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

氮是植物最重要的营养元素。大豆植株可以利用根瘤与根瘤菌共生的根瘤所固定的大气N_2中的氮素，本研究的目的是对根瘤中的N_2固定及其向地上部分的运输进行可视化和定量分析。一个更高的放射性的示踪剂，超过100 MBq，需要可视化的固定氮运输到地上部分。本研究采用一种全新的示踪气体生产方法，即用质子离子辐照靶气体，通过~（13）O（p，α）~（13）N核反应获得~（13）N核素，直接将N_2O还原为N_2，得到高放射性、高纯度的示踪气体<13><16><13>。将经辐照的气体引入新的净化系统的管线。剩余的CO_2被碱石灰吸收，而<13>辐照的副产物^ N_2O被加热的Cu珠立即还原为^<13>N_2。将生成的纯N_2<13>与非放射性N_2、O_2和Ar适当混合。我们将示踪气体通入大豆植株的地下部分10分钟，并通过PETIS收集连续图像1小时。最终成功地获得了输送到空中的图像。

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.