Non-invasive imaging and characterization of absorption, transport and accumulation of cadmium in an intact rice plant
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.