Comparison of Nitrate-N Uptake between Nodulating and Non-nodulating Soybean Isolines

Nodulating and non-nodulating soybeans from the 3 pairs of near isogenic lines (Norin 2 and Tosan 89, T 202 and T 201, and Fujimijiro and Tosan 90) were grown in a culture solution containing 15N-nitrate and the amounts of 15N accumulated in whole plants were compared. Up to the flowering stage, the amounts of 15N taken up by each pair of nodulating and non-nodulating isolines were similar. At maturity, however, nodulating isolines accumulated considerably less 15N than the non-nodulating counterparts, with the differenccs being statistically significant excepting the isolines, T 202 and 201. Differential 15N uptake between these isolines was not due to the alteration of root development, because no differences in root dry weight were found between them. In consequence of these results, total amounts of N2 fixed, as measured by the N balance method (difference method) for Norin 2 and for Fujimijiro, were only one third and one half, respectively, of those estimated by the 15N isotope dilution method. These results point out the need of excercising caution in applying the N balance method even when near isogenic lines are used.