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UBC Theses and Dissertations

EPR investigation of free radicals in excised and attached leaves subjected to ozone and sulphur dioxide air pollution Vaartnou, Manivalde


The X-band EPR spectrometry system was modified to allow for the in situ monitoring of free radical changes in attached, intact plant leaves, which were caused by stress factors such as exposure to excessive photon flux density, ozone or sulphur dioxide. This was done through use of the dewar insert of the variable temperature accessory as a guide, the construction of 'T' shaped cellulose acetate holders to which leaves could be attached with adhesive tape, and modification of the gas flow system used for controlled temperature studies. Kinetic studies of free radical formation were possible with leaves which had minimal underlying Fe⁺⁺ and Mn⁺⁺ signals. In leaves with large underlying signals a Varian software program was used to subtract overlapping signals from each other, thereby revealing the free-radical signal changes which occurred under different light regimes and stress conditions. Preliminary investigation disclosed the formation of a new signal upon prolonged exposure to far-red light and the effect of oxygen depletion upon photosynthetic Signals I and II. Leaves subject to high photon flux density reveal an unreported free-radical signal, which decays upon exposure to microwave radiation; and concomitant damage to Photosystems I and II. Upon elimination of this signal leaves return to the undamaged state or reveal permanent damage to either photo-system, depending upon the degree of damage. Kentucky bluegrass and perennial ryegrass leaves subject to low levels of ozone (up to 80ppb) for periods of 8 hours show no changes in free-radical signal formation. At intermediate levels of ozone (80-250ppb) a new free-radical signal was formed within 3 hours of fumigation, Signal II was decreased and Signal I decayed. These changes were reversible if fumigation was terminated. At fumigation levels exceeding 250ppb a different new irreversible free-radical signal was formed in darkness within 1.5 hours of fumigation. Radish, Kentucky bluegrass and perennial ryegrass leaves subject to high levels of sulphur dioxide (10-500ppm) reveal the formation of Signal I upon irradiation with broad-band white or 650nm light, thereby indicating an interruption of normal electron flow from PSII to PSI. Damage to the oxygen-evolving complex and reaction centre of PSII is also revealed through changes in Signal II and the Mn⁺⁺ signal. These changes in the normal EPR signals are dose-dependent. Leaves subject to low levels of sulphur dioxide (600-2000ppb) reveal the disappearance of Signal I after 3 hours of fumigation and the formation of a new free-radical signal with parameters similar to the sulphur trioxide free-radical signal. These latter changes are partially reversible upon termination of fumigation. After prolonged exposure to either ozone or sulphur dioxid a free-radical signal with parameters similar to the superoxid anion free-radical signal is formed in plant leaves.

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