Analysis of kinetics of nitrogen fixation in soybean plants by positron imaging system

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Nitrogen is the most important nutrient for plants. Soybean plants can utilize nitrogen (N) from atmospheric N_2 fixed by nodules which is symbiotic organ of leguminous plants with rhizobium.The purpose of our study is to visualize and analyze the N2 fixation in the nodules and transport to the aerial part noninvasively and quantitatively. A much higher radioactivity of the tracer, more than 100 MBq, was required for visualization of the transport of fixed nitrogen to the aerial part. In this study, we started to develop a fully new production method to produce a tracer gas with high radioactivity and purity by reduction of N_2O to N_2 directly.The target CO_2 gas was irradiated with proton ions as conventional methods to obtain ^<13>N nuclide through the ^<16>O (p, α) ^<13>N nuclear reaction. The irradiated gas was introduced to a line of the new purification system. The remaining CO_2 was absorbed in soda-lime, and ^<13>N_2O, a byproduct of the irradiation, was instantly reduced to ^<13>N_2 by heated Cu beads. The resultant pure ^<13>N_2 was mixed properly with nonradioactive N_2, O_2 and Ar. We fed the tracer gas to the underground part of soybean plants for 10 min and collected the serial images for one hour by PETIS. As a result, we finally obtained the image of transport to the aerial part successfully.