Carbon Dioxide Fixation by Lupin Root Nodules

Publisher Website
Google Scholar
Abstract
In vivo CO2 fixation and in vitro phosphoenolpyruvate (PEP) carboxylase levels were 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[Rhizobium]-containing zone of mature nodules. 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 h-1 mg fresh weight-1, similar to rates of N2 fixation and reported values for amino acid translocation. The results suggest that the oxaloacetate used as the primary Ca skeleton acceptor for ammonia assimilation and amino acid synthesis in lupin nodules is provided via the PEP carboxylase reaction rather than through the tricarboxylic acid cycle. The source of PEP is presumably glycolysis; the major source of CO2 is presumably respiration.