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Carbon Dioxide Fixation by Lupin Root Nodules

II. Studies with ¹⁴C-labeled Glucose, the Pathway of Glucose Catabolism, and the Effects of Some Treatments That Inhibit Nitrogen Fixation

Plant Physiology Division, Department of Scientific and Industrial Research, Palmerston North, New Zealand

Labeling studies using detached lupin (Lupinus angustifolius) nodules showed that over times of less than 3 minutes, label from [3,4-¹⁴C]glucose was incorporated into amino acids, predominantly aspartic acid, to a much greater extent than into organic acids. Only a slight preferential incorporation was observed with [1-¹⁴C]- and [6-¹⁴C]glucose, while with [U-¹⁴C]-glucose more label was incorporated into organic acids than into amino acids at all labeling times. These results are consistent with a scheme whereby the "carbon skeletons" for amino acid synthesis are provided by the phosphoenolpyruvate carboxylase reaction.

A comparison of ¹⁴CO₂ release from nodules supplied with [1-¹⁴C]- and [6-¹⁴C]glucose indicated that the oxidative pentose phosphate pathway accounted for less than 6% of glucose metabolism. Several enzymes of the oxidative pentose phosphate and glycolytic pathways were assayed in vitro using the 12,000g supernatant fraction from nodule homogenates. In all cases, the specific activities were adequate to account for the calculated in vivo fluxes.

Three out of four diverse treatments that inhibited nodule nitrogen fixation also inhibited nodule CO₂ fixation, and in the case of the fourth treatment, replacement of N₂ with He, it was shown that the normal entry of label from exogenous ¹⁴CO₂ into the nodule amino acid pool was strongly inhibited.