Master of Science / Department of Plant Pathology / William W. Bockus / The effects of tan spot on lipid profiles in wheat leaves were quantified by mass spectrometry. Inoculation with Pyrenophora tritici-repentis significantly reduced the amount of many lipids, including the major lipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), in leaves over time. These two lipids accounted for 89% of the mass spectral signal of detected lipids in wheat leaves. Reductions in amounts of lipids were at much higher rates over time for susceptible cultivars compared with resistant cultivars. Furthermore, data show that cultivars resistant to tan spot have different lipid profiles when compared with susceptible cultivars. Resistant cultivars had more MGDG and DGDG than susceptible ones, even in non-inoculated leaves. Using linear models that were fit to data, non-inoculated cultivars with a rating of 1 (highly resistant to tan spot) were calculated to have 66.1% more MGDG and 52.7% more DGDG signal than cultivars with a rating of 9 (highly susceptible). These latter findings are indirect evidence that the amounts of some lipids in wheat leaves may be determining factors in the resistance response of cultivars to tan spot.

O caquizeiro (Diospyros kaki L.) é uma planta caducifólia pertencente à família Ebenaceae, tendo como centro de origem a região central da China. Entre os estados brasileiros, São Paulo é o que apresenta maior produção de caquis com 49.727 t. A cercosporiose (Pseudocercospora kaki), também conhecida como mancha angular, é a principal doença foliar da cultura. Alguns autores atribuem à cercosporiose a queda precoce de folhas, maturação antecipada dos frutos e a redução da produção na safra seguinte. Entretanto, não existem trabalhos com o patossistema P. kaki - D. kaki que deem suporte a essas pressuposições. Diante disso, os objetivos desse trabalho foram comparar a susceptibilidade de quatro cultivares de caquizeiro e avaliar o efeito da cercosporiose na fisiologia da planta. Os experimentos foram conduzidos em condições naturais de campo, com as cultivares Fuyu, Rama Forte, Taubaté e Giombo, por serem as mais cultivadas no estado de São Paulo. Nos experimentos desenvolvidos buscou-se conhecer o grau de resistência das quatro cultivares à cercosporiose, avaliando a incidência, severidade e queda de folhas ao longo de dois ciclos de cultivo, assim como avaliar o controle químico na redução da doença em cvs. Taubaté e Rama Forte e, consequentemente, no acúmulo de reservas na raiz e na qualidade dos frutos colhidos. Foram avaliadas também os danos na fotossíntese em folhas das quatro cultivares com diferentes intensidades de cercosporiose. Como resultados pode-se constatar, que a cv. Fuyu é a mais susceptível à cercosporiose, seguida com resistência média \'Giombo\' e \'Rama Forte\' e com maior resistência a cv. Taubaté. A intensidade da cercosporiose, para severidades da doença de até 7% não influencia na queda de folhas. O controle químico reduziu a intensidade de doença nas cvs. Taubaté e Rama Forte, entretanto, entre as intensidades de cercosporiose avaliadas em plantas tratadas e não tratadas, não há redução no acúmulo de reservas no sistema radicular, assim como na qualidade físico-química dos frutos colhidos. Mesmo a cercosporiose afetando a fotossíntese, no processo fotoquímico, tanto na área lesionada como na área adjacente à lesão, a intensidade da doença observada no campo não foi suficiente para causar redução da produção à cultura. / The persimmon (Diospyros kaki L.) is a deciduous plant of the botanical family Ebenaceae, native to central China. Among the Brazilian states, São Paulo has the highest production of persimmon with 49,727 ton/year. The angular leaf spot of persimmon (Pseudocercospora kaki), also known as cercosporiosis, is the main leaf disease of the crop. Some authors attribute to the cercosporiosis the premature defoliation, early maturation of the fruits and the reduction of the yield in the next crop cycle. However, there are no previous studies with the Pseudocercospora kaki pathosystem that support these hypotheses. Therefore, the objectives of this study were to compare the susceptibility of four persimmon cultivars and evaluate the effect of angular leaf spot on plant physiology. A field experiment was conducted with Fuyu, Rama Forte, Taubaté and Giombo cultivars, the four most cultivated in the state of São Paulo. The degree of resistance to cercosporiosis was determined by the incidence and severity of the disease and by defoliation of the plants in two crop cycles. Chemical spraying for the disease reduction was also evaluated in cvs. Taubaté and Rama Forte. The reduction of the carbohydrate reserve accumulation in the roots and the quality of the fruits harvested were tested. The photosynthesis reduction was also evaluated in leaves of the four cultivars with different intensities of cercosporiosis. The Fuyu cultivar was the most susceptible to cercosporiosis, followed by moderate resistance of \'Giombo\' and \'Rama Forte\' and high resistance of cv. Taubaté. The intensity of cercosporiosis, for disease severity up to 7% does not influence leaf drop. Though the chemical control reduced the disease intensity in cvs. Taubaté and Rama Forte, treated and untreated plants showed no differences in the reserves accumulation in the roots, as well as in the physicochemical quality of the harvested fruits. Even though cercosporiosis affects photosynthesis in the photochemical process on both the necrotic and remaining green leaf area, the intensity of the disease observed in the field was not enough to cause yield reduction to the culture.

Orientador: Adriano Sakai Okamoto / Resumo: Uma das principias doenças transmitida por alimento é a salmonelose, causada pela bactéria Salmonella spp., ela está presente em vários aviários, e com o aumento da produção avícola, se disseminou nos aviários, tornando-se um problema pertinente e de grande importância na saúde pública e animal. Com isso medidas de controle foram instituídas, entre elas o uso de antimicrobiano, causando diversas discussões, principalmente em relação a resistência bacteriana á antimicrobianos utilizados na medicina humana, onde medidas alternativas estão sendo desenvolvidas para evitar o uso dos antimicrobianos. Dessa forma o objetivo do projeto foi avaliar a capacidade dos Lactobacillus spp., isolados na inibição da Salmonella Heidelberg (SH), pelo mecanismo de quorum sensing in vitro e in vivo. Foram colhidos de suabes cloacal de frangos de corte que passaram por provas fenotípica e molecular. Esses isolados foram submetidos ao teste de inibição em placa para avaliar a capacidade de inibição da SH. Em seguida foram produzidos os filtrados, a partir dos Lactobacillus sp. (ATCC), separadamente, com contato com a SH. Esses isolados foram cultivados com os filtrados, separadamente e em seguida submetidos novamente ao teste de inibição em placa. Dessa maneira, foi possível medir e comparar os halos de inibição, para a verificação da ocorrência da comunicação bacteriana entre os Lactobacillus sp. (ATCC) e os isolados. Foi realizada a análise estatística selecionando um Lactobacillus spp. isolado d... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: One of the main food-borne diseases is salmonellosis, caused by the bacterium Salmonella spp., It is present in several aviaries, and with the increase of poultry production, it has spread in aviaries, becoming a relevant problem and of great importance in health public and animal. With this, control measures were instituted, including the use of antimicrobial, causing several discussions, mainly regarding bacterial resistance to antimicrobials used in human medicine, where alternative measures are being developed to avoid the use of antimicrobials. Thus, the objective of the project was to evaluate the ability of Lactobacillus spp., isolated in the inhibition of Salmonella Heidelberg (SH), by in vitro and in vivo quorum sensing mechanism. They were harvested from cloacal swabs of chickens that underwent phenotypic and molecular tests. These isolates were subjected to plaque inhibition test to assess the inhibitory capacity of SH. Filtrates were then produced from Lactobacillus sp. (ATCC), separately, with SH. These isolates were cultured with the filtrates separately and then replate inhibited. In this way, it was possible to measure and compare inhibition halos to verify the occurrence of bacterial communication between Lactobacillus sp. (ATCC) and the isolates. Statistical analysis was performed by selecting a Lactobacillus spp. isolated from chickens and a filtrate which obtained better results in the in vitro test. The in vivo test was divided into four treatments, Lacto... (Complete abstract click electronic access below) / Mestre

Neste trabalho é proposto e avaliado um sistema capaz de monitorar de forma não-destrutiva, não-invasiva, individualizada, em tempo real e em ambiente industrial, a qualidade de soldas produzidas através do processo de solda a ponto resistiva (PSPR), diminuindo ou mesmo eliminando a necessidade dos testes destrutivos, reduzindo custos e aumentando a produtividade. Este sistema de monitoramento é baseado em reconhecimento de padrões, através de redes neurais artificiais do tipo Perceptron multicamadas. As características do processo usadas na entrada da rede neural são os parâmetros ajustados de um modelo matemático parametrizável, criado com o intuito de refletir as propriedades fundamentais da grandeza do processo passível de ser medida e monitorada em tempo real, neste caso a curva de resistência dinâmica. Estes valores ajustados dos parâmetros do modelo são ainda relacionados com os estados ou condições do processo, de forma a permitir a identificação de possíveis causas para falhas detectadas. Para avaliar e validar este sistema, usaram-se dados reais obtidos na produção de lotes de contatos elétricos através do PSPR. Os resultados obtidos mostram que o sistema proposto é capaz de monitorar satisfatoriamente a qualidade do processo investigado, com erro médio quadrático de 16,5 N, na estimação da força de cisalhamento suportada pela solda, no pior caso. O sistema também mostrou-se capaz de identificar a causa para soldas cuja qualidade estimada foi considerada baixa, com taxa de acerto acima de 97%. Esse sistema proposto não contém especificidades de nenhum processo produtivo e, portanto, tem potencial para ser aplicado em outros processos, além do PSPR. / In this work a non-destructive, non-invasive, individualized, real-time system has been proposed and evaluated to monitor the quality of welds produced by resistance spot welding process (RSWP) in industrial environment. This system is able to reduce or eliminate the need for destructive tests, leading to cost reduction and increase in productivity. This monitoring system is based on pattern recognition with multilayer Perceptron artificial neural networks (ANN). The process features used as input of the ANN are adjusted parameters of a parametric mathematic model created to reflect the fundamental properties of the process variable that is measurable in real time, in this work, the dynamic resistance curve. The adjustable model parameters values are related with the process states and conditions, so that it is possible to identify the causes for detected bad quality. In order to evaluate and validate the proposed system, real data obtained in the production of electric contacts by RSWP were used. The results show that the proposed system is capable of properly monitoring the investigated process quality, with a mean square error of 16.5 N, in the estimation of the shear force supported by the weld, in the worst case. The system proved to be able to identify the causes for detected bad quality, with a reliability of more than 97%. The proposed system contains no productive process specificities, and, therefore, can be applied to other processes.

Em três experimentos realizou-se o mapeamento da mancha cega do olho direito para se investigar se o completamento perceptivo que ocorre nela distorce a percepção de tamanho de barras. Em todos os experimentos, o mapeamento da mancha cega foi realizada pelo método dos estímulos constantes; os tamanhos das barras podiam nunca interceptar a mancha cega, interceptá-la parcialmente ou interceptá-la por completo; e as barras eram apresentadas pelo método PEST para se determinar o ponto de igualdade subjetiva, e conseqüentemente, o erro constante. No Experimento I as barras eram horizontais e comparadas no nível do ponto de fixação em condição monocular com o olho direito. Elas eram apresentadas por 100, 300 ou 400 ms, aos pares, simultânea e simetricamente localizadas no hemicampo direito e esquerdo em relação ao ponto de fixação. Os resultados desse experimento indicaram uma assimetria lateral nas comparações de tamanho da barra menor e que o completamento perceptivo na mancha cega não distorce efetivamente o tamanho percebido. No Experimento II as barras eram verticais e podiam ser apresentadas também por 100, 300 ou 400 ms, aos pares, simultânea e simetricamente em relação ao ponto de fixação em cinco excentricidades nos hemicampos do olho direito em condição monocular. As barras nas excentricidades menor e maior não interceptavam a mancha cega e as barras de tamanhos maior e intermediário nas excentricidades intermediárias incidiam nas regiões periféricas e central da mancha cega. Os resultados indicaram também que o completamento perceptivo na mancha cega não distorce a percepção de tamanho e que em algumas excentricidades em que a barra padrão foi apresentada no hemicampo esquerdo evidencia-se uma assimetria lateral na comparação de tamanhos. No experimento III, o mapeamento da mancha cega do olho direito e as comparações das barras verticais foram realizados em condição dicótica. As barras foram apresentadas aos pares e seqüencialmente, expostas por 100 ms cada e com um intervalo de 200 ms entre suas apresentações. Elas foram apresentadas em cinco excentricidades no hemicampo direito de cada olho, similar ao Experimento II, de maneira que as comparações de tamanho envolvessem somente o hemisfério cerebral esquerdo. Os resultados indicaram também que o completamento perceptivo na mancha cega não distorce o tamanho percebido. Os três experimentos sugerem que existe assimetria ao se comparar tamanhos, embora tênue, quando se envolvem ambos hemisférios cerebrais e que a explicação associativa proposta por Trypathy et al. (1995) é a alternativa que melhor explica o completamento perceptivo na mancha cega. / Three experiments were carried out to investigate whether the filling-in in the blind spot can distort perceived size. In these experiments, the method of constant stimuli was applied to map the blind spot of the right eye; the lengths of the bars could never intercept the blind spot, intercept it partially or intercept it totally; and the PEST method was applied to determine the point of subjective equality, and then, the constant error. In Experiment I, the lengths of two horizontal bars were compared at the level of fixation point of the right eye under monocular viewing conditions. These bars were presented simultaneously for 100, 300 or 400ms and they were located symmetrically to each other from the fixation point. Results suggested that the filling-in in the blind spot did not distort perceived length, and that there was lateral asymmetry in comparing bars which lengths do not intercept the blind spot. In Experiment II, two vertical bars were presented simultaneously also for 100, 300 or 400ms and symmetrically from the fixation point at five eccentricities in the visual hemifields of right eye in monocular viewing conditions. Bars at the farther and closest eccentricity never intercepted the blind spot, and the bars at intermediate eccentricities could intercept the central and peripheral regions of the blind spot. Results also indicated that the filling-in in the blind spot did not distort perceived size, and that there was lateral asymmetry in comparing size as the standard bar was at the left visual hemifield. In Experiment III, the blind spot of the right eye was mapped and length bar comparisons were done in dichotic conditions in such way that only the left brain was involved. Two vertical bars were presented at five eccentricities like in Experiment II, but they were presented sequentially for 200 ms with an interval of 400 ms between presentations. Results also indicated that the filling-in did not affect perceived size. These three experiments suggest that there is a weak lateral asymmetry in size comparisons due to the inter-hemispheric brain effect and that the associative explanation proposed by Trypathy et al. (1995) is the best alternative to explain the filling-in in the blind spot.

Em três experimentos realizou-se o mapeamento da mancha cega do olho direito para se investigar se o completamento perceptivo que ocorre nela distorce a percepção de tamanho de barras. Em todos os experimentos, o mapeamento da mancha cega foi realizada pelo método dos estímulos constantes; os tamanhos das barras podiam nunca interceptar a mancha cega, interceptá-la parcialmente ou interceptá-la por completo; e as barras eram apresentadas pelo método PEST para se determinar o ponto de igualdade subjetiva, e conseqüentemente, o erro constante. No Experimento I as barras eram horizontais e comparadas no nível do ponto de fixação em condição monocular com o olho direito. Elas eram apresentadas por 100, 300 ou 400 ms, aos pares, simultânea e simetricamente localizadas no hemicampo direito e esquerdo em relação ao ponto de fixação. Os resultados desse experimento indicaram uma assimetria lateral nas comparações de tamanho da barra menor e que o completamento perceptivo na mancha cega não distorce efetivamente o tamanho percebido. No Experimento II as barras eram verticais e podiam ser apresentadas também por 100, 300 ou 400 ms, aos pares, simultânea e simetricamente em relação ao ponto de fixação em cinco excentricidades nos hemicampos do olho direito em condição monocular. As barras nas excentricidades menor e maior não interceptavam a mancha cega e as barras de tamanhos maior e intermediário nas excentricidades intermediárias incidiam nas regiões periféricas e central da mancha cega. Os resultados indicaram também que o completamento perceptivo na mancha cega não distorce a percepção de tamanho e que em algumas excentricidades em que a barra padrão foi apresentada no hemicampo esquerdo evidencia-se uma assimetria lateral na comparação de tamanhos. No experimento III, o mapeamento da mancha cega do olho direito e as comparações das barras verticais foram realizados em condição dicótica. As barras foram apresentadas aos pares e seqüencialmente, expostas por 100 ms cada e com um intervalo de 200 ms entre suas apresentações. Elas foram apresentadas em cinco excentricidades no hemicampo direito de cada olho, similar ao Experimento II, de maneira que as comparações de tamanho envolvessem somente o hemisfério cerebral esquerdo. Os resultados indicaram também que o completamento perceptivo na mancha cega não distorce o tamanho percebido. Os três experimentos sugerem que existe assimetria ao se comparar tamanhos, embora tênue, quando se envolvem ambos hemisférios cerebrais e que a explicação associativa proposta por Trypathy et al. (1995) é a alternativa que melhor explica o completamento perceptivo na mancha cega. / Three experiments were carried out to investigate whether the filling-in in the blind spot can distort perceived size. In these experiments, the method of constant stimuli was applied to map the blind spot of the right eye; the lengths of the bars could never intercept the blind spot, intercept it partially or intercept it totally; and the PEST method was applied to determine the point of subjective equality, and then, the constant error. In Experiment I, the lengths of two horizontal bars were compared at the level of fixation point of the right eye under monocular viewing conditions. These bars were presented simultaneously for 100, 300 or 400ms and they were located symmetrically to each other from the fixation point. Results suggested that the filling-in in the blind spot did not distort perceived length, and that there was lateral asymmetry in comparing bars which lengths do not intercept the blind spot. In Experiment II, two vertical bars were presented simultaneously also for 100, 300 or 400ms and symmetrically from the fixation point at five eccentricities in the visual hemifields of right eye in monocular viewing conditions. Bars at the farther and closest eccentricity never intercepted the blind spot, and the bars at intermediate eccentricities could intercept the central and peripheral regions of the blind spot. Results also indicated that the filling-in in the blind spot did not distort perceived size, and that there was lateral asymmetry in comparing size as the standard bar was at the left visual hemifield. In Experiment III, the blind spot of the right eye was mapped and length bar comparisons were done in dichotic conditions in such way that only the left brain was involved. Two vertical bars were presented at five eccentricities like in Experiment II, but they were presented sequentially for 200 ms with an interval of 400 ms between presentations. Results also indicated that the filling-in did not affect perceived size. These three experiments suggest that there is a weak lateral asymmetry in size comparisons due to the inter-hemispheric brain effect and that the associative explanation proposed by Trypathy et al. (1995) is the best alternative to explain the filling-in in the blind spot.

A white spot lesion (WSL) is defined as enamel porosity, which could be due to an enamel defect or an initial carious lesion. ICON® resin is a resin infiltrant that penetrates into the enamel porosity and seals the lesion, thus inhibiting the progression of caries. This method is micro-invasive; however, the resin is radiolucent and the clinician cannot detect the material using radiographs. In order to develop a radiopaque resin, understanding the pore size and pore structure in WSLs is helpful. Therefore, the aim of this study was to characterise: (1) the pore size and structure, (2) the incorporation of radiopaque agents into ICON® resin. Brunauer-Emmett-Teller (BET) and focused ion beam-scanning electron microscopy (FIB-SEM) were used to characterise the porosity in WSLs. The data showed the enamel prisms become more pronounced in the advanced areas of the WSLs and demineralisation occurs within or/and between the enamel prisms. The pore size could be as small as an enamel crystallite, 28x48nm. Nano-strontium hydroxyapatite (non-coated and coated) and strontium bioglass were made as radiopaque fillers and characterised using different techniques including fourier transform infrared spectroscopy (FTIR), BET, transmission electron microscopy (TEM), X-ray diffraction (XRD) and particle size measurements. The radiopaque fillers had micron-sized particles, which made them unsuitable for infiltration into WSLs despite their possible ability to remineralise the WSLs. Radiopaque monomers including bromine-methacrylate and tin-methacrylate were able to make ICON® resin radiopaque and they showed a similar viscosity, wettability and biocompatibility compared to ICON® resin. The X-ray microtomography (XMT) showed that the experimental radiopaque resins were able to infiltrate into the WSLs, artificial and natural lesions, and they could be detected using image subtraction. Backscattered electron (BSE) imaging after each step of application of materials showed that the etching pattern and etchant gel could be barriers for a successful infiltration of the resins into WSLs.

Two different aluminium alloys, AA6111 (Al-Mg-Si) and AA7055 (Al-Mg-Zn), were chosen as the aluminium alloys to be welded with DC04, and two welding methods (USW and FSSW) were selected to prepare the welds. Selected pre-welded joints were then annealed at T=400 - 570oC for different times. Kinetics growth data was collected from the microstructure results, and the growth behaviour of the IMC layer was found to fit the parabolic growth law. A grain growth model was built to predict the grain size as a function of annealing time. A double-IMC phase diffusion model was applied, together with grain growth model, to predict the thickness of each phase as a function of annealing time in the diffusion process during heat treatment. In both material combinations and with both welding processes a similar sequence of IMC phase formation was observed during the solid state welding. η-Fe2Al5 was found to be the first IMC phase to nucleate. The IMC islands then spread to form a continuous layer in both material combinations. With longer welding times a second IMC phase, θ-FeAl3, was seen to develop on the aluminium side of the joints. Higher fracture energy was received in the DC04-AA6111 joints than in the DC04-AA7055 joints. Two reasons were claimed according to the microstructure in the two joints. The thicker IMC layers were observed in the DC04-AA7055 joints either before or after heat treatment, due to the faster growth rate of the θ phase. In addition, pores were left in the aluminium side near the interface as a result of the low melting point of AA7055.The modelling results for both the diffusion model and grain growth model fitted very well with the data from the static heat treatment. Grain growth occurred in both phases in the heat treatment significantly, and was found to affect the calculated activation energy by the grain boundary diffusion. At lower temperatures in the phases with a smaller grain size, the grain boundary diffusion had a more significant influence on the growth rate of the IMC phases. The activation energies for the grain boundary diffusion and lattice diffusion were calculated as 240 kJ/mol and 120 kJ/mol for the η phase, and 220 kJ/mol and 110 kJ/mol for the θ phase, respectively. The model was invalid for the growth of the discontinuous IMC layers in USW process. The diffusion model only worked for 1-Dimensional growth of a continuous layer, which was the growth behaviour of the IMC layer during heat treatment. However, due to the highly transient conditions in USW process, the IMC phases were not continuous and uniform even after a welding time of 2 seconds. Therefore, the growth of the island shaped IMC particles in USW was difficult to be predicted, unless the nucleation stage was taken into consideration.

Hot spotting of automotive disc brakes is an undesired thermal localisation phenomenon, which is a challenge for numerical modelling in terms of both accuracy and efficiency especially for complex disc geometry. In this research, the aim was to develop a computationally efficient finite element (FE) approach for 2-piece pin-mounted ventilated disc hot spot prediction with acceptable accuracy enabling parametric studies to contribute to the knowledge of the complex mechanisms. A time reduction strategy for the simulations was established by incorporating an axisymmetric brake pad assumption with material scaling factor and the friction characteristics were defined by a user-subroutine. The computing accuracy and efficiency of this method were then verified by comparing with traditional FE models. 2D in-plane, 2D out-of-plane, and 3D models were performed to investigate the effects of ventilated disc hot spotting, radial hot spot/band migration, and hot spotting of realistic complex disc geometry respectively. Both 2D and 3D results were validated using experimental results based on a laboratory dynamometer and showed good correlation. The results suggested that adequate modelling of friction pair contact pressure distribution and the subsequent non-uniform heat generation is essential for hot spot simulation; speed was identified as the determinant for the number of hot spots, whereas hot spot temperature was determined by energy level. Furthermore, recommendations for vent design, pins, disc run-out, cooling, material selection, wear rate, pad length and loading distribution were given. Finally, hot spotting and hot band migration cause-effect chains were established based on the results and discussion.

In the first model, we study the optimal procurement strategy in a two-period framework when both the spot market and the forward contract are considered. The forward contract is agreed upon in the first period, and is then delivered in the second period, when the spot market is also available. This is followed by production and demand. The objective of the buyer is to minimize his expected cost. We study the problem for two scenarios: the buyer cannot and can sell to the spot market. Through our analysis, when the buyer can not sell to the spot market, there exists a threshold forward price, under which the buyer will enter into the forward contract. This threshold is lower than the expected spot price. Furthermore, we analytically show that the optimal order quantities via forward contract increase in the mean of the spot price, but decrease in the variability of the spot price. However, the buyer only speculates using the forward contract when he can sell to spot market. / In the second model, we consider a problem in which a buyer makes procurement decisions when he faces periodic random demand and two supply sources, one is a long-term contract supplier and the other is a spot market. When he procures from the contract supplier, a fixed unit price is charged and a predetermined minimum quantity for each period must be committed, and when he procures from the spot market, a stochastic spot price plus a fixed setup cost is charged. The spot price is only realized at the beginning of each period. We show that the optimal policy consists three different (s, S) type policies. More important, we identify certain conditions under which there exist monotone properties between the policy parameters and the current spot price for a general Markov spot price process. Then, we can divide the price space into three regions, each of which corresponds to a specific policy, for each period. We also conduct numerical analysis to gain more insights into how the spot market impacts the buyer's performance. We find the buyers benefits from a more volatile market. / The last model extends the second model by incorporating an important feature that is widely seen; i.e., the procurement from the contract supplier should fulfill a total order quantity commitment (TOQC). The TOQC requires the buyer to procure no less than the predetermined commitment during the contract period, which we call the planning horizon. Thus, in each period, the buyer trades off between the possible lower cost now (by procuring from the spot market) and the reduced cost in the future (by reducing the remaining commitment). Two types of commitment contracts are considered: a minimal TOQC contract and a definite quantity contract. Our analysis characterizes an optimal procurement policy which depends on the spot price in each period and an optimal virtual remaining commitment level. Such a structured policy can be viewed as a combination of some policies of base-stock type, each of which can be computed through an equivalent system without any commitment. Moreover, some of these equivalent systems are of simple multiple-period newsvendor type. This greatly simplifies the computation of the optimal policies. We also numerically analyze how the TOQC and the spot market affects the buyer's performance. / This research develops mathematical models for inventory and procurement management in the presence of spot markets. More specifically, we consider those models by incorporating different types of supply contracts. Particular attention is paid to the quantity flexible contracts. This research is an attempt to understand how firms should adopt their operating policies in the presence of fluctuating commodity prices. In this thesis, we mainly consider the following three models. / Xue, Weili. / Adviser: Youhua Chen. / Source: Dissertation Abstracts International, Volume: 72-11, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2009. / Includes bibliographical references (leaves 122-134). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web.