No description available.

by Choi, Chi Fung. / "September 2007." / Thesis (Ph.D.)--Chinese University of Hong Kong, 2007. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese.

No description available.

Thesis (Ph. D.)--University of Hong Kong, 1980.

It has been proposed that the protective systems (photorespiration, the anti-oxidant system and non-radiative energy dissipation) alleviate or reduce photoinhibitory damage under high light conditions. To investigate the role of these mechanisms in C4 photosynthetic species, nine day old Zea mays seedlings were photoinhibited (30 minutes of 2500 J,Lmol m-2 s-1 PPFD) in the presence of various concentrations of 02 or CO2; or by photoinhibiting leaves in N2 after they had been fed glycolate or phosphoglycerate via the transpiration stream. The extent of the photoinhibition and the subsequent recovery from the photoinhibitory treatments was monitored with both CO2 gas exchange and chlorophyll fluorometry. Photoinhibitory treatments resulted in both a decrease in the rate of CO2 fixation and an interruption of PSII electron transport. CO2 response curves were used to monitor the efficiency of the carboxylation processes and the level of carbon metabolism substrate cycling during recovery following photoinhibitory treatments. Both were decreased by the treatment and recovered once leaves were returned to normal conditions. Low concentrations of 02 (2%) markedly reduced the extent of the photoinhibition. This protection could not be accounted for by photorespiration, which would be inoperative at such a low 02 concentration. Leaves fed glycolate exhibited enhanced photoinhibtion. It is also unlikely that the anti-oxidant system (Mehler reaction and associated glutathione and ascorbate reactions) could utilize sufficient reductant at such low 02 concentrations to produce the observed protection. Leaves inhibited in the presence of 02 had decreased maximum fluorescence yields (Fm) and little altered initial fluorescence yields (F0)' resulting in decreased PSlI efficiency (Fv/Fm)' Photoinhibition resulted in a small increase in the slow relaxing component (60 minute) of non-radiative energy dissipation. This component became more predominant as the 02 concentration was increased. The rate constant for photochemistry was also decreased by the inhibitory treatment. Leaves supplied with CO2 at a concentration above 50 J,Lmol mol-1 exhibited little photoinihibition suggesting that the protection was not due to a quantitative utilization of energy. PGA, fed via the transpiration strea~ enhanced the photoinhibition, suggesting that more than just the Benson-Calvin cycle is required to protect C4 plants from photoinhibition. At CO2 concentrations below this, the Fv/FID ratio was decreased due to large increases in the F0 values. Fm was little altered. These changes are characteristic of a decrease in the rate constant for photochemistry. The rate constant for non-radiative energy dissipation was little altered by the photoinhibition. The protection observed in the presence of either CO2 or 02 was not due to a quantitative utilization of energy and the different responses of F0' Fm and the rate constants KD and Kp, suggest that different mechanisms were operative in the presence or absence of oxygen. / Thesis (M.Sc.)-University of Natal, Durban, 1990.

Thesis (Ph.D.)--Brown University, 2005. / Vita. Thesis advisor: Samuel I. Beale. Includes bibliographical references (leaves 123-146). Also available online.

published_or_final_version / Biochemistry / Doctoral / Doctor of Philosophy

Two separate aspects involving the thiophenes of the Tageteae (Asteraceae) were investigated. The first concerned the distribution of four thiophenes in Tagetes patula L. which were examined by high pressure liquid chromatography (HPLC). The derivatives were found to be differentially distributed throughout hydroponically grown plants. The predominant thiophenes in roots were 5-(4-acetoxy-1-butenyl)-2,2'-bithienyl (BBT-OAc) and 5-(buten-3-ynyl)-2,2'-bithienyl (BBT). BBT-OAc was the main derivative in shoots, whereas 2,2':5',2''-terthienyl (alpha-T) was the major compound in flower petals. BBT and one unidentified compound were found to occur in leaf glands. The levels of BBT-OAc in shoots and BBT-OAc and BBT in roots increased over the life of the plant and reached a plateau following flowering. Alpha-T in roots and both alpha-T and BBT in shoots remained at low levels over the life of the plant while 5-(4-hydroxy-1-butenyl)-2,2'-bithienyl (BBT-OH) was found to be an minor component of roots or shoots. Fourteen species from four genera of the tribe Tageteae were also screened for the presence of thiophenes by HPLC. Representatives of Dyssodia, Porophyllum, and Tagetes all contained thiophenes, but none were detected in species of Pectis. The second part of this study concerned the photobiocidal effects of isolated thiophenes on Escherichia coli B which was used as a model biological system. Alpha-terthienyl (alpha-T), in the presence of UV-A irradiation (320nm-400nm), was found to be a Type II photosensitizer which required oxygen for the expression of biological activity. Scavenger studies with sodium azide and BHT suggested that both singlet oxygen and superoxide were generated by the photosensitized reaction. Cellular inactivation by alpha-T was sensitive to temperature and studies with recombination deficient mutants of E. coli K-12 did not indicate that damage to cellular DNA occurred. Proteins were found to be substantially affected by the photoactivated reaction. SDS-gel electrophoresis revealed that both cytoplasmic and membrane-associated proteins might be. crosslinked following treatment with alpha-T and UV-A. / Science, Faculty of / Botany, Department of / Graduate

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants known for their photoinduced toxicity. There are two mechanisms through which this may occur: photosensitization and photomodification. Photosensitization generally leads to the production of singlet oxygen, a reactive oxygen species (ROS), which is highly damaging to biological molecules. Photomodification of PAHs, usually via oxygenation, results in the formation of new compounds (oxyPAHs), and can occur under environmentally relevant levels of actinic radiation. <br /><br /> PAHs and oxyPAHs readily adsorb to the organic phase of particulate matter in the environment such as sediments. It is logical to conclude that sediment transport will also facilitate the transport of these contaminants, and it has been shown that in the course of transport, degradative processes evoke a change in the profile of the PAHs present. Sediment samples taken along a transect from Hamilton Harbour were fractionated, and analyzed using a 2D HPLC method. All sediments contained intact and modified PAHs, although a marked change was noted in the profile of compounds present in the samples, which differ in distance from shore. Fractions of sediment extract were tested for toxicity using a bacterial respiration assay. Toxicity was observed in fractions containing modified PAHs, and was similar to that of intact PAH-containing fractions. <br /><br /> Subsequently, the toxicities of 16 intact PAHs were assessed to <i>Daphnia magna</i> under two ultraviolet radiation (UV) conditions. The toxicity of intact PAHs generally increased in the presence of full spectrum simulated solar radiation (SSR), relative to visible light plus UVA only. To expand the existing data on the effects of PAH photoproducts to animals, fourteen oxyPAHs were also assayed with <i>D. magna</i>, most of which were highly toxic without further photomodification. The data presented highlight the effects of UV radiation on mediating PAH toxicity. The importance of the role of photomodification is also stressed, as several oxyPAHs were highly toxic to <i>D. magna</i>, a key bioindicator species in aquatic ecosystems. <br /><br /> A QSAR model previously developed for <i>Lemna gibba</i> showed that a photosensitization factor (PSF) and a photomodification factor (PMF) could be combined to describe toxicity. To determine whether it was predictive for <i>D. magna</i>, toxicity was assessed as both EC50 and ET50. As with <i>L. gibba</i> and <i>Vibrio fischeri</i>, neither the PSF nor the PMF alone correlated to D. magna toxicity. However, a PSF modified for <i>D. magna</i> did in fact exhibit correlation with toxicity, which was further improved when summed with a modified PMF. The greatest correlation was observed with EC50 toxicity data. This research provides further evidence that models that include factors for photosensitization and photomodification will likely be applicable across a broad range of species. To gain further knowledge of the roles that the variables contributing to the photosensitization and photomodification, a structural equation model was constructed based on the <i>D. magna</i> QSAR. This model accounted for a high amount of variance in six sets of toxicity data, as well as insight into the mechanisms of phototoxicity affecting different aquatic organisms.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants known for their photoinduced toxicity. There are two mechanisms through which this may occur: photosensitization and photomodification. Photosensitization generally leads to the production of singlet oxygen, a reactive oxygen species (ROS), which is highly damaging to biological molecules. Photomodification of PAHs, usually via oxygenation, results in the formation of new compounds (oxyPAHs), and can occur under environmentally relevant levels of actinic radiation. <br /><br /> PAHs and oxyPAHs readily adsorb to the organic phase of particulate matter in the environment such as sediments. It is logical to conclude that sediment transport will also facilitate the transport of these contaminants, and it has been shown that in the course of transport, degradative processes evoke a change in the profile of the PAHs present. Sediment samples taken along a transect from Hamilton Harbour were fractionated, and analyzed using a 2D HPLC method. All sediments contained intact and modified PAHs, although a marked change was noted in the profile of compounds present in the samples, which differ in distance from shore. Fractions of sediment extract were tested for toxicity using a bacterial respiration assay. Toxicity was observed in fractions containing modified PAHs, and was similar to that of intact PAH-containing fractions. <br /><br /> Subsequently, the toxicities of 16 intact PAHs were assessed to <i>Daphnia magna</i> under two ultraviolet radiation (UV) conditions. The toxicity of intact PAHs generally increased in the presence of full spectrum simulated solar radiation (SSR), relative to visible light plus UVA only. To expand the existing data on the effects of PAH photoproducts to animals, fourteen oxyPAHs were also assayed with <i>D. magna</i>, most of which were highly toxic without further photomodification. The data presented highlight the effects of UV radiation on mediating PAH toxicity. The importance of the role of photomodification is also stressed, as several oxyPAHs were highly toxic to <i>D. magna</i>, a key bioindicator species in aquatic ecosystems. <br /><br /> A QSAR model previously developed for <i>Lemna gibba</i> showed that a photosensitization factor (PSF) and a photomodification factor (PMF) could be combined to describe toxicity. To determine whether it was predictive for <i>D. magna</i>, toxicity was assessed as both EC50 and ET50. As with <i>L. gibba</i> and <i>Vibrio fischeri</i>, neither the PSF nor the PMF alone correlated to D. magna toxicity. However, a PSF modified for <i>D. magna</i> did in fact exhibit correlation with toxicity, which was further improved when summed with a modified PMF. The greatest correlation was observed with EC50 toxicity data. This research provides further evidence that models that include factors for photosensitization and photomodification will likely be applicable across a broad range of species. To gain further knowledge of the roles that the variables contributing to the photosensitization and photomodification, a structural equation model was constructed based on the <i>D. magna</i> QSAR. This model accounted for a high amount of variance in six sets of toxicity data, as well as insight into the mechanisms of phototoxicity affecting different aquatic organisms.