Carbon Dioxide Fixation by Lupin Root Nodules: I. Characterization, Association with Phosphoenolpyruvate Carboxylase, and Correlation with Nitrogen Fixation during Nodule Development

In vivo CO₂ fixation and in vitro phosphoenolpyruvate (PEP) carboxylase levels have been measured in lupin (Lupinus angustifolius L.) root nodules of various ages. Both activities were greater in nodule tissue than in either primary or secondary root tissue, and increased about 3-fold with the onset of N₂ fixation. PEP carboxylase activity was predominantly located in the bacteroid-containing zone of mature nodules, but purified bacteroids contained no activity. Partially purified PEP carboxylases from nodules, roots, and leaves were identical in a number of kinetic parameters. Both in vivo CO₂ fixation activity and in vitro PEP carboxylase activity were significantly correlated with nodule acetylene reduction activity during nodule development. The maximum rate of in vivo CO₂ fixation in mature nodules was 7.9 nmol hour⁻¹ mg fresh weight⁻¹, similar to rates of N₂ fixation and reported values for amino acid translocation.