Assimilation and transport of nitrate, nitrite, and ammonia absorbed by nodulated soybean plants

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Abstract
Nodulated soybean plants were treated with a culture solution containing 15N03 -, 15N02 -, or 15NH4 +, and the fates of these N sources were compared in the roots, nodules, stems, and leaves. The absorption rates of N and their partitioning patterns in the shoots were very similar for 15N03- and 15NH4+. Either N source was rapidly transported to the stems and leaves, and readily incorporated into proteins. However, l5NO2 absorption was only half as high as that of 15NO3- or 15NH4+. In addition, the N originating from 15NO2- was transported very slowly to the shoot organs, and accumulated in the roots especially in the form of glutamine and proteins. It was observed that the 15NH4 + or 15N02 - applied was rapidly translocated into the nodules without a lag-phase. However when 15NO3 - was added, a time-lag was observed in the incorporation of 15N into the nodules. The oxidation of nitrite to nitrate was assumed to have taken place in the nodules, and the activity was also detected in the isolated bacteroid suspension. 15N03 - administration resulted in the accumulation of ureides and asparagine in the nodules. On the other hand in the 15NH4 + or 15N02 - treatment, the accumulation of ureides and asparagine was observed in the stems. The ureide content in the leaves was markedly increased by the ammonium treatment.