<p>Over the past 200 years, the Earth's atmospheric carbon dioxide (CO<sub>2</sub>) concentration has increased by more than 35%, and climate experts predict that CO<sub>2</sub> levels may double by the end of this century. Understanding the mechanisms of resource management in plants is fundamental for predicting how plants will respond to the increase in atmospheric CO<sub>2</sub>. Plant productivity sustains life on Earth and is a principal component of the planet's system that regulates atmospheric CO<sub>2</sub> concentration. As such, one of the central goals of plant science is to understand the regulatory mechanisms of plant growth in a changing environment. Short-lived positron-emitting radiotracer techniques provide time-dependent data that are critical for developing models of metabolite transport and resource distribution in plants and their microenvironments. To better understand the effects of environmental changes on resource transport and allocation in plants, we have developed a system for real-time measurements of metabolite transport in plants using short-lived positron-emitting radiotracers. This thesis project includes the design, construction, and demonstration of the capabilities of this system for performing real-time measurements of metabolite transport in plants.</p><p>The short-lived radiotracer system described in this dissertation takes advantage of the combined capabilities and close proximity of two research facilities at Duke University: the Triangle Universities Nuclear Laboratory (TUNL) and the Duke University Phytotron, which are separated by approximately 100 meters. The short-lived positron-emitting radioisotopes are generated using the 10-MV tandem Van de Graaff accelerator located in the main TUNL building, which provides the capability of producing
short-lived positron-emitting isotopes such as carbon-11 (<sup>11</sup>C; 20 minute half-life), nitrogen-13 (<sup>13</sup>N; 10 minute half-life), fluorine-18 (<sup>18</sup>F; 110 minute half-life), and oxygen-15 (<sup>15</sup>O; 2 minute half-life). The radioisotopes may be introduced to plants as biologically active molecules
such as <sup>11</sup>CO<sub>2</sub>, <sup>13</sup>NO<sub>3</sub><sup>-</sup>, <sup>18</sup>F<sup>-</sup>-[H<sub>2</sub>O], and H<sub>2</sub><sup>15<\sup>O. Plants for these studies are grown in controlled-environment chambers at the Phytotron. The chambers offer an array of control for temperature, humidity, atmospheric CO<sub>2</sub> concentration, and light intensity. Additionally, the Phytotron houses one large reach-in growth chamber that is dedicated to this project for radioisotope labeling measurements.</p><p>There are several important properties of short-lived positron-emitting radiotracers that make them well suited for use in investigating metabolite transport in plants. First, because the molecular mass of a radioisotope-tagged compound is only minutely different from the corresponding stable compound, radiotracer substances should be metabolized and transported in plants the same as their non-radioactive counterparts. Second, because the relatively high energy gamma rays emitted from electron-positron annihilation are attenuated very little by plant tissue, the real-time distribution of a radiotracer can be measured <em>in vivo</em> in plants. Finally, the short radioactive half-lives of these isotopes allow for repeat measurements on the same plant in a short period of time. For example, in studies of short-term environmental changes on plant metabolite dynamics, a single plant can be labeled multiple times to measure its responses to different environmental conditions. Also, different short-lived radiotracers can be applied to the same plant over a short period of time to investigate the transport and allocation of various metabolites.</p><p>This newly developed system provides the capabilities for production of <sup>11</sup>CO<sub>2</sub> at TUNL, transfer of the <sup>11</sup>CO<sub>2</sub> gas from the target area at TUNL to a radiation-shielded cryogenic trap at the Phytotron, labeling of photoassimilates with <sup>11</sup>C, and <em>in vivo</em> gamma-ray detection for real-time measurements of the radiotracer distribution in small plants. The experimental techniques and instrumentation that enabled the quantitative biological studies reported in this thesis were developed through a series of experiments made at TUNL and the Phytotron. Collimated single detectors and coincidence counting techniques were used to monitor the radiotracer distribution on a coarse spatial scale. Additionally, a prototype Versatile Imager for Positron Emitting Radiotracers (VIPER) was
built to provide the capability of measuring radiotracer distributions
in plants with high spatial resolution (~2.5 mm). This device enables detailed
quantification of real-time metabolite dynamics on fine spatial scales.</p><p>The full capabilities of this radiotracer system were utilized in an
investigation of the effects of elevated atmospheric CO<sub>2</sub> concentration and root nutrient availability on the transport and allocation of recently fixed carbon, including that released from the roots via exudation or respiration, in two grass species. The <sup>11</sup>CO<sub>2</sub> gas was introduced to a leaf on the plants grown at either ambient or elevated atmospheric CO<sub>2</sub>. Two sequential measurements were performed per day on each plant: a control nutrient solution labeling immediately followed by labeling with a 10-fold increase or decrease in nutrient concentration. The real-time distribution of <sup>11</sup>C-labeled photoassimilate was measured <em>in vivo</em> throughout the plant and root environment. This measurement resulted
in the <em>first</em> observation of a rapid plant response to short-term changes in nutrient availability via correlated changes in the photoassimilate allocation to root exudates. Our data indicated that root exudation was consistently enhanced at lower nutrient concentrations. Also, we found that elevated atmospheric CO<sub>2</sub> increased the velocity of
photoassimilate transport throughout the plant, enhanced root exudation in an annual crop grass, and reduced root exudation in a perennial native grass.</p> / Dissertation
| Identifer | oai:union.ndltd.org:DUKE/oai:dukespace.lib.duke.edu:10161/708 |
| Date | 29 July 2008 |
| Creators | Kiser, Matthew Ryan |
| Contributors | Howell, Calvin R |
| Source Sets | Duke University |
| Language | en_US |
| Detected Language | English |
| Type | Dissertation |
| Format | 6907695 bytes, application/pdf |
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