Assimilation and transport of nitrogenous compounds originated from15N2fixation and15NO2absorption

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Abstract
Nodulated leguminous plants utilize both combined nitrogen absorbed by roots and gaseous nitrogen fixed by root nodules. In order to elucidate the physiological role of N2 fixation compared with NO2 absorption, the assimilation and transport of nitrogenous compounds from these nitrogen sources were studied. One part of soybean plants was administered with labeled nitrogen gas and unlabeled nitrate, and another part with unlabeled nitrogen gas and labeled nitrate. After feeding of labeled compounds to roots, ammonia, nitrate, amino acids, amides and allantoin in nodules, roots and stems were separated, and 15N content of them were determined optically using JASCO NIA-l 15N-analyzer. In nodules supplied with 15N2, high 15N contents were found in glutamate, alanine, serine, γ-amino-butyrate, but in roots supplied with 15NO2, asparagine showed the highest 15N content after 8 hr 15N feeding. In stems, aIIantoin showed the highest 15N content in the case of 15N2 treatment, and the ratio (15N from 15N2/15N from 15NO2) of allantoin showed 10-50 fold higher than the ratios of other compounds, suggesting that allantoin was actively synthesized in nodules.