Amino acid composition in xylem sap of soybean [Glycine max] related to the evaluation of N2 fixation by the relative ureide method

For the estimation of N2 fixation of soybean (Glycine max (L.) Merr.) by the relative ureide method, we modified the original equation [ureide-N / (ureide-N + alpha-amino-N + nitrate-N)] proposed by Herridge et al. to [ureide-N / (ureide-N + 2 x alpha-amino-N + nitrate-N9], based on the observation that asparagine which has two N atoms in one molecule was the major amino acid in the xylem sap of soybean plants. However, since the amino acid composition may change with the growth stages or environmental factors such as N availability, it is necessary to determine whether the factor "2 x" is appropriate. In the present paper, the experiments were carried out to analyze the levels and relative composition of amino acids and amides in the xylem sap of soybean cv. Enrei which was grown in the field with three fertilizer treatments, i.e. control with a small amount of basal dressing of ammonium sulfate, deep placement or top dressing of coated urea (slow release N fertilizer). Although the composition of the amino acids and amides changed with the fertilizer treatments or growth stages, the major form was always asparagine. The concentration of asparagine as well as glutmine, arginine, and histidine increased with growth till the R5 stage. The levels of valine, lysine, threonine, isoleucine, leucine, phenylalamine, and serine rose remarkably at the maturation stage. In contrast, the levels of glutamic acid, GABA, alanine, and glycine decreased through maturation. In the control treatment, the proportion of asparagine accounted for about 30-40% of total amino-N till the R3 stage, then increased to about 50% at maturation. The proportion of asparagine exceeded 70% at the R5 stage with either deep placement or top dressing of coated urea