The Role of Dark Carbon Dioxide Fixation in Root Nodules of Soybean

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Abstract
The magnitude and role of dark CO2 fixation were examined in nodules of intact soybean plants (Harosoy 63 × Rhizobium japonicum strain USDA 16). The estimated rate of nodule dark CO2 fixation, based on a 2 minute pulse-feed with 14CO2 under saturating conditions, was 102 micromoles per gram dry weight per hour. This was equivalent to 14% of net nodule respiration. Only 18% of this CO2 fixation was estimated to be required for organic and amino acid synthesis for growth and export processes. The major portion (75-92%) of fixed label was released as CO2 within 60 minutes. The labeling pattern during pulse-chase experiments was consistent with CO2 fixation by phosphoenolpyruvate carboxylase. During the chase, the greatest loss of label occurred in organic acids. Exposure of nodulated roots to Ar:O2 (80:20) did not affect dark CO2 fixation, while exposure to O2:CO2 (95:5) resulted in 54% inhibition. From these results, it was concluded that at least 66% of dark CO2 fixation in soybean may be involved with the production of organic acids, which when oxidized would be capable of providing at least 48% of the requirement for ATP equivalents to support nitrogenase activity.