REAL-TIME MONITORING AND ANALYSIS OF NUTRIENT TRANSPORTATION IN A LIVING PLANT USING A POSITRON EMITTING TRACER IMAGING SYSTEM (PETIS)

摘要

The radioisotope (RI) is a powerful tool in life-science studies that is used for the observation of substance transportation and metabolism. In the physiological study of plants, the application of radioisotope allows the dynamic analysis of assimilation, transportation, and accumulation of nutrients or other substances in plants. Generally, only a few radioisotopes are used in plant science. Carbon-14 (~(14)C) and tritium (~3H) are used for radiographs to observe the distribution of substances in a plant, and phosphorus-32 (~(32)P) and iodine-131 (~(131)I) are used for molecular biological experiments to determine the role of genes. Application of these radioisotopes provides static information on plant physiology. These radioisotopes decay with β-mode and single weak β radiation was emitted by the decay. It is necessary to prepare a thin slice or an extraction of the sample for the detection of these radioisotopes. Therefore, it is quite difficult to detect these radioisotopes in living things non-invasively. The number of radioisotopes used for biological studies is not very high. Every element has isotopes. Some of them are stable, and others are radioactive. For example, carbon has isotopes ~(10)C, ~(11)C, ~(12)C, ~(13)C, ~(14)C, and ~(15)C. ~(12)C and ~(13)C are stable, and ~(10)C, ~(11)C, ~(14)C, and ~(15)C are radioactive. A radioisotope has an intrinsic decay mode and a decay constant (half-life time). The main decay mode of ~(10)C is β+, and the half-life time is 19.4 sec. The main decay mode and half-life time of ~(11)C, ~(14)C, and ~(15)C are β+ and 20.4 min, β- and 5730 year, and β - and 2.4 sec, respectively. ~(11)C- or ~(14)C-labeled compounds, such as carbon dioxide, are being used for plant physiology studies, especially in the study of the mechanism of photosynthesis and transportation of photoassimilates in plants. The half-lives of ~(10)C and ~(15)C are too short; therefore, they cannot easily be used for plant physiology experiments. The long half-life of ~(14)C allows the chase of ~(14)C-labeled compounds for a long period in plants, but it is difficult to detect the radiation of ~(14)C non-invasively. The half-life of ~(11)C is shorter than that of ~(14)C, but the decay mode of ~(11)C allows non-invasive monitoring of ~(11)C-labeled compounds in living plants. ~(11)C is called a positron-emitting nuclide. A positron-emitting nuclide emits a positron (β + particle) when it decays. A positron annihilates with an electron, and the mass of the particles converts to a pair of 511 keV γ-rays that are emitted at 180 degrees from each other. The annihilation γ-rays easily escape from the plant body and may be detected externally. Coincidence detection of a pair of annihilation γ -rays provides non-invasive monitoring of the process of transportation of a positron-emitting nuclide-labeled substance in a living plant with high spatial resolution. In medicine, positron-emitting nuclei, such as ~(11)C, ~(13)N, ~(15)O, and ~(18)F, are exploited for understanding the biochemical basis of normal and abnormal functions within the body and biochemical examination of patients as part of their clinical care.