The absorption, transport and accumulation of cadmium (Cd) in rice (Oryza sativa L. cv. Nipponbare) were visualized and characterized quantitatively using the positron-emitting tracer imaging system (PETIS). We developed a method to produce a positron-emitting Cd-107 (half-life: 6.5 hr) tracer by ion-beam bombardment and chemical purification. The tracer was fed to the hydroponic culture and serial images of Cd distribution in the intact rice plants at vegetative stage and grain-filling stage were obtained by PETIS in every four minutes for 36 hours. The images showed that Cd moved up through the root and leaf sheath (vegetative stage) and through the culm to panicles (grain-filling stage) with velocities of a few cm/h. The most characteristic feature of the Cd dynamics was intensive accumulation in the nodes both at vegetative and grain-filling stages. It was found Cd moved from the shoot base into crown roots at vegetative stage. In contrast, no Cd was detected in the leaf blades. These results suggest that xylem-to-phloem transfer is a pivotal step in long-distance transport of Cd from the soil to the grains and the nodes are the most likely organ where the transfer takes place.

We analyzed the real-time translocation of 107Cd from root to shoot among 6 rice cultivars using a positron-emitting tracer imaging system (PETIS). 107Cd first accumulated at the basal region of the shoot within 2 h after the exposure and the signals were much stronger in high-Cd-accumulating cultivars (indica) than in low-Cd-accumulating ones (japonica). 107Cd accumulated increasingly at the upper portion of the shoot with time, and the signals were obviously strong in high-Cd-accumulating cultivars. These results suggest that the different ability of root-to-shoot translocation of Cd is closely related to the genotypic variation in shoot Cd accumulation in rice.

We developed analytical methods for monitoring carbon translocation and nitrogen fixation in intact plants using short-lived radioactive tracer gas and the positron-emitting tracer imaging system (PETIS). In the analysis of carbon translocation, we fed ¹¹C (half life: 20.4 min)-labeled radioactive carbon dioxide gas to leaf blades of rice plants, and serial images of ¹¹C-photoassimilate were obtained non-invasively using PETIS. In order to understand source-sink relations, we manipulated source and sink strength by treating tested rice plants with p-chlorobenzenesulfonic acid (PCMBS), an inhibitor of sucrose transporters. We developed an analytical algorithm to estimate the velocity of ¹¹C-photoassimilate flow from the serial images. As a result, a decrease in the velocity after the manipulation was successfully detected. In the analysis of nitrogen fixation, we newly developed a rapid method to produce and purify ¹³N (half life: 10.0 min)-labeled radioactive nitrogen gas and fed the gas to the underground part of nodulated soybean plants. Serial images of distribution of ¹³N were obtained and obvious signal of ¹³N was observed at the nodules. To our knowledge, this is the first example of real-time imaging of nitrogen fixation in an intact plant.

We characterized the transport of cadmium (Cd) in soybean (Glycine max [L.] Merr.). A positron-emitting ¹⁰⁷Cd (half-life: 6.5 hr) tracer and a gamma-emitting ¹⁰⁹Cd (half-life: 453 d) tracer with non-radioactive Cd (final conc. 0.1 µM Cd) were fed as a mixture to the hydroponic culture. ¹⁰⁷Cd distribution in the intact test plants was imaged non-invasively using the positron-emitting tracer imaging system (PETIS). The test plants were sampled and separated to roots, stems, petioles, leaves, pods and seeds at about 2, 3 and 5 days after Cd feeding. After the sufficient decay of ¹⁰⁷Cd, ¹⁰⁹Cd accumulation in each part was analyzed and quantified using autoradiography and well-type counter. The PETIS images showed that the Cd reached shoot base about a few hours after feeding and was transported to upper nodes through the stem. The autoradiographic images revealed that a part of Cd was transported to the pods and seeds without passing through leaves within 2 days after Cd feeding. And the results obtained from well-type counter showed that a part of Cd absorbed by the roots moved and accumulated into the seeds, pods, leaves and petioles gradually within 5 days.

Reduction of cadmium (Cd) accumulation in farm products has become more and more important in order to produce them in a safe and sustainable manner. It is necessary to elucidate mechanisms of Cd distribution in plants. However, these mechanisms are not fully understood. The purpose of our work is to clarify these mechanisms by visualizing Cd absorption, transport and accumulation non-invasively using positron-emitting tracer imaging system (PETIS). ¹⁰⁷Cd (half-life: 6.5 hr) was used as a positron-emitting tracer in the PETIS experiments. We succeeded to obtain serial images of cadmium distribution in oilseed rape plants (Brassica napus L.). Strong ¹⁰⁷Cd signals were observed in the basal region of the shoot. We also could see strong signals in the node of oilseed rape plants. Cd distribution in oilseed rape plants will be discussed quantitatively using the results from PETIS experiments.