It is well known that photosynthetic health impacts the overall fitness of the mature plant. This study aims to determine photosynthetic vigour of spring wheat cultivars during field development as well as their biomass composition at maturity to determine which cultivars/varieties would be optimum for cellulosic ethanol production. Additionally, specimens were grown at non-acclimating (20˚C), cold acclimating (5˚C), non-acclimating high CO2 (20˚C/750 µmol mol-1 CO2) and cold-acclimating high CO2 (5˚C/750 µmol mol-1 CO2) to resolve photosynthetic responses to different environments. Plants were photoinhibited under high irradiance (5 fold growth irradiance) and low temperature (5˚C) while photochemical efficiency of PSII was monitored throughout using chlorophyll fluorescence imaging. Vegetative production was monitored using normalised difference vegetation index. De-epoxidation of xanthophyll photoprotective pigments were also recorded using HPLC and photochemical reflectance index. Additionally, carbon assimilation rate was recorded with infra-red gas analysis methods. It was discovered that no one wheat cultivar demonstrated any photosynthetic advantage in the field or under photoinhibitory conditions. However, photosynthetic differences were observed between wheat grown in different environments. Plants that were cold-acclimated or grown under high CO2 were more resilient to photoinhibitory stress. This was also reflected by most cold-acclimated cultivars having increased triose phosphate utilization, electron transport and zeaxanthin induction. Plants acclimated to high CO2 at room temperature also displayed increased electron transport and triose phosphate utilization but had decreased zeaxanthin induction. It is hypothesized increased excitation pressure in cold acclimated and high CO2 cultivars allowed for their increase in the development of photoinhibitory tolerance.

Rhododendron maximum L. is a dominant subcanopy species in the southern Appalachian Mountains. R. maximum undergo distinct thermonastic leaf movements (TLM). The purpose of these movements has not yet been determined. Previous studies have suggested TLM are a photoprotective mechanism for the dynamic light environment of the subcanopy in a deciduous forest during winter. The present study aimed to determine the effects of restricting TLM on photoinhibition, net photosynthesis, and other gas exchange parameters, particularly during the early spring. After restricting TLM on certain leaves, we observed the above parameters from autumn 2005 to late spring 2006. Our results indicated that photoinhibition increased (lower Fv/Fm) in treatment leaves over reference leaves throughout the winter. The difference became greater during the early spring, when reference leaves began to return to normal levels of photochemical efficiency and treatment leaves sustained low Fv/Fm. Net photosynthesis was lower for treatment leaves than reference leaves. This became most significant during the early spring, when maximum carbon gain is possible. Finally, gas exchange parameters as measured by light and CO2 response curves did not indicate any significant difference between treatment and reference leaves post canopy closure. Out results suggest that TLM are an important mechanism for photoprotection, allowing leaves of R. maximum to recover quickly during the early spring and maximize their early spring carbon gain. / Master of Science

Submitted by Jorge Silva (jorgelmsilva@ufrrj.br) on 2017-08-22T19:49:00Z No. of bitstreams: 1 2016 - Gepatrik Rodrigues Lima.pdf: 1199140 bytes, checksum: a162c3135ebd07b66364dfd0d3a3b7fd (MD5) / Made available in DSpace on 2017-08-22T19:49:00Z (GMT). No. of bitstreams: 1 2016 - Gepatrik Rodrigues Lima.pdf: 1199140 bytes, checksum: a162c3135ebd07b66364dfd0d3a3b7fd (MD5) Previous issue date: 2016-02-29 / The aim of the study was to evaluate the effects of herbicides applied on post- emergence on common bean cultivation, using chlorophyll a fluorescence parameters and visual evaluation, just after theirs application. The selectivity of the herbicide in the crop was also evaluated. The experiment was conducted in the crop science experimental field at the Federal Rural University of Rio de Janeiro. The treatments were the following herbicides, bentazon (720 g ha-1), fluazifop-p-butil (187,5 g ha-1), fluazifop-p-butil + fomesafen (187,5 + 250 g ha-1), fomesafen (250 g ha-1), and two other treatments with or without weeds mechanical control, and the experimental design used was on randomized blocks, with six treatments and three replications. The bentazon was the only herbicide that induced a reduction in the maximum quantum yield of PS II (Fv/Fm), in photochemical quenching (qP) and in effective quantum yield of PS II (?FSII), while it promoted an increase in non photochemical quenching (NPQ) in the first day after its application. The Fv/Fm was the best discriminatory parameter for the evaluation of the effects of the herbicide on photosynthetic apparatus of plants in the field. All the herbicides used did not promote phytotoxicity or bean yield reduction. / O trabalho teve como objetivos avaliar os efeitos da aplica??o de herbicidas p?s-emergentes na fisiologia da cultura do feijoeiro, utilizando par?metros de fluoresc?ncia de clorofila a e a seletividade do herbicida na cultura. O experimento foi conduzido no campo experimental de Fitotecnia, da Universidade Federal Rural do Rio de Janeiro. Os tratamentos constaram das aplica??es dos seguintes herbicidas: bentazon (720 g ha-1), fluazifop-p-butil (187,5 g ha-1), fluazifop-p-butil + fomesafen (187,5 + 250 g ha-1), fomesafen (250 g ha-1) e mais duas testemunhas sem aplica??o de herbicida, sendo uma capinada e outra mantida sem capina, no delineamento casualiza??o por bloco, com quatro repeti??es. Foram avaliadas as vari?veis de fluoresc?ncia da clorofila a, conte?do de prote?na sol?vel foliar e a fitotoxicidade a cultura. O bentazon foi o ?nico que causou redu??es significativas no rendimento qu?ntico m?ximo do fotossistema II (Fv/Fm), quenching fotoqu?mico (qP), rendimento qu?ntico efetivo do fotossistema II (?FSII) e promoveu incremento no quenching n?o fotoqu?mico (NPQ) nos primeiros dias ap?s sua aplica??o. O Fv/Fm foi a vari?vel mais indicada para avaliar efeitos decorrentes da aplica??o de herbicidas no aparato fotossint?tico de plantas no campo. Todos os herbicidas testados n?o ocasionaram fitotoxicidade elevada e queda no rendimento dos gr?os do feijoeiro comum.

PhD / The waratah, Telopea speciosissma and its hybrids with other Telopea species, is an Australian native species grown for domestic and export cut flower markets. The showy floral bracts surrounding the inflorescence often suffer from bract browning, reducing the market value and export potential of the blooms. Prior to this project, the physiological cause of the disorder was not known, although bract browning had been attributed to water stress, heat stress, high light (particularly after frost), wind and mechanical damage. Bract browning was reportedly minimised when waratahs were grown in shaded conditions, although the reduction in browning by shade had not been quantified. The aim of this project was to examine the physiological cause of the bract browning disorder and investigate methods for control. The appearance, timing, and severity of the bract browning disorder was initially characterised by dissecting waratah buds from commercial growers throughout NSW. Bract browning became evident in the six to eight weeks prior to harvest, coinciding with rapid bract and flower expansion. A survey of commercial waratah growers, initiated by NSW Agriculture and the Waratah Industry Network and analysed by the author, corroborated these results. The survey showed that bract browning was observed in all years between 1999 and 2003, with relatively high severity (scores from three to five out of a possible five) in three of those years. Scores or counts of brown bracts were used to assess the severity of the disorder, the latter including the number of senesced floral bracts following browning as a measure of browning severity. The position and timing of browning suggest light damage or localised calcium deficiency could play a role in the development of browning. The bract browning disorder was studied in further experiments on potted red waratahs of cultivars ‘Fire and Brimstone’, ‘Olympic Flame’ and ‘Sunflare’ at the Mount Annan Botanic Garden; on commercially grown ‘Wirrimbirra White’ waratahs at Jervis Bay; and on natural populations in the Royal National Park. The effect of calcium nutrition on bract browning was studied at Mount Annan in 2001 and 2002, testing the hypothesis that browning may be caused by a localised calcium deficiency similar to lettuce tipburn or poinsettia bract necrosis. Waratah bracts had significantly less calcium in all fractions than leaves, with the procedure of Ferguson et al. (1980) used to separate physiologically active, oxalate associated and residual calcium. Calcium chloride sprays applied to developing bracts increased total bract calcium by about 25% in ‘Sunflare’ and ‘Olympic Flame’ cultivars, but not in ‘Fire and Brimstone’. However, application of calcium as a spray to the developing bracts, or as gypsum to the potting medium did not significantly reduce bract browning scores. These results and the development of bract browning in exposed, rather than enclosed tissue, suggest that factors other than calcium are involved in the development of bract browning. The light environment (full sun or 50% shade cloth) had a greater effect than irrigation frequency on bract browning of ‘Sunflare’ and ‘Olympic Flame’ waratahs in 2001. Waratahs grown under 50% shade cloth showed less bract browning at flower maturity than waratahs grown in full sun. This result was corroborated by subsequent experiments in 2002 and 2003. For example, in 2002, shade cloth reduced browning and bract loss by 30-60% at flower maturity, compared to waratahs grown in full sun. Shading waratahs from bud initiation in late summer (December-January) or bud opening in late winter (July-August) was equally effective in reducing browning. Shade cloth (50%) significantly reduced the light intensity experienced by waratah plants throughout the day, as well as reducing the daily maximum temperature and minimum relative humidity. Natural shade conditions at the Royal National Park effectively prevented browning of floral bracts, although the smaller basal bracts still turned brown and senesced. The development of bract browning as waratahs matured was linked to the development of chronic photoinhibition, measured as a decrease in predawn photosynthetic efficiency using chlorophyll fluorescence techniques. Waratah bracts were unable to maintain efficient photosynthesis in full sun conditions and reached saturation of non-photochemical quenching at lower light intensities than leaves. This suggests that bract tissue is adapted to a lower light environment than leaf tissue. Outer bracts had a significantly lower photosynthetic efficiency (Fv/Fm) than leaves early in flower development, as they were exposed to the environment for a prolonged period. Outer bracts also began to senesce towards flower maturity, particularly in full sun, increasing their susceptibility to damage. Inner waratah bracts were able to maintain a high photosynthetic efficiency prior to exposure, but photosynthetic efficiency decreased significantly at the intermediate stage of floral development, as inner bracts were no longer protected by outer bracts. Waratah leaves were more resilient than bracts, and did not suffer from chronic photoinhibition or browning during flower development. The increased susceptibility of bracts to photoinhibition and browning parallels results in other species, such as Dendrobium, where floral tissue experiences photoinhibition, bleaching and necrosis at lower light intensities than leaf tissue. Bracts on shaded waratahs maintained higher chlorophyll, carotenoid and anthocyanin concentrations than sun-exposed bracts, giving more intense flower colour and higher quality blooms. The significant decrease in bract pigmentation in the sun is likely to be a result of pigment destruction following photoinhibition, and has been noted in susceptible tissues of other species, such as Illicium (star anise) leaves. The presence of anthocyanins did not reduce bract browning in waratahs, with the concentration of UV-absorbing compounds showing a stronger positive correlation with protection from photoinhibition than the concentration of anthocyanins. However, anthocyanin concentrations were significantly lower in sun-exposed bracts, and brown compounds appeared to replace anthocyanins in the epidermal cells of brown bracts. Thus, it seems likely that browning in waratah bracts is the visible manifestation of oxidative damage to cell components, following chronic photoinhibition. Light-induced oxidative damage can lead to yellowing and pigment bleaching, lipid peroxidation, the development of necrotic lesions and senescence. However, lipid peroxidation as measured by the malionaldehyde assay gave no indication of oxidative damage to waratah bract tissue. This was probably due to the presence of anthocyanins and other flavonoids and sugars other than sucrose in bract tissue interfering with the colourimetric measurement of thiobarbituric acid reactive substances. The extensive planting of waratahs in NSW in the last five years suggests that the total value of lost production due to bract browning is likely to increase in the future. The browning disorder may also prevent the establishment of waratahs in other markets, as international cut-flower markets demand high quality blooms free from blemishes. The results of this study show that bract browning, photoinhibition and pigment loss are minimised by protecting waratahs from high light intensities from bud opening until harvest. However, the consequences of shading waratahs throughout the year require further investigation, as does the use of different percentages of shade cloth or other methods to reduce incident light.

PhD / The waratah, Telopea speciosissma and its hybrids with other Telopea species, is an Australian native species grown for domestic and export cut flower markets. The showy floral bracts surrounding the inflorescence often suffer from bract browning, reducing the market value and export potential of the blooms. Prior to this project, the physiological cause of the disorder was not known, although bract browning had been attributed to water stress, heat stress, high light (particularly after frost), wind and mechanical damage. Bract browning was reportedly minimised when waratahs were grown in shaded conditions, although the reduction in browning by shade had not been quantified. The aim of this project was to examine the physiological cause of the bract browning disorder and investigate methods for control. The appearance, timing, and severity of the bract browning disorder was initially characterised by dissecting waratah buds from commercial growers throughout NSW. Bract browning became evident in the six to eight weeks prior to harvest, coinciding with rapid bract and flower expansion. A survey of commercial waratah growers, initiated by NSW Agriculture and the Waratah Industry Network and analysed by the author, corroborated these results. The survey showed that bract browning was observed in all years between 1999 and 2003, with relatively high severity (scores from three to five out of a possible five) in three of those years. Scores or counts of brown bracts were used to assess the severity of the disorder, the latter including the number of senesced floral bracts following browning as a measure of browning severity. The position and timing of browning suggest light damage or localised calcium deficiency could play a role in the development of browning. The bract browning disorder was studied in further experiments on potted red waratahs of cultivars ‘Fire and Brimstone’, ‘Olympic Flame’ and ‘Sunflare’ at the Mount Annan Botanic Garden; on commercially grown ‘Wirrimbirra White’ waratahs at Jervis Bay; and on natural populations in the Royal National Park. The effect of calcium nutrition on bract browning was studied at Mount Annan in 2001 and 2002, testing the hypothesis that browning may be caused by a localised calcium deficiency similar to lettuce tipburn or poinsettia bract necrosis. Waratah bracts had significantly less calcium in all fractions than leaves, with the procedure of Ferguson et al. (1980) used to separate physiologically active, oxalate associated and residual calcium. Calcium chloride sprays applied to developing bracts increased total bract calcium by about 25% in ‘Sunflare’ and ‘Olympic Flame’ cultivars, but not in ‘Fire and Brimstone’. However, application of calcium as a spray to the developing bracts, or as gypsum to the potting medium did not significantly reduce bract browning scores. These results and the development of bract browning in exposed, rather than enclosed tissue, suggest that factors other than calcium are involved in the development of bract browning. The light environment (full sun or 50% shade cloth) had a greater effect than irrigation frequency on bract browning of ‘Sunflare’ and ‘Olympic Flame’ waratahs in 2001. Waratahs grown under 50% shade cloth showed less bract browning at flower maturity than waratahs grown in full sun. This result was corroborated by subsequent experiments in 2002 and 2003. For example, in 2002, shade cloth reduced browning and bract loss by 30-60% at flower maturity, compared to waratahs grown in full sun. Shading waratahs from bud initiation in late summer (December-January) or bud opening in late winter (July-August) was equally effective in reducing browning. Shade cloth (50%) significantly reduced the light intensity experienced by waratah plants throughout the day, as well as reducing the daily maximum temperature and minimum relative humidity. Natural shade conditions at the Royal National Park effectively prevented browning of floral bracts, although the smaller basal bracts still turned brown and senesced. The development of bract browning as waratahs matured was linked to the development of chronic photoinhibition, measured as a decrease in predawn photosynthetic efficiency using chlorophyll fluorescence techniques. Waratah bracts were unable to maintain efficient photosynthesis in full sun conditions and reached saturation of non-photochemical quenching at lower light intensities than leaves. This suggests that bract tissue is adapted to a lower light environment than leaf tissue. Outer bracts had a significantly lower photosynthetic efficiency (Fv/Fm) than leaves early in flower development, as they were exposed to the environment for a prolonged period. Outer bracts also began to senesce towards flower maturity, particularly in full sun, increasing their susceptibility to damage. Inner waratah bracts were able to maintain a high photosynthetic efficiency prior to exposure, but photosynthetic efficiency decreased significantly at the intermediate stage of floral development, as inner bracts were no longer protected by outer bracts. Waratah leaves were more resilient than bracts, and did not suffer from chronic photoinhibition or browning during flower development. The increased susceptibility of bracts to photoinhibition and browning parallels results in other species, such as Dendrobium, where floral tissue experiences photoinhibition, bleaching and necrosis at lower light intensities than leaf tissue. Bracts on shaded waratahs maintained higher chlorophyll, carotenoid and anthocyanin concentrations than sun-exposed bracts, giving more intense flower colour and higher quality blooms. The significant decrease in bract pigmentation in the sun is likely to be a result of pigment destruction following photoinhibition, and has been noted in susceptible tissues of other species, such as Illicium (star anise) leaves. The presence of anthocyanins did not reduce bract browning in waratahs, with the concentration of UV-absorbing compounds showing a stronger positive correlation with protection from photoinhibition than the concentration of anthocyanins. However, anthocyanin concentrations were significantly lower in sun-exposed bracts, and brown compounds appeared to replace anthocyanins in the epidermal cells of brown bracts. Thus, it seems likely that browning in waratah bracts is the visible manifestation of oxidative damage to cell components, following chronic photoinhibition. Light-induced oxidative damage can lead to yellowing and pigment bleaching, lipid peroxidation, the development of necrotic lesions and senescence. However, lipid peroxidation as measured by the malionaldehyde assay gave no indication of oxidative damage to waratah bract tissue. This was probably due to the presence of anthocyanins and other flavonoids and sugars other than sucrose in bract tissue interfering with the colourimetric measurement of thiobarbituric acid reactive substances. The extensive planting of waratahs in NSW in the last five years suggests that the total value of lost production due to bract browning is likely to increase in the future. The browning disorder may also prevent the establishment of waratahs in other markets, as international cut-flower markets demand high quality blooms free from blemishes. The results of this study show that bract browning, photoinhibition and pigment loss are minimised by protecting waratahs from high light intensities from bud opening until harvest. However, the consequences of shading waratahs throughout the year require further investigation, as does the use of different percentages of shade cloth or other methods to reduce incident light.

Plants absorb light for photosynthesis, but not all is used. Excess light energy may lead to photoinhibition of photosynthesis and irreversible photooxidative damage. Plants have evolved mechanisms for energy dissipation under high light stress. One such response may involve production of ELIP. It is of interest to know what signal(s) may be involved in ELIP expression. My hypothesis is that redox status of the chloroplast photosynthetic electron transport Chain (PETC) and/or chlororespiration may induce ELIP expression. Using the Arabidopsis thaliana immutans (im) chlororespiratory mutant, this hypothesis was tested. Etiolated seedlings of this variegated mutant were subjected to various light intensities over 0-24 hr period and ELIP mRNA levels were analyzed. These were compared with the wild type plants treated in the same manner. It was found that mature thylakoids may not be required for ELIP expression, and that both photoreceptor-dependent and independent components may be involved in ELIP expression.

<p> 3D printing has established itself as a critical tool for manufacturing in all areas. It has evolved from a purely rapid prototyping technique into a feasible process for large-scale processing. A wide variety of 3D printing processes exist across an extreme range of size, from meters to nanometers. Much of the current technological advances come from pushing fabrication techniques to smaller and smaller scales. For 3D printing this has led to the rise of two-photon polymerization, a direct laser writing process with submicron structuring capabilities. Two-photon polymerization has proven its worth as a nanoscale 3D fabrication technique but is often considered slow and expensive, two undesirable qualities for high throughput manufacturing. Parallelization methods such as projection lithography are potential solutions to increasing the throughput capabilities of two-photon polymerization 3D printing. Additionally, the drive for further reducing the print size has inspired printing resolution enhancing strategies in two-photon polymerization printing by processes such as stimulated emission depletion (STED) and other STED-inspired pathways. This work will explore avenues for improving two-photon polymerization printing throughput and resolution.</p> <p> First, a two-photon polymerization printing system is constructed with a secondary laser for controlling polymerization inhibition. Through a STED process, a 65 nm wide printed line feature was achieved. Alongside this, a characterization and verification methodology for choosing new photoinitiator molecules for similar inhibition lithography processes is presented. Through implementation of tests such as Z-scan, fluorescence depletion, ultrafast transient spectroscopy and UV-Vis absorption and fluorescence measurements a promising new photoinitiator with 5-factor improvement in printing efficiency is found. </p> <p> Second, a projection lithography scheme is developed for rapid two-photon 3D printing. A digital micro-mirror device (DMD) is utilized for dynamic pattern generation and the effects of its dispersion properties are considered. Through a spatiotemporal focusing process, continuous 3D printing is achieved at vertical prints speeds of 1 mm s-1. Simulations performed representing this rapid printing process indicate a ~1 µm layer print feature size for large areas of exposure. Comparably, a printed vertical feature size of ~ 1 µm was achieved. Lateral feature sizes ~200 nm were also demonstrated in fabrication. A variety of complex 3D structures are printed for demonstration of the spatiotemporal focusing projection lithography process including millimeter scale objects with micrometer scale 3D features.</p> <p> Finally, resolution enhancing strategies are implemented into the continuous, projection two-photon lithography technique. An investigation of the inhibition properties of a variety of photoinitiator systems for inhibiting polymerization achieved with low repetition rate laser exposure is presented. A planar polymerization inhibiting region is generated by creating a light sheet propagating perpendicularly to the projection printing plane. </p>

Previous studies indicate that the degree of UV-Binduced photosynthetic inhibition may be highly dependent upon the photosynthetic photon flux density (PPFD, total quantum flux in the waveband 400-700 nm) incident on a plant. This study illustrates that Essex soybean leaves (Glycine max) preconditioned under high PPFD suffered less UV-B-induced photoinhibition than when preconditioned under low PPFD. However, sensitivity to UV-B increased when soybean leaves received high-PPFD as a concomitant treatment. The relative msgnitude of UV-B-induced damage was similar for both light-limited and light-saturated photosynthesis. This probably indicates that UV-B is inhibiting fundamentally different photosynthetic processes. Soybean leaves preconditioned under high PPFD had greater specific-leaf-weight, chlorophyll a/b ratio, and crude flavonoid content. The total chlorophyll concentration of soybean leaves preconditioned under high PPFD increased slightly over a UV-B irradiation period of five hours. Total chlorophyll concentration of leaves preconditioned under low PPFD decreased slightly in response to the same irradiation period.

Damage to primary photosynthetic reactions caused by environmental stress can be assessed by measurement of chlorophyll fluorescence induction in intact leaves. This approach was applied in studies of ultraviolet-B photoinhibition of photosynthesis in Pisum sativum L. and Rumex patientia L. leaves. At ultraviolet-B dose rates insufficient to cause inhibition of net photosynthesis, changes in the magnitude of fluorescence transients did occur, which suggested direct effects on chloroplast pigments in Pisum and inhibition of photosynthetic electron transport between the photosystems in both species. Leaves of these two species subjected to a much higher dose rate had a significant reduction of net photosynthesis and changes in the magnitude of fluorescence transients that indicated partial loss of water-splitting capability and direct effects on chloroplast pigments. Ultraviolet radiation-induced changes of photosynthetic thylakoid membranes may be ultimately responsible for these disruptions of the primary photosynthetic reactions.

<p dir="ltr">Photosynthesis is the basis of primary productivity on Earth. Oxygenic photosynthesis utilizes the nearly inexhaustible energy of radiant solar light to fix atmospheric carbon dioxide into usable forms of chemical energy and produces dioxygen as a product. Central to this process are several large hetero-oligomeric protein complexes that comprise the photosynthetic electron transport chain. Photosystem II (PSII) initiates electron transport through the light-driven oxidation of water, in-turn relinquishing protons and oxygen. Through this reaction, electrons are used to form the reductant NADPH, while protons form a proton-motive gradient that is used to drive synthesis of ATP. As a result of this highly energetic reaction, PSII is often subject to oxidative photodamage due to the production of reactive oxygen species. Inevitably, accumulation of oxidative photodamage disrupts the catalytic activity of PSII, resulting in a loss of photosynthetic activity. To deal with the nearly constant incurred photodamage to PSII, oxygenic photoautotrophs undergo a disassembly and repair cycle that results in the complete turnover of the damaged D1 subunit of PSII. Due to its high tendency for damage, the D1 subunit has a half-life of under one hour in high light intensity. Despite our current understanding of photoinhibition and PSII repair, it is still unclear how D1 is replaced so rapidly in response to damaging conditions. Previous research has indicated a role for phosphorylation of PSII in D1 turnover, however the mechanism has not been totally resolved. In the first chapter of this thesis, our current understanding of PSII phosphorylation and oxidative damage is reviewed in the context of PSII repair. In the second chapter, the role of protein phosphorylation in the PSII repair cycle is investigated in the model organism <i>Arabidopsis</i>. Using several PSII phosphorylation mutants, we demonstrate that phosphorylation seems to mediate disassembly of large PSII supercomplexes and dimers into smaller subcomplexes. In the third chapter, the role of oxidative photodamage is investigated in mediating PSII disassembly. Here, we use several <i>in vitro</i> assays to demonstrate that photodamage is sufficient to induce the disassembly of smaller PSII subcomplexes. In the fourth chapter, a technique for determining the stoichiometry of photosynthetic complexes is examined, with implications for understanding PSII repair. Finally, in the fifth chapter, several conclusions and unanswered questions from this thesis are discussed.</p>