As the world’s population increases, the demand for energy also grows. The strain on our limited resources of fossil fuels is unsustainable in the long term. An alternative, renewable method of energy generation must be implemented. Solar energy has good potential as an environmentally sound, unlimited energy source, but solar devices are not yet able to efficiently store energy for later use. A device has been proposed which uses direct sunlight to split water into hydrogen and oxygen. The hydrogen can then be harvested and stored as fuel, solving the question of how to effectively store energy generated during times of peak sunlight for use when sunlight levels are low. The prototype device incorporates arrays of doped silicon microwires which function as light absorbers and current-carriers, driving the chemical reactions that evolve hydrogen from water. This work aims to quantify and characterize the reduction in microwire resistivity that is achievable through application of silicon’s piezoresistive properties. Silicon displays a change in electrical resistance as a function of applied mechanical strain. This electromechanical effect has been studied extensively in bulk and top-down (etched) microstructures, but studies on microstructures grown bottom-up have been limited. A simple method is presented for piezoresistive characterization of individual, released, bottom-up silicon microwires. It is shown that these n-type microwires display a consistent negative piezoresistive response which increases in magnitude with increasing doping concentration. It was found that harnessing the piezoresistive response of moderately-doped (∼10^17 cm^−3) n-type wires allowed for a maximum observed reduction in resistivity of 49%, which translated to a 1% reduction in overall system resistance of a prototype unit cell of the artificial photosynthesis device, if all other components therein remained unchanged. / February 2015

In a warm, dry grassland in southeastern Arizona dominated by C₄ grasses the only C₃ grass found was restricted to dry, exposed ridge crests within the hottest and driest part of the region. This was precisely the opposite of what one would predict from physiological and biogeographic considerations, which would lead one to expect a C₃ grass in this environment to be growing on cooler or moister areas that would mitigate the effects of the inhospitable climate. Cover of C₄ grasses was very low on these ridge crests, and increased downslope with the greater volume of water available to high values on the lower slopes and in washes. It was suggested that this C₃ grass, Stipa neomexicana, had a very high tolerance of water stress, but a very poor tolerance of competition, and was limited to unfavorably dry sites by its competitively superior C₄ neighbors. Most species, regardless of photosynthetic type, could not survive in the harsh ridge crest environment, which therefore offered a refuge from competition. The hypothesis of competitive exclusion was tested by removal experiments conducted at ridge crest, midslope and lower slope positions along the topographic gradient of decreasing Stipa neomexicana and increasing C₄ grass cover. The predictions made under this hypothesis were confirmed. The presence of competitors limited the growth of mature plants, flower production, seedling establishment and seedling survival. The beneficial effects of the removal of competitors increased downslope. Competition depressed estimated finite rates of population increase for Stipa neomexicana. This depression was most severe on the lower slope. It was concluded that increasing competition from C₄ grasses along the topographic gradient was responsible for restricting Stipa neomexicana to the unfavorable ridge-crest sites.

No description available.

No description available.

No description available.

Little is known of the physiological and biochemical mechanisms underlying competitive interactions between alien invasive grasses and native taxa, and how these are affected by resource supply. Consequently, this study compared photosystem II (PS II) function, photosynthetic gas and water exchange, enzyme and pigment concentrations, flowering and biomass accumulation in an indigenous geophyte, Tritonia crocata (L.) Ker. Gawl., grown in monoculture and admixed with the alien grass, Lolium multiflorum Lam., at different levels of water and nutrient supply. Diminished stomatal conductances were the primary cause of reduced net C02 assimilation rates, and consequent biomass accumulation in T. crocata admixed with L. multiflorum at all levels of water and nutrient supply with one exception. These corresponded with decreased soil water contents induced presumably by more efficient competition for water by L. multiflorum, whose biomass was inversely correlated with soil water content. Biochemical impairments to photosynthesis were also apparent in T. crocata admixed with L. multiflorum at low levels of water and nutrient supply. These included a decline in the density of working photosystems (reaction center per chlorophyll RC/ABS), which corresponded with a decreased leaf chlorophyll a content and a decreased efficiency of conversion of excitation energy to electron transport (¥0 / l-^o), pointing to a reduction in electron transport capacity beyond QA~, a decline in apparent carboxylation efficiency and Rubisco content. At low nutrient levels but high water supply, non-stomatal induced biochemical impairments to photosynthesis (decreased RC/ABS, chlorophyll a and Rubisco content) were apparent in T. crocata admixed with L. multiflorum. These attributed to a reallocation of fixed carbohydrate reserves to floral production which increased significantly in T. crocata under these conditions only and associated with a corresponding reduction in the mass of its underground storage organ (bulb). The results of this study did not support the hypothesis that under conditions of low water and low nutrient supply invasive annual grasses would have a lesser impact on the growth and physiology of native geophytes than under resource enriched conditions that favor growth of these grasses. Unresolved is whether resource limitation and allelopathic mechanisms functioned simultaneously in the inhibition of the native geophyte by the alien grass. / Thesis (M. Environmental Science (Ecological Remediation and Sustainable Utilisation))--North-West University, Potchefstroom Campus, 2008.

This research was an attempt to characterize previously constructed mutants with a specifically altered psbO gene which encodes a 33 kDa protein active in photosynthesis. This polypeptide was believed to function in stabilization of manganese ions during photolysis of water at the photosystem II. The initial phase of this work was concerned with determining the manganese content of the genetically manipulated PS II particles of the photosynthetically active cyanobacteria.We found however, that the results of the isolation procedure for PS II particles of photosynthetically active cyanobacteria as described by Burnap et al. was not reproducible in our research organism. This prevented the chemical characterization of function of these particles as had been planned.In the second phase of the research sequencing of the mutated gene was to be performed for several clones in order to determine the kinds of specific alterations that had been made. The mutated genes had been cloned into both pUC1 20 and pPGV5 vectors which were transformed into Escherichia OR (EQQJi) and the cyanobacterium Synechococcus PCC 7942, respectively.Several attempts were mad o isolate plasmid DNA from both the transformed E QQJI and cyanobacterium. Isolation of pUC120 DNA was not achieved due to the toxicity of the 33 kDa protein product of the psbO gene in sgJj. The pPGV5 plasmid isolation was successful and PCR-sequencing was performed. However, the sequencing did not result in a readable sequence. Instead, banding patterns showed more than one nucleotide per lane. Since pPGV5 contains a strong constitutive promoter, a large amount of mutant protein was being produced. Our findings suggested that transformed cyanobacteria may have been under pressure to revert the altered gene to wild-type. Thus, upon growth of a single colony to a larger volume, a heterogeneous population of cells with different sizes of plasmids may have resulted. Restriction analysis of isolated plasmid DNA confirmed the presence of multiple-sized plasmid molecules. Therefore, this research has shown that the previously constructed mutants are not stable enough to characterize for alterations in manganese binding. / Department of Biology

Rapid climate change in Arctic regions is of concern due to important feedbacks between the Arctic land surface and the global climate system. A large amount of organic carbon (C) is currently stored in Arctic soils; if decomposition is stimulated under warmer conditions additional release of CO2 could result in an accelerating feedback on global climate. The strength and direction of Arctic C cycle - climate feedbacks will depend on the growth response of vegetation; if plant growth increases some or all of the extra CO2 emissions may be offset. Currently the Arctic is thought to be a small net sink for CO2, the expected balance of terrestrial C sinks and sources in the future is unknown. In this thesis I explore some of the critical unknowns in current understanding of C cycle dynamics in Arctic vegetation. Quantifying gross primary productivity (GPP) over regional scales is complicated by large spatial heterogeneity in plant functional type (PFT) in Arctic vegetation. I use data from five Arctic sites to test the generality of a relationship between leaf area index (LAI) and canopy total foliar nitrogen (TFN). LAI and TFN are key drivers of GPP and are tightly constrained across PFTs in Low Arctic Alaska and Sweden, therefore greatly simplifying the task of up-scaling. I use data from Greenland, Barrow and Svalbard to asses the generality of the LAI-TFN relationship in predicting GPP at higher Arctic latitudes. Arctic ecosystems are unique among biomes in the large relative contribution of bryophytes (mosses, liverworts and hornworts) to plant biomass. The contribution of bryophytes to ecosystem function has been relatively understudied and they are poorly represented in terrestrial C models. I use ground based measurements in Northern Sweden to fill an existing data gap by quantifying CO2 fluxes from bryophytes patches in early spring and summer, and develop a simple model of bryophyte GPP. Using the model I compare bryophyte GPP to that of vascular plants before, during and after the summer growing season, finding that productive bryophyte patches can contribute up to 90 % of modelled annual GPP for typical vascular plant communities at the same site, and that the relative magnitude of bryophyte GPP is greatest in spring whilst the vascular plant canopy is still developing. Understanding how GPP relates to plant growth is important in relating remotely sensed increases in Arctic ‘greenness’ to changes in plant C stocks. I use a 13C pulselabelling techniques to follow the fate of recently fixed C in mixed vascular and bryophyte vegetation, with a focus on quantifying the contribution of bryophytes to ecosystem carbon use efficiency (CUE). I show that bryophytes contribute significantly to GPP in mixed vegetation, and act to increase ecosystem CUE. I highlight the importance of including bryophytes, which do not have roots, in aboveground: belowground partitioning schemes in C models. To further explore C turnover in bryophytes, I use the results of a second 13C labelling experiment to develop a model of C turnover in two contrasting Arctic mosses (Polytrichum piliferum and Sphagnum fuscum). I find significant differences in C turnover between Polytrichum piliferum which respires or translocates about 80 % of GPP, while Sphagnum fuscum respires 60 %. This analysis is the first to explicitly model differences in C partitioning between Arctic bryophyte species. Finally, I discuss the implications of each chapter for our understanding of Arctic C dynamics, and suggest areas for further research.

Strawberry production in the United States is almost entirely done in open-fields. Recently, interest in off-season strawberry production using controlled environment (CE) systems such as greenhouses and soilless cultivation has increased in the US. However, strawberry production in greenhouses is relatively new in North America and available information about greenhouse strawberry production is limited. Plant physiological responses to the environment must be well understood to maximize production using CE systems. In the present research, photoperiodic and photosynthetic responses of strawberry plants in greenhouse were studied. To evaluate photoperiodic response eight cultivars of strawberry widely cultivated in North America were subjected to varied photoperiods under an average daily temperature of 17 °C. Short-day (SD) cultivars included 'Radiance', 'F-127', 'Shuksan' and 'Chandler', and day-neutral/ever-bearing (DN/EB) cultivars included 'Albion', 'Portola', 'Monterey' and 'San Andreas'. SD cultivars were subjected to treatments of 11-h, 12-h, 13-h and 14-h photoperiod for 8 weeks. DN/EB cultivars were subjected to separated treatments of 8-h, 11-h, 14-h and 17-h photoperiod for up to 10 weeks. After 8 and 10 weeks of photoperiodic treatments in SD and DN/EB cultivars respectively, shoot apical meristems (SAM) were observed under microscope and classified into one of twelve developmental stages (Indices: 0-11). All SD cultivars examined showed a critical photoperiod between 13 h and 14 h. DN/EB cultivars 'San Andreas', 'Albion' and 'Monterey' presented facultative long-day response with positive correlation between SAM developmental indices and photoperiod after 8 weeks of treatment. 'Portola' showed non-significant influences of photoperiodic treatments in flower primordial development, suggesting day neutral response. However, further experiments must be conducted to confirm cultivar responses and identify possible interactions between photoperiod and temperature. The effect of light intensity and plant sink/source balance on strawberry plant photosynthesis was also investigated. Measurements of leaf net photosynthetic rate (Pn), stomatal conductance, intercellular CO2 concentration (Ci) and transpiration rate under near-saturated photosynthetically photon flux of 1,000ï ­mol m-2 s-1 and ambient CO2 concentration in strawberry cultivars 'Albion' and 'Nyoho'; and tomato cultivar 'Speedella'(comparison purposes) were conducted monthly in greenhouse during May 2014, and monthly from September 2015 through May 2016 (winter/spring production season). Hourly measurements were recorded hourly from 9 AM to 4 PM. Potential source strength was determined by the number of leaves and the daily light integral (DLI, 400-700 nm) and sink load from flowers and fruits was represented as the number of flowers and fruits. Seasonal changes in daily maximum Pn were observed, as well as diurnal change in Pn in both strawberry cultivars. A significant positive correlation was determined between the estimated ratios of sink/source and the slopes representing diurnal linear decline of Pn. Also, Pn was negatively correlated with Ci but not significantly correlated with vapor pressure deficit (VPD) in greenhouse, suggesting diurnal decline in Pn was likely due to negative feedback of photosynthesis caused by unbalance of sink and source, and not to water stress from high VPD. The photosynthetic capacity as affected by seasonal changes in greenhouse environment and its diurnal change as affected by sink/source balance could help develop more effective practices in CE strawberry production to maximize production. Also, photoperiodic response revealed in this study for American strawberry cultivars will specify conditions to induce flowering in these economically important cultivars off-season.

Thesis advisor: Dunwei Wang / Solar water splitting refers to the reaction that converts solar energy into chemical fuel. It is an attractive means to store solar energy. This process, analogous to nature photosynthesis, uses semiconductor to capture and convert solar irradiation and, as such, is called artificial photosynthesis. Despite its promising prospect, the lack of materials that can satisfy all requirements to achieve efficient solar water splitting becomes an important challenge. In this thesis, we aim to develop a strategy of forming heteronanostructure to tackle the challenge faced by metal oxide-based photoanode for water oxidation. The challenge associated with metal oxide-based photoanodes and current approach to alleviate the challenge is first discussed. We propose a strategy of combining multiple components to form heteronanostructure to meet the challenges, in particular the charge transport issue. By introducing a dedicated charge transporter, we fabricate various heteronanostructure including TiO₂/TiSi₂, Fe₂O₃/TiSi₂ and Fe₂O₃/AZO nanotubes to improve the charge collection and therefore overall efficiency. Additionally, the growth of several important metal oxides by atomic layer deposition is developed and its utilization as photoanode for water splitting is studied for the first time. Because this strategy is based on the rational design and synthesis of materials, it has the potential to produce electrodes with a combination of properties that have not been exhibited simultaneously by single-component materials. In addition, the strategy is highly versatile and can incorporate the latest developments produced by parallel efforts. We are confident that the rational design and synthesis of materials such as the strategy proposed here will play an increasingly more important role in energy research. / Thesis (PhD) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.