Nitrogen assimilation in soybean nodules

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Abstract
The fate of nitrogen originating from N2 and NO3 in soybean plants was elucidated through continuous supply of 15N labeled nitrate. Soybean plants were harvested at the ini tial flowering stage and the initial pod filling stage. The percentages of nitrogen originating from NO3 in allantoin in the stems^petioles and leaf blades were almost the same as in the nodules; however, a higher percentage was observed in the roots than in the other organs. These results indicate that most of the allantoin in the shoots derives from nodules, and allantoin formed in the roots from NO3 contributes very little to nitrogen transportation to the shoots. Allantoin was not detected in peeled stem bark of 40-day-old plants, suggesting that allantoin was transported mainly via xylem vessels of the stems. The allantoin concentration was exclusively high in the pods, and it is suggested that the allantoin nitrogen was used for pod protein synthesis. The effects of various nitrogen fertilizers on the nodule formation and the concentrations of allantoin and amino acids in the shoots of soybean plants were investigated. The concentration of amino acids was basically unaffected by nodulation; however, a low concentration of allantoin was detected in all plants including non-noduaated ones, and after a certain critical stage of nodule growth, the content of allantoin increased in proportion to the increase in nodule weight.