Quantitative analysis of the initial transport of fixed nitrogen in nodulated soybean plants using 15N as a tracer

The quantitative analysis of the initial transport of fixed isotope 15-nitrogen (N-15) in intact nodulated soybean plants (Glycine max [L.] Merr. cv. Williams) was investigated at the vegetative stage (36 days after planting, DAP) and pod-filling stage (91 DAP) by the N-15 pulse-chase experiment. The nodulated roots were exposed to N-2 gas labeled with a stable isotope N-15 for 1h, followed by 0, 1, 3 and 7h of exposure with normal air. Plant roots and shoots were separated into three sections (basal, middle and distal parts) with the same length of the main stem or primary root. Approximately 80 and 92% of fixed N was distributed in the basal part of the nodulated roots at the vegetative and pod-filling stages by the end of 1h of N-15(2) exposure, respectively. In addition, about 90% of fixed N-15 was retained in the nodules and 10% was exported to root and shoot after 1 h of N-15(2) exposure at 91 DAP. The percentage distribution of N-15 in the nodules at the pod-filling stage decreased from 90% to 7% during the 7h of the chase period, and increased in the roots (14%), stems (54%), leaves (12%), pods (10%) and seeds (4%). The N-15 distribution was negligible in the distal root segment, suggesting that N fixation activity was negligible and recycling fixed N from the shoot to the roots was very low in the initially short time of the experiment.