Η υδατική καταπόνηση αποτελεί έναν από τους κυριότερους περιβαλλοντικούς παράγοντες περιορισμού της φυτικής ανάπτυξης και απόδοσης στα ξηρά και ημίξηρα οικοσυστήματα, συμπεριλαμβανομένων κι αυτών με μεσογειακού τύπου κλίμα. Σκοπός της παρούσας ερευνητικής εργασίας ήταν η εκτίμηση των επιδράσεων της υδατικής καταπόνησης στη φωτοχημική ικανότητα του PSII και τις φωτεινές αντιδράσεις της φωτοσύνθεσης, σε τυπικά μεσογειακά είδη. Η υδατική καταπόνηση επιτεύχθηκε τόσο εργαστηριακά εντός διαστήματος λίγων ωρών, όσο και σε ημι-φυσικές συνθήκες περιβάλλοντος σε διάστημα περίπου δύο εβδομάδων. Μετά τις δύο εβδομάδες τα φυτά επανυδατώθηκαν, με στόχο την εκτίμηση της ικανότητας ανάκαμψης από το έντονο υδατικό stress. Η ανάλυση της κινητικής της επαγωγής του φθορισμού της χλωροφύλλης a μέσω του JIP-test αποκάλυψε την άμεση επίδραση της έλλειψης νερού στη ροή των ηλεκτρονίων, όπως αυτή εκφράζεται από τις παραμέτρους ψΕο, φΕο, δRo και φRo. Επιπλέον, παρατηρήθηκε αρνητική συσχέτιση του δείκτη RWC με το συνολικό απόθεμα των ενεργών κέντρων του PSI και τους τελικούς υποδοχείς του PSI, καθώς και με τους δείκτες φωτοσυνθετικής απόδοσης PIABS & PItotal. Η μέγιστη φωτοχημική ικανότητα του PSII, φPo(=Fv/Fm) μειώθηκε μόνο σε πολύ χαμηλές τιμές RWC στις εργαστηριακές μετρήσεις που πραγματοποιήθηκαν, ενώ στις ημι-φυσικές συνθήκες η μείωση του φPo την δεύτερη εβδομάδα της υδατικής καταπόνησης συνοδεύτηκε από αυξημένη θερμική απόσβεση της απορροφούμενης ενέργειας διεγέρσεως. Η ανάπτυξη του υδατικού stress σε ημι-φυσικές συνθήκες οδήγησε επίσης στη μεταβολή των ειδικών ενεργειακών ροών ανά δραστικό ενεργό κέντρο του PSII, πιθανώς λόγω της μερικής αδρανοποίησης ενεργών κέντρων στις συνθήκες αυτές. Επιπλέον, καταγράφηκε αύξηση του φθορισμού στα 300 μsec (Σημείο Κ), ένα φαινόμενο που συνδέεται με καταστολή της λειτουργίας του συμπλόκου έκλυσης οξυγόνου. Οι εν λόγω μεταβολές δεν παρατηρήθηκαν στον ίδιο βαθμό στις εργαστηριακές μετρήσεις, γεγονός που πιθανώς οφείλεται στη σταδιακή ανάπτυξη της ξηρασίας και τη συνδυαστική δράση της έλλειψης νερού, της υψηλής θερμοκρασίας και των υψηλών εντάσεων φωτός κάτω από φυσικές συνθήκες περιβάλλοντος. Από την πρώτη κιόλας ημέρα μετά την επανυδάτωση των φυτών, όλες οι παράμετροι του JIP-test άρχισαν να επανακάμπτουν. Ωστόσο, εντοπίστηκαν διαφορές στην ταχύτητα επαναφοράς μεταξύ των διαφορετικών ειδών. Συνολικά, τα ευρήματα μας υποδεικνύουν καταστολή των φωτεινών αντιδράσεων της φωτοσύνθεσης σε συνθήκες υδατικής καταπόνησης, μεταβολή η οποία αντιστράφηκε γρήγορα στα πειράματά μας. Θα μπορούσε λοιπόν, να θεωρηθεί ως ένας μηχανισμός προσαρμογής που εξασφαλίζει στα μεσογειακά είδη που μελετήθηκαν την διατήρηση της λειτουργικότητας της φωτοσυνθετικής συσκευής, ακόμη και σε συνθήκες έντονης υδατικής καταπόνησης. / Water stress is one of the most important environmental factors limiting plant growth and yield in arid and semi-arid ecosystems, including those with Mediterranean climate. The objective of this study was to evaluate the effects of water stress on the photosynthetic capacity of PSII and on the light reactions of photosynthesis of typical Mediterranean species. Water stress experiments were conducted both under laboratory conditions, through dehydration of leaf-disks within hours and under semi-natural conditions by withholding water for about two weeks. Drought period was followed by six days of re-watering, in order to assess the recovery of the photosynthetic apparatus. Analysis of the polyphasic OJIP fluorescence transient through JIP-test, revealed direct limitations in electron transport, as expressed by the parameters ψΕο, φΕο, δRo and φRo. Furthermore, a negative correlation was found between RWC and the pool of the RCs of PSI, the pool of electron end-acceptors, and the photosynthetic efficiency, as expressed by the parameters PIABS & PItotal. The maximum quantum yield of primary photochemistry φPο (=Fv/Fm) was affected only under very low RWC values during the laboratory experiments. On the other hand, a reduction was found during the second week of drought stress treatment, along with an increased conversion rate of absorbed light into thermal energy. Under semi-natural conditions, water stress also caused a shift in the specific energy flows per RC of PSII, caused possibly by inactivation of RCs. Moreover, there was an increase in fluorescence intensity at 300 microseconds (K-band), a phenomenon associated with inhibition of the oxygen evolving complex. These alterations were not observed to the same extent in the laboratory experiments, so they can be attributed to the gradual onset of drought and the combined effect of dehydration, high temperature and high light intensities under natural conditions. Within one day after rehydration, all the parameters of the JIP-test began to recover. However, differences in the recovery rate were observed between different species. In conclusion, our findings indicate down-regulation of light reactions of photosynthesis under water stress. This phenomenon was quickly reversed in our experiments. Therefore, it could be regarded as an adaptive response, which maintains the functionality of the photosynthetic apparatus of the Mediterranean species studied, even under severe water deficit.

Photoinduced electron transfer is involved in a number of photochemical and photobiological processes. One example of this is photosynthesis, where the absorption of sunlight leads to the formation of charge-separated states by electron transfer. The redox equivalents built up by successive photoabsorption and electron transfer is further used for the oxidation of water and reduction of carbon dioxide to sugars. The work presented in this thesis is part of an interdisciplinary effort aiming at a functional mimic of photosynthesis. The goal of this project is to utilize sunlight to produce renewable fuels from sun and water. Specifically, this thesis concerns photoinduced electron transfer in donor(D)-photosensitizer(P)-acceptor(A) systems, in mimic of the primary events of photosynthesis. The absorption of a photon typically leads to transfer of a single electron, i.e., charge separation to produce a single electron-hole pair. This fundamental process was studied in several molecular systems. The purpose of these studies was optimization of single electron transfer as to obtain charge separation in high yields, with minimum losses to competing photoreactions such as energy transfer.Also, the lifetime of the charge separated state and the confinement of the electron and hole in three-dimensional space are important in practical applications. This led us to explore molecular motifs for linear arrays based on Ru(II)bis-tridentate and Ru(II)tris-bidentate complexes. The target multi-electron catalytic reactions of water-splitting and fuel production require a build-up of redox equivalents upon successive photoexcitation and electron transfer events. The possibilities and challenges associated with such processes in molecular systems were investigated. One of the studied systems was shown to accumulate two electrons and two holes upon two successive excitations, without sacrificial redox agents and with minimum yield losses. From these studies, we have gained better understanding of the obstacles associated with step-wise photoaccumulation of charge and how to overcome them.

No description available.

Moss dominated ecosystems are an important part of the global terrestrial carbon cycle. Over large areas, remote sensing can be useful to provide an improved understanding of these ecosystems. Two boreal mossess (Pleurozium and Sphagnum) were assessed using remote sensing based spectral vegetation indices for estimating biochemical capacity and photosynthetic efficiency by varying net photosynthesis rate via changes in water content. In the laboratory, changes in the normalized difference vegetation index (NDVI) and chlorophyll index coincided with declining photosynthetic capacity due to desiccation. This effect was more dramatic in Sphagnum. The photochemical reflectance index (PRI) did not vary with changes in CO2 supply as anticipated, possibly due to overriding effects of changing water content. The water band index (WBI) was strongly related to water content but this relationship showed an uncoupling in the field. Bi-directional reflectance measurements indicated what WBI was sensitive to sensor, sun, and moss surface slope angles. / xi, 110 leaves : ill. (some col.) ; 29 cm.

Complex biogenic habitats created by large canopy-forming macroalgae on intertidal and shallow subtidal rocky reefs worldwide are increasingly affected by degraded environmental conditions at local scales and global climate-driven changes. A better understanding of the mechanisms underlying the impacts of complex suites of anthropogenic stressors on algal forests is essential for the conservation and restoration of these habitats and of their ecological, economic and social values. This thesis tests physical and biological mechanisms underlying the impacts of different forms of natural and human-related disturbance on macroalgal assemblages dominated by fucoid canopies along the east coast of the South Island of New Zealand. A field removal experiment was initially set up to test assemblage responses to mechanical perturbations of increasing severity, simulating the impacts of disturbance agents affecting intertidal habitats such as storms and human trampling. Different combinations of assemblage components (i.e., canopy, mid-canopy and basal layer) were selectively removed, from the thinning of the canopy to the destruction of the entire assemblage. The recovery of the canopy-forming fucoids Hormosira banksii and Cystophora torulosa was affected by the intensity of the disturbance. For both species, even a 50% thinning had impacts lasting at least eighteen months, and recovery trajectories were longer following more intense perturbations. Independently of assemblage diversity and composition at different sites and shore heights, the recovery of the canopy relied entirely on the increase in abundance of these dominant fucoids in response to disturbance, indicating that functional redundancy is limited in this system. Minor understory fucoids, which could have provided functional replacement for the dominant habitat formers, had reduced rates of growth or recruitment when the overlying canopy was disturbed. I then used a combination of field and laboratory experiments to test the impacts of physical and biotic stress sources on the dominant fucoids H. banksii and C. torulosa. The large fucoid Durvillaea antarctica was also included in one of the laboratory investigations. I assessed how altered physical and biotic conditions affect these important habitat formers, both separately and in combination. Physical stressors included increased sedimentation, nutrient enrichment and warmer water temperatures. Biotic stress originated from interspecific competition with turfs of articulated coralline algae and ephemeral, fast-growing green and brown algae. Sediment deposition severely reduced the survival and growth of recently settled H. banksii, C. torulosa and D. antarctica germlings in laboratory experiments. In the field, the recruitment of H. banksii on unoccupied substrates was significantly higher than in treatments in which sediments or mats of turf-forming coralline algae covered the substrate. This shows that sediment deposition and space pre-emption by algal turfs can synergistically affect the development of fucoid beds. Further impacts of sediment accumulation in the benthic environment were investigated using in situ and laboratory photorespirometry techniques to assess the contribution of coralline algae to assemblage net primary productivity (NPP), both in the presence and absence of sediment. The NPP of articulated corallines was reduced by sediment. Sediment accumulation among the thalli limited the access of the corallines to the light and induced photoinhibitive mechanisms. In the absence of sediment, however, coralline algae enhanced the NPP of assemblages with a fucoid canopy, showing the importance of synergistic interactions among the components of multi-layered assemblages in optimizing light use. Nutrient enrichment had a less pervasive influence on the dominant fucoids H. banksii and C. torulosa than sedimentation. In laboratory experiments, nutrients stimulated the growth of H. banksii and C. torulosa germlings. However, negative impacts of high nutrient levels were observed for the early life stages of D. antarctica. The abundance of opportunistic, fast-growing algae rapidly increased in response to nutrient enrichment both in the laboratory and in the field. Impacts of ephemeral species on fucoid early life stages were only evident in laboratory contexts, where green algae of the genus Ulva impaired both the settlement of H. banksii zygotes and the growth of its germlings. Fucoid recruitment in the field was not affected by increased covers of ephemeral algae caused by enhanced nutrient regimes, indicating that H. banksii and C. torulosa may be resistant to short-term (one year) nutrient pollution. In the laboratory, increased temperatures within the range predicted for the end of the 21st century caused increased mortality in the H. banksii, C. torulosa and D. antarctica germlings. In a separate experiment, a combination of warmer water temperatures and nutrient enrichment enhanced the growth of ephemeral green algae. These results suggest that opposite responses to altered climate conditions may contribute to shifts from complex biogenic habitats dominated by macroalgal canopies to simplified systems monopolized by a limited number of stress-tolerant species. This research contributes to a clearer mechanistic understanding of biotic and physical mechanisms shaping the structure of coastal marine hard bottom communities under increasingly stressful conditions worldwide. These findings may provide insights for other studies investigating the complex mosaic of challenges facing marine coastal ecosystems.

Strong light can cause damage and be lethal for photosynthetic organisms. An increase of thermal dissipation of excess absorbed energy at the level of photosynthetic antenna is one of the processes protecting against deleterious effects of light. In cyanobacteria, a soluble photoactive carotenoid binding protein, Orange Carotenoid Protein (OCP) mediates this process. The photoactivated OCP by interacting with the core of phycobilisome (PB; the major photosynthetic antenna of cyanobacteria) triggers the photoprotective mechanism, which decreases the energy arriving at the reaction centres and PSII fluorescence. The excess energy is dissipated as harmless heat. To regain full PB capacity in low light intensities, theFluorescence Recovery Protein (FRP) is required. FRP accelerates the deactivation of OCP.In this work, I present my input in the understanding of the mechanism underlying the OCPrelated photoprotection. I further characterized the FRP of Synechocystis PCC6803, the model organism in our studies. I established that the Synechocystis FRP is shorter than what it was proposed in Cyanobase and it begins at Met26. Our results also revealed the great importance of a high OCP to FRP ratio for existence of photoprotection. The most remarkable achievement of this thesis is the in vitro reconstitution of the OCPrelated mechanism using isolated OCP, PB and FRP. I demonstrated that light is only needed for OCP photoactivation but OCP binding to PB is light independent. Only the photoactivated OCP is able to bind the PB and quench all its fluorescence. Based on our in vitro experiments we proposed a molecular model of OCP-related photoprotection. The in vitro reconstituted system was applied to examine the importance of a conserved salt bridge (Arg155-Glu244) between the two domains of OCP and showed that this salt bridge stabilises the inactive form of OCP. During photoactivation this salt bridge is broken and Arg155 is involved in the interaction between the OCP and the PB. The site of OCP binding in the core of a PB wasalso investigated with the in vitro reconstituted system. Our results demonstrated that the terminal energy emitters of the PB are not needed and that the first site of fluorescence quenching is an APC trimer emitting at 660 nm. Finally, we characterised the properties of excited states of the carotenoid in the photoactivated OCP showing that one of these states presents a very pronounced charge transfer character that likely has a principal role in energy dissipation. Our results strongly suggested that the OCP not only induces thermal energy dissipation but also acts as the energy dissipator.

Fourier transform infrared difference spectroscopy (FTIR DS) is widely used to study the structural details of electron transfer cofactors in photosynthetic protein complexes. In photosynthetic proteins quinones play an important role, functioning as a cofactor in light-driven electron transfer. In photosystem I (PS I) phylloquinone (PhQ) functions as an intermediary in electron transfer. To investigate the properties of PhQ that occupies the, so called, A1 binding site in PS I, time-resolved step-scan FTIR DS, with 5µs time resolution at 77K has been used. By replacing PhQ in the A1 binding site with specifically isotope labeled version, information on the vibrational frequencies associated specifically with the quinone in the binding site were obtained, which could be compared to the vibrational properties of quinone in solution or quinones in other protein binding sites. To further aid in assessing the origin of bands in the spectra, quantum mechanics /molecular mechanics (QM/MM) ONIOM type calculations were undertaken. ONIOM is an acronym for Our own N-layered Integrated molecular Orbital and molecular Mechanics. We find that the phytyl tail of PhQ does not play an important role in the orientation of PhQ in the A1 binding site. We also find that PhQ, in both neutral and reduced states, is strongly hydrogen bonded. To test and verify the applicability of our QM/MM approach, ONIOM calculations were also undertaken for ubiquinone and a variety of other quinones incorporated into the, so called, QA binding site in purple bacteria photosynthetic reaction centers. The calculated and experimental spectra agree well, demonstrating the utility and applicability of our ONIOM approach. Hydrogen bonding to the carbonyl groups of quinones in the QA binding site was shown to be relatively weak, and it was found that hydrogen bonding to neutral ubiquinone in purple bacterial reaction centers can be considered in purely electrostatic terms, contrary to the widely held belief that the hydrogen bonding amino acids should be treated quantum mechanically.

The main aim of this study was to investigate the effects of water availability and SO2 pollution, imposed separately or simultaneously, on the photosynthetic metabolism of Augea capensis Thunb., a succulent of the Namib Desert in the region of Skorpion Zinc mine, Namibia. The main driver for this investigation was the need to distinguish between the effects of water availability on plants native to a desert environment, where water availability dominates plant response, but where the possibility of anthropogenic SO2 pollution poses a new threat to the unique succulent vegetation. Fifteen measuring sites were selected in the vicinity of the mine to determine how rainfall influenced the physiological status of the vegetation. Chlorophyll a fluorescence measurements, and analysis of recorded OJlP fluorescence transients with the JIP-test, were used for this purpose. A series of laboratory experiments were also conducted on A. capensis to determine the precise physiological response that water deprivation and SO2 pollution had under controlled growth conditions. Potted plants were exposed to water deprivation or SO2 fumigation in the light or dark. Besides chlorophyll a fluorescence, photosynthetic gas exchange and Rubisco activity were also measured. Changes in fast fluorescence rise kinetics observed under field conditions suggest considerable modulation of photosystem II function by rainfall with concomitant involvement of a heat stress component as well. In both the field and laboratory experiments, one of the JIP-test parameters, the so-called performance index (PIABS), was identified as a very sensitive indicator of the physiological status of the test plants. Moreover, under laboratory conditions, a good correlation existed between the water deprivation-induced decline in CO2 assimilation rates and the decline in PIABS values. The JIP-test in general, and the PIABS in particular, shows considerable potential for application in the investigation of water availability influences on desert ecosystems. In the laboratory experiments, water deprivation caused stomatal closure but also a slight elevation in intercellular C02 concentration and inhibition of Rubisco activity, suggesting that mesophyll limitation was the dominant factor contributing to the decrease in C02 assimilation rates. Following re-watering, A. capensis showed remarkable recovery capacity. Fumigation of A. capensis with 1.2 ppm SO2 in the dark or light revealed relatively small effects on C02 assimilation. The inhibitory effects on photosynthesis were also fully reversible, indicating no permanent metabolic/structural damage. The effects on photosynthesis were more pronounced when fumigation occurred in the dark. This phenomenon might be related to diurnal differences in cellular capacity for SO2 detoxification. When long-term moderate water deprivation was combined with simultaneous SO2 fumigation, there was no additional inhibitory effect on photosynthesis. These findings suggest that water deprivation do not increase sensitivity towards SO2 pollution in A. capensis. Fumigation with SO2, singly or in combination with water deprivation also had no major effect on chloroplast ultrastructure. It appears that A. capensis is remarkably resistant to SO2 pollution even in the presence of low water availability, which is a common phenomenon in desert ecosystems. Since A, capensis seems to be highly tolerant to S02, its suitability as an indicator species for the detection of SO2 pollution effects at Skorpion Zinc mine is questionable. Because water availability dominates the physiological/biochemical response in this species, subtle SO2 pollution effects might be difficult to detect against this dominant background. The high water content of A. capensis and similar succulents might act as a substantial sink for SO2 and could convey considerable tolerance against this form of air pollution. / Thesis (M.Sc. (Botany))--North-West University, Potchefstroom Campus, 2006.

Species diversity may simultaneously be a cause and a consequence of variability in population, community and ecosystem properties. Ecology has traditionally focused on elucidating the causes of biodiversity. However, in the last decade and a half ecologists have asked the opposite question: What are the consequences of species diversity? The majority of these studies elucidated the effects of species diversity within single trophic levels. Incorporating trophic complexity is the next step in this research program. In this dissertation I investigated the causes of resource species diversity, as well as the impacts that resource diversity has on rates of consumption and the stability of population, community and ecosystem properties over time in planktonic food webs. The high diversity of phytoplankton found in nature appears to defy the competitive exclusion principle, and elucidating the mechanisms which maintain this diversity continues to be a challenge. In general, variability in limiting factors is required to maintain non-neutral species diversity, but this variability can be generated by forces outside of the competitive community (i.e. exogenous), or may be the outcome of competitive interactions themselves (i.e. endogenous). Using microcosm experiments, I showed that endogenously generated variability in limiting factors was more effective at maintaining phytoplankton species diversity over the long-term, although the strength of this effect depended on the composition of the phytoplankton community. Existing resource diversity has been proposed to generally weaken consumer-resource interaction strengths and limit consumer control of resource biomass. This is because more diverse resource communities are more likely to contain inedible, unpalatable, toxic or non-nutritious species. However, when resource communities contain multiple palatable species, diversity may also accelerate consumption. Using grazing experiments with multiple zooplankton consumer species, I found that the mechanism, direction and magnitude of modulation of consumption depended on the feeding selectivity of the consumer and the composition of the resource community. By altering consumer-resource interaction strengths in the short-term, resource species diversity may impact the stability of consumer-resource dynamics in the long-term. In separate microcosm experiments, I investigated the influence of resource species diversity, community composition and consumer feeding selectivity on population, community, and ecosystem properties over time. Diversity had positive effects on phytoplankton population biomass, resource community biomass, the rate of photosynthesis, the standing stock of particulate nutrients, and the generalist consumer’s population density. It also stabilized resource community biomass and the stocks of particulate nutrients over time. Unexpectedly, diversity did not stabilize either of the consumer populations, regardless of feeding selectivity. This suggests that effects of diversity on resource community properties do not impact consumer dynamics linearly. Resource community composition was generally more important than resource species diversity in determining food web properties. The importance of community composition in determining both the causes and consequences of resource diversity in these experiments points to the importance of species’ traits and the outcomes of their interactions. I suggest that the use of complex adaptive systems theory and trait-based approaches in the future will allow a consideration of the feedbacks between the causes and consequences of species diversity in food webs. / Graduate

The work presented in this thesis was aimed to investigate the structure-function relationship of several photosynthetic Light-Harvesting Complexes (LHCs) including Chlorophyll Protein 29 (CP29) and Light-Harvesting Complex II (LHCII) of green plants, and Fenna-Matthews-Olson (FMO) complex of green sulfur bacterium Chlorobium tepidum. Based on the exciton calculations, a model was proposed to the electronic excited states (EES) of both CP29 and LHCII complexes by incorporating a considerable part of the current information offered by structure determination, mutagenesis analysis and spectroscopy in the modeling. The essential parameters for characterizing the excited states, Qy dipole orientations and site energies were assigned by suggesting a model that can explain both the key features of the linear (polarized) absorption spectra and the time scales of the energy transfer processes in CP29 and LHCII. The idea of offering structural information through setting connection between the spectroscopy and the spectral simulations were supported by the presented results on CP29 and LHCII. New spectroscopic measurements (absorption, linear dichroism (LD) and circular dichroism), carried out at 4 K on the FMO complex were presented, and also the LD spectrum was corrected for the degree of orientation of the sample, in order to provide comparison of not only the shape but also the size of the simulated and experimental spectra. The EES structure of the FMO complex was studied by simulating the measured optical spectra with more realistic model than the previously applied models. Simulations have been carried out with a computer program based on exciton model, which includes inhomogeneous, homogeneous and lifetime broadenings explicitly.