴C incorporation into aspartic acid.

A study of the intramolecular distribution of radioactivity showed that 71 to 76% of the ¹⁴C was located in the C₄ (-carboxyl) of malate and aspartate and the remainder in the C₁. This intramolecular labeling pattern did not change during the 5- to 60-second dark ¹⁴CO₂ fixation period and was scarcely altered by the presence of O₂. Three degradation techniques led to similar data.

The significance of these results is discussed taking into account the known possible carboxylation pathways. It is concluded that ribulose 1,5-bisphosphate can be a primary acceptor of CO₂ when maize leaves are preilluminated in the absence of O₂.