Symbiotic Performance of Supernoclulating Soybean (Glycine max(L.) Merrill) Mutants during Development on Different Nitrogen Regimes

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Abstract
Growth and symbiotic performance of soybean (Glycine max (L.) Merrill) cv. Bragg and three of its induced nodulation mutants (nod49, non-nodulating; ntsl 116, intermediate supernodulator; nts1007, extreme supernodulator) were compared throughout development under different nitrogen regimes (0, 2, 5 and 10 mol nitrate m−3). Nitrogen fixation was assessed using 15N-isotope dilution and xylem sap analysis for ureide content. Both techniques confirmed a complete lack of N2 fixation activity in nod49. Plant reliance on nitrogen fixation by the other genotypes was dependent on the nitrate regime and the developmental stage. The ntsl007 and ntsl 116 mutants fixed more nitrogen than the parent cultivar in the presence of 10 mol m−3 nitrate in the nutrient solution, but higher input of symbiotically derived nitrogen was still insufficient to offset the amount of nitrogen removed in the harvested seed. However, the mutants utilized less nitrate for growth than Bragg. Comparison of estimates of N2 fixation derived from the 15N-dilution technique with those based on relative ureide content of xylem sap indicated that the latter offered a simple and reliable procedure for evaluating the symbiotic performance of supernodulating plants.