Characteristics of Asparagine Pool in Soybean Nodules in Comparison with Ureide Pool

The changes in the content of nitrogenous compound in intact soy bean nodules were investigated by inhibiting nitrogen fixation with acetylene application. The contents of ureides (allantoin and allantoic acid), asparagine, and glutamine decreased by the acetylene treatment. Incorporation of 15N into asparagine, ureides, and other nitrogenous compounds in intact soybean nodules was also investigated by the long term feeding of gaseous 15N2. Approximately 68% nitrogen in the ureide pool and 35% nitrogen in the asparagine pool were derived from gaseous 15N2 at 5.5 h after 15N2 exposure. The flow rate of newly fixed nitrogen via the asparagine pool was from 17 to 22% of that via the ureide pool, which was estimated based on the data of the acetylene inhibition experiment and 15N tracer experiment. A larger quantity of UN was incorporated into the amido-N of asparagine than into its amino-N, and 15N incorporation into ammonia was almost similar to that into asparagine at 5.5 h after 15N2 exposure. The characteristics of the asparagine, aspartate, ammonia, and ureide pools in soybean nodules are discussed based on these results.