Early remobilization: A possible source of error in the ureide assay method for N2fixation measurement by early maturing soybean

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Abstract
The ureide assay method has been found to be suitable for soybean (Glycine max) to assess nitrogen ixation in a wide range of genotypes and Bradyrhizobium strains. The relationships between the relative abundance of ureide-N in xylem-sap and the proportion of plant N derived from nitrogen fixation were studied for early maturing soybeans grown under greenhouse and field conditions. Plants grown under greenhouse conditions were either supplemented with N-free nutrients or supplied with different levels of 15N-nitrate. In the field, soybeans were grown with or without inoculation, in the presence or absence of nitrogen fertilizer. Treatments were applied in order to obtain a wide range of nitrogen nutrition levels. The effect of the source and level of nitrogen nutrition on growth and nitrogen accumulation in plants, proportion of fixed nitrogen in the xylem-sap and in the shoots, and the proportions of N-compounds in the xylem-sap was determined. The greenhouse study demonstrated that the relative abundance of ureide-N was closely correlated with the proportion of fixed nitrogen measured in the xylem-sap from V2V3 until R7 stages. However, in both greenhouse and field experiments a positive correlation between the relative abundance of ureide-N in the xylem-sap and fixed nitrogen measured in the shoots was only observed until the beginning of seed filling. Since the disappearance of the relationship between relative ureides and nitrogen fixation corresponded to the beginning of the filling of the grain, and to the onset of remobilization in this early maturity genotype, it is suggested that interferences from ureides synthesized from breakdown products during senescence or released from storage pools may explain the lack of correlations later in the growth cycle. Further investigations on the origin of measured ureides at different development stages should therefore be carried out.