EARLY REMOBILIZATION - A POSSIBLE SOURCE OF ERROR IN THE UREIDE ASSAY-METHOD FOR N-2 FIXATION MEASUREMENT BY EARLY-MATURING SOYBEAN

The ureide assay method has been found to be suitable for soybean (Gly cine max) to assess nitrogen fixation in a wide range of genotypes and Bradyrhizobium strains. The relationships between the relative abunda nce of ureide-N in xylem-sap and the proportion of plant N derived fro m nitrogen fixation were studied for early maturing soybeans grown und er greenhouse and held conditions, Plants grown under greenhouse condi tions were either supplemented with N-free nutrients or supplied with different levels of N-15-nitrate. In the field, soybeans were grown wi th or without inoculation, in the presence or absence of nitrogen fert ilizer. Treatments were applied in order to obtain a wide range of nit rogen nutrition levels. The effect of the source and level of nitrogen nutrition on growth and nitrogen accumulation in plants, proportion o f fixed nitrogen in the xylem-sap and in the shoots, and the proportio ns of N-compounds in the xylem-sap was determined. The greenhouse stud y demonstrated that the relative abundance of ureide-N was closely cor related with the proportion of fixed nitrogen measured in the xylem-sa p from V2V3 until R(7) stages. However, in both greenhouse and field e xperiments a positive correlation between the relative abundance of ur eide-N in the xylem-sap and fixed nitrogen measured in the shoots was only observed until the beginning of seed filling. Since the disappear ance of the relationship between relative ureides and nitrogen fixatio n corresponded to the beginning of the filling of the grain, and to th e onset of remobilization in this early maturity genotype, it is sugge sted that interferences from ureides synthesized from breakdown produc ts during senescence or released from storage pools may explain the la ck of correlations later in the growth cycle. Further investigations o n the origin of measured ureides at different development stages shoul d therefore be carried out.