Visualization of the initial fixed nitrogen transport in nodulated soybean plant using N2 tracer gas

The observation of initial transport of fixed nitrogen in intact soybean plants in real-time was conducted by using the positron-emitting tracer imaging system (PETIS). Soybean root nodules were fed with [13N]N2 for 10 minutes, and the radioactivity of [13N]N tracer was recorded for 60 minutes. The serial images of nitrogen fixation activity and translocation of fixed nitrogen in the soybean plant were reconstructed to estimate the fixed-N transport to the upper shoot. As a result, the signal of nitrogen radiotracer moving upward through the intact stem was successfully observed. This is the first report that the translocation of fixed-N is visualized in real-time in soybean plant by a moving image. The signal of nitrogen radiotracer appeared at the base stem at about 20 minutes after the feeding of tracer gas and it took 40 minutes to reach the upper stem. The velocity of fixed nitrogen translocation was estimated approximately at 1.63 cm min-1. The autoradiography taken after PETIS experiment showed a clear picture of transport of fixed 13N in the whole plant that the fixed-N moved not only via xylem system but also via the phloem system to the shoot after transferring from xylem to phloem in the stem although it has been generally considered that the fixed-N in nodule is transported dominantly via xylem by transpiration stream toward mature leaves. This result also suggests that the initial transport of fixed-N was mainly into the stem and subsequently translocated to young leaves and buds via the phloem system. These new findings in the initial transport of fixed nitrogen of soybean by PETIS observation will become the basis for future study of fixed-N transport in the whole legume plants.