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

Abstract

The magnitude and role of dark CO(2) fixation were examined in nodules of intact soybean plants (Harosoy 63 x Rhizobium japonicum strain USDA 16). The estimated rate of nodule dark CO(2) fixation, based on a 2 minute pulse-feed with (14)CO(2) 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 CO(2) 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 CO(2) within 60 minutes. The labeling pattern during pulse-chase experiments was consistent with CO(2) fixation by phosphoenolpyruvate carboxylase. During the chase, the greatest loss of label occurred in organic acids. Exposure of nodulated roots to Ar:O(2) (80:20) did not affect dark CO(2) fixation, while exposure to O(2):CO(2) (95:5) resulted in 54% inhibition. From these results, it was concluded that at least 66% of dark CO(2) 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.