Philosophiae Doctor - PhD / Thoron Rn-220 is a radioactive gas with a half-life of 55.6 s. It has been identified as a possible health concern in specific places such as monazite processing plants and (rare-earth) mines. The short half-life of Rn-220 makes Rn-220 calibration sources and chambers less common than for the isotope Rn-222. There are many Rn-220 standard sources and chambers that are widely described in the literature and used for different applications and calibration. However, some of these chambers and sources are not easy to set up in typical nuclear environmental laboratories. In this project, we developed a Rn-220 standard source using a thorium nitrate solution (Th(NO3)4.6H2O). The solution was split into a large volume which was used in a Marinelli beaker to characterize its strength using a Hyper Pure Germanium (HPGe) detector, and a smaller volume of around 30 ml which was poured into a small bottle. The Rn-220 is extracted by bubbling air through the solution in the small bottle using an aerator. Gamma rays from the solution were measured simultaneously using a 76.2 mm × 76.2 mm NaI(Tl) detector. The gamma rays were measured for 66 hours. The accumulated spectra were thereafter analysed using an Excel spreadsheet where the counts in the Tl-208(2614 keV) peak were extracted and used to obtain the percentage of Rn-220 pumped out of the solution in the small bottle.

Sink limitations in plants reduce the potential for photosynthesis and yield, particularly under conditions that favour enhanced source activity such as elevated CO2 (EC). Dark respiration, considered catabolic, has rarely been exploited to enhance sink activity in plants. Arabidopsis thaliana L. lines with partially-suppressed mitochondrial pyruvate dehydrogenase (mtPDH) kinase (mtPDHK), a negative post-translational regulator of the mtPDH complex, was shown previously to have both elevated mtPDH complex activity and increased seed weight and oil content at ambient CO2 (AC), suggesting an enhancement of sink activity. The mtPDH links glycolysis with the tricarboxylic acid (TCA) cycle. It was hypothesized that Arabidopsis having suppressed mtPDHK will display their greatest plant productivity at EC through a combined enhancement of source and sink activities. Control and transgenic Arabidopsis having either constitutive or seed-specific expression of antisense mtPDHK were grown at either AC or EC. Expression of mtPDHK and mtPDH complex activity in rosette leaves and reproductive tissues were measured, which required the development of an assay to quantify mtPDH activity. Vegetative and reproductive growth over time, seed oil parameters, and leaf net C exchange were also quantified. A parabolic relationship was found between mtPDHK expression and mtPDH activity, reflecting a role for mtPDH in balancing photosynthetic and respiratory processes. A number of growth and seed oil parameters were improved in transgenic lines, particularly at EC; many of these parameters showed a significant linear or quadratic correlation with mtPDHK expression and mtPDH activity. The proportion of very long chain fatty acids was increased in transgenic lines. Leaf net C exchange was enhanced at AC and EC, and particularly in lines showing repression of mtPDHK. The greatest enhancement in total seed and oil productivity was found for the constitutive lines 104 and 31 at EC (up to 2.8 times). These two lines exhibited a significant increase in inflorescence size, an increase in leaf water use efficiency, the lowest rate of mtPDH complex inactivation by ATP, and an intermediary enhancement of mtPDH complex activity in seeds. Thus, it is concluded that the mtPDH plays a key role in regulating sink and source activities in plants. / Natural Sciences and Engineering Research Council (NSERC) through the Green Crop Networks Research Network; Ontario Graduate Scholarship; Syngenta Graduate Scholarship; Ball Farm Services and Agrico Canada Ltd. Scholarship; Mrs. Fred Ball Scholarship; Arthur D. Latornell Scholarship; Hoskins Scholarship; Robb Travel Grant; Registrars and the Deans Scholarship and travel awards and bursaries from the University of Guelph, and the Ontario Agricultural College.