Dark CO2 fixation by cell-free preparations of the wood of Robinia pseudoacacia

Abstract

In the presence of phosphoenolpyruvate, cell-free preparations from distinct annual ring tissues of the trunk and the root of Robinia pseudoacacia L. incorporate 14CO2 into organic compounds. The CO2 fixation process exhibits a pH optimum at 7.8 and is activated by Mg2+. Aspartic acid contained 97% of the radioactivity incorporated. Oligosaccharides, malic acid, fumaric acid, glutamic acid, and three substances not identified with certainty show minor labeling. The activities of malate dehydrogenase (EC. 1.1.1.37), aspartate aminotransferase (EC. 2.6.1.1), and "malic enzyme" (EC. 1.1.1.40), involved in the further metabolism of the primary CO2 fixation product oxaloacetate, were detected spectrophotometrically in preparations from different annual rings of the trunk. Compared with the outer zones, the innermost part of the sapwood shows only little activity of these enzymes. The capacity to transfer CO2 into organic compounds decreases in the radial direction of both trunk and root wood. Heartwood preparations exhibit no CO2 fixation. In comparison to the trunk, the wood of the root shows only little dark CO2 fixation. The possible physiological role of this nonautotrophic CO2 binding in the wood of Robinia is discussed.