Nitrogen fixation, amino acid, and ureide associations in chickpea

The metabolic products of nitrogen fixation (N-2) in a legume can be either amides (asparagine, glutamine) or ureides (allantoin and allantoic acid), which are then exported to the shoot via the xylem. Ureides are synthesized solely in the nodules. Chickpea (Cicer arientinum L.) has been separately classified as an amide and as a ureide exporter; however, the exact shoot metabolic products resulting from N-2 fixation are not known. The objectives of this study were (i) to determine the metabolites of N-2 fixation, namely free amino acids and ureides and (ii) to quantify the differences in N, fixation for chickpea cultivars in the field by N-15 natural abundance. Leaf ureide concentrations were analyzed at Weeks 6, 7, 8, 9, 10, and 11 after emergence. Free amino acid concentrations were analyzed at Weeks 7, 9, and 11 after emergence. Flax (Linum usitatissimum L., CDC-Bethune) was used as the reference crop for assessment of percentage nitrogen derived from the atmosphere. Two chickpea cultivars, CDC-Anna and Myles, had significantly higher N-2 fixation than the other three tested cultivars. Myles also maintained ureides and amides at a moderate concentration from flowering through reproductive growth. Overall, we found that asparagine and alanine were the major free amino acids, along with ureides, representing likely metabolites from N-2 fixation. Therefore, chickpea should be classified as both an amide and ureide exporter, on the basis of the concentration of both types of N product found in the shoot.