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.

When dark (14)CO(2) fixation in maize leaves was carried out under anaerobic conditions after preillumination in the absence of O(2), the (14)C incorporation in aspartic acid was transient; its maximum level was very low compared with that of malic acid. The addition of 5% O(2) during the dark fixation period increased the total uptake of (14)CO(2) and the (14)C incorporation into aspartic acid.A study of the intramolecular distribution of radioactivity showed that 71 to 76% of the (14)C was located in the C(4) (beta-carboxyl) of malate and aspartate and the remainder in the C(1). This intramolecular labeling pattern did not change during the 5- to 60-second dark (14)CO(2) fixation period and was scarcely altered by the presence of O(2). 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 CO(2) when maize leaves are preilluminated in the absence of O(2).