Amino acid composition in xylem sap of soybean related to the evaluation of N2fixation by the relative ureide method

Abstract

Abstract 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 + α-amino-N + nitrate-N)] proposed by Herridge et al. to [ureide-N/(ureide-N + 2 × α-amino-N + nitrate-N)], 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×” 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 ...