Non-invasive imaging of nitrogen fixation and carbon translocation in intact soybean plants using a positron-emitting tracer imaging system

Ishii, Satomi; Ito, Sayuri*; Suzui, Nobuo; Ishioka, Noriko; Kawachi, Naoki; Otake, Norikuni*; Oyama, Takuji; Fujimaki, Shu

The positron-emitting tracer imaging system (PETIS) is one of the non-invasive imaging tools using specialized for plant nutrition. PETIS can visualize radiotracer in a living organism and provides real-time 2D images like a video camera. We visualized nitrogen fixation and carbon translocation in intact nodulated soybean plants. We developed a rapid method to produce and purify N-labeled nitrogen gas, and fed it as a mixture of N, O and He (10:20:70) to the underground part. As a result, serial images of distribution of the fixed N were obtained, and the N fixation rates were estimated from the PETIS data. Allocation of photosynthates to root nodules was analyzed non-invasively using CO. C-photosynthates were transported to the root base within about 20 min and to the root tips within one hour after feeding of CO to the leaves. Most of the C-photosynthates in the underground part were localized to the root base where many nodules were formed. It was shown that larger amount of C-photosynthates was transported into the nodules on the root base than into those on the distal regions. This suggested that the position of nodules in the roots may be a dominant determining factor for the sink activity of the photoassimilates, and that the basal nodules may have higher nitrogen fixation activity.