15N study on the partitioning of the nitrogen taken by soybeans from atmospheric dinitrogen, medium nitrate or ammonium

Soybean plants (Glycine max, cv. Akisengoku) were water-grown in a greenhouse with a low concentration of nitrate in the culture solution. Under these conditions root nodulation mostly on primary roots was profuse. At the vegetative and pod-filling stages, plants were fed 15N-labeled dinitrogen, nitrate, or ammonium for 7 to 8 hr in the daytime. Partitioning of the 15N from the three nitrogen sources showed distinct characteristics. The nitrogen (N) from dinitrogen preferentially distributed to the developing organs, young leaves, and developing pods, in comparison with N from nitrate. N from ammonium showed distribution pattern similar to that of N from dinitrogen. The nodule N was supplied by both directly fixed-N and transferred nitrogen, and fixedN was inferred to play a major role in this process. The pro tein N in the roots could be furnished by the recycled N of dinitrogen and nitrate through the shoots along with direct incorporation of amino acids produced in the roots from nitrate