Metabolic changes in soybean nodules after inhibition of nitrogen fixation by treatment with oxygen

Metabolites that accumulated in soybean [Glycine max (L.) Merr.]nodules after inhibition of nitrogen fixation were analysedto determine what carbon compounds the bacteroids might obtainfrom their host. Exposure of roots of intact soybean plantsto 100% O₂ for 5 min caused a decrease in acetylene reductionactivity within 10 min and then the activity recovered onlyslowly. Analysis of carbohydrates, organic acids, volatile compoundsand amino acids in extracts of nodules revealed that succinate,malate and alanine all accumulated within 10 min after treatmentwith O₂. The concentrations of sucrose, acetone, tyrosine, valine,isoleucine, leucine, and ornithine in the nodules increasedslowly after such treatment. The results are discussed in termsof carbon sources for supporting nitrogen fixation of soybeanbacteroids. Key words: Glycine max, carbon metabolism, nitrogen fixation, nodules