Allium white rot (AWR) is a soilborne disease that seriously damages commercial cultivation of onion (Allium cepa) and garlic (Allium sativum) crops. The disease has been found everywhere onions are cultivated and at present no system of control has been found that fully prevents the occurrence of the disease. The fungus responsible for the disease, Sclerotium cepivorum, uses oxalic acid to kill Allium bulb and root tissue in growing onion and garlic plants. Research suggests recombinant oxalate oxidase and oxalate decarboxylase enzymes may be able to degrade this acid and confer resistance against pathogens that rely on it, such as Sm. cepivorum or Sclerotinia sclerotiorum. To test the efficacy of these enzymes against white rot pathogens, three transgenes for wheat oxalate oxidase, barley oxalate oxidase and Flammulina oxalate decarboxylase were transformed into onions and garlic by Agrobacterium-mediated transformation. Allium species are highly recalcitrant to transformation, so these three transgenes were also transformed into tobacco to provide fast-recovering, easy to test transformants to assess the efficacy of the transgenes. Transformed garlic and tobacco lines were analysed to assess the integration and expression of the transgenes, then challenged with Sm. cepivorum or Sa. sclerotiorum, respectively, to assess the bioactivity of recombinant wheat oxalate oxidase, barley oxalate oxidase, and Flammulina oxalate decarboxylase against oxalic acid-dependent pathogens. Results show that one line of tobacco expressing the Flammulina oxalate decarboxylase enzyme was found to be consistently resistant to Sclerotinia sclerotiorum. Garlic lines transformed with this transgene failed to display stable transgene expression or disease resistance, possibly due to silencing of the transgene in recovered transformant tissue.

Allium white rot (AWR) is a soilborne disease that seriously damages commercial cultivation of onion (Allium cepa) and garlic (Allium sativum) crops. The disease has been found everywhere onions are cultivated and at present no system of control has been found that fully prevents the occurrence of the disease. The fungus responsible for the disease, Sclerotium cepivorum, uses oxalic acid to kill Allium bulb and root tissue in growing onion and garlic plants. Research suggests recombinant oxalate oxidase and oxalate decarboxylase enzymes may be able to degrade this acid and confer resistance against pathogens that rely on it, such as Sm. cepivorum or Sclerotinia sclerotiorum. To test the efficacy of these enzymes against white rot pathogens, three transgenes for wheat oxalate oxidase, barley oxalate oxidase and Flammulina oxalate decarboxylase were transformed into onions and garlic by Agrobacterium-mediated transformation. Allium species are highly recalcitrant to transformation, so these three transgenes were also transformed into tobacco to provide fast-recovering, easy to test transformants to assess the efficacy of the transgenes. Transformed garlic and tobacco lines were analysed to assess the integration and expression of the transgenes, then challenged with Sm. cepivorum or Sa. sclerotiorum, respectively, to assess the bioactivity of recombinant wheat oxalate oxidase, barley oxalate oxidase, and Flammulina oxalate decarboxylase against oxalic acid-dependent pathogens. Results show that one line of tobacco expressing the Flammulina oxalate decarboxylase enzyme was found to be consistently resistant to Sclerotinia sclerotiorum. Garlic lines transformed with this transgene failed to display stable transgene expression or disease resistance, possibly due to silencing of the transgene in recovered transformant tissue.

Submitted by Natália Cecília Sartarelli (natalia_sartarelli@yahoo.com.br) on 2018-09-04T19:59:14Z No. of bitstreams: 1 Defesa Mestrado Natália C. Sartarelli - PPG Alimentos e Nutrição.pdf: 1871672 bytes, checksum: 8f9bb6f688029216469fbd2871954f16 (MD5) / Approved for entry into archive by Maria Irani Coito null (irani@fcfar.unesp.br) on 2018-09-04T22:53:10Z (GMT) No. of bitstreams: 1 sartarelli_nc_me_arafcf_par.pdf: 704578 bytes, checksum: ff79c882ecba32f7207c61c8873bc82f (MD5) / Made available in DSpace on 2018-09-04T22:53:11Z (GMT). No. of bitstreams: 1 sartarelli_nc_me_arafcf_par.pdf: 704578 bytes, checksum: ff79c882ecba32f7207c61c8873bc82f (MD5) Previous issue date: 2018-07-03 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Objetivos: Analisar quali e quantitativamente algumas hortaliças e frutos, quanto ao teor de ácido oxálico e avaliar o comportamento químico do ácido oxálico frente a agentes indutores e inibidores de cristalização in vitro. Métodos: A análise qualitativa morfológica de cristais de oxalato de cálcio foi feita sob microscopia ótica de luz polarizada e, quantitativamente foram determinados por método espectrofotométrico, os teores de ácido oxálico após extração com água e ácido clorídrico. Foram realizados também ensaios de indução de cristalização com cloreto de cálcio e inibição com sulfato de magnésio e citrato de sódio, respectivamente. Resultados: Foram identificados cristais de oxalato de cálcio do tipo drusas na alface; ráfides no cará e kiwi e, na rúcula e laranja não foi possível identificar o tipo morfológico do cristal presente. No estudo quantitativo observou-se diferença significativa entre as médias obtidas nas extrações aquosa e ácida somente para o cará, sendo superior na extração ácida. Observou-se ainda significância somente para a couve no ensaio quantitativo de cristalização; o acréscimo de sulfato de magnésio como agente inibidor da cristalização não foi significativo para todas as amostras avaliadas no estudo, no entanto, com o acréscimo de citrato de sódio houve aumento do conteúdo de ácido oxálico para o cará, inhame, laranja e kiwi. No estudo qualitativo dos ensaios de cristalização com cloreto de cálcio em microscopia ótica de luz polarizada observou-se presença de arranjos cristalinos; no ensaio com sulfato de magnésio observou-se presença de cristais poliédricos no kiwi, couve e laranja e, com o acréscimo de citrato de sódio observou-se presença de cristais menores somente em alface, cará, couve e inhame. Conclusão: Neste contexto, os teores de oxalato nas hortaliças e frutos estudados apresentam variações próprias conforme a matriz constitucional e, podem ser modificados com o acréscimo de cálcio e citrato. / Objectives: Analyze qualitatively and quantitatively some vegetables and fruits, regarding in the content of oxalic acid and evaluate the chemical behavior of the oxalic acid against inducers and inhibitors agents of the in vitro crystallization. Methods: The qualitative morphological analysis of calcium oxalate crystals was done under optical microscopy of polarized light and, quantitatively, the values of oxalic acid after extraction with water and hydrochloric acid were determined by spectrophotometric method. Tests inducting the crystallization with calcium chloride and inhibitioning with magnesium sulphate and sodium citrate, respectively, were also performed. Results: Druses calcium oxalate crystals were identified in lettuce; raphides in yam and kiwifruit; and in the arugula and orange it was not possible to identify the morphological type of the present crystal. In the quantitative study a significant difference was observed between the averages obtained in the aqueous and acid extractions only for the yam, being superior in the acid extraction. It was also observed a significance only for the wild cabbage in quantitative crystallization test. The addition of magnesium sulphate as a crystallization inhibitor was not significant for all the samples evaluated in the study. However, with the addition of sodium citrate there was an increase in the content of oxalic acid for yam, taro, orange and kiwifruit. Crystalline arrangements were observed in the qualitative study of crystallization tests with calcium chloride under optical microscopy of polarized light; in the test with magnesium sulphate it was noticed the presence of polyhedral crystals in kiwifruit, wild cabbage and orange and, with the addition of sodium citrate, small crystals were present only in lettuce, taro, wild cabbage and yam. Conclusion: In this context, oxalate contents in the vegetables and fruits that were studied have their own variations according to the constitutional matrix and, the oxalate content can be modified with the addition of calcium and citrate.

Made available in DSpace on 2014-06-11T19:24:08Z (GMT). No. of bitstreams: 0 Previous issue date: 2005-02-23Bitstream added on 2014-06-13T19:51:24Z : No. of bitstreams: 1 lima_dm_me_arafo.pdf: 826526 bytes, checksum: ff1d690d6b0676f46d0b484cb8adef94 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O objetivo deste trabalho foi o de avaliar comparativamente in vitro, a resistência de união ao cisalhamento de uma resina composta à dentina bovina em função da combinação de três diferentes sistemas adesivos e de dois diferentes agentes dessensibilizantes. Foram utilizados os sistemas adesivos: Single Bond, Master Bond e Prime & Bond 2.1 e os seguintes agentes dessensibilizantes: o Oxagel e o G. H. F. Dessensibilizador. Para a confecção dos corpos de prova, 90 incisivos bovinos (n=10) foram utilizados, os quais tiveram suas superfícies vestibulares desgastadas, até a exposição dentinária, de tal forma que, a área de adesão padronizada, tivesse 4 mm de diâmetro. Após a realização dos tratamentos de superfície, onde os Grupos controle eram indicados apenas pela aplicação do sistema adesivo, e os Grupos experimentais, pela aplicação dos agentes dessensibilizantes testados após o condicionamento ácido, foram confeccionados cilindros de resina composta Z-100, com o auxílio de uma matriz bipartida. Os corpos-de-prova foram então armazenados em água destilada a 37°C por 24 horas, e submetidos em seguida ao ensaio de cisalhamento, a uma velocidade do atuador de 0,5 mm/min. Os resultados obtidos foram submetidos à análise de variância, e ao teste de Tukey, ao nível de 5% de significância. Observou-se que houve influência dos agentes dessensibilizantes, quando estes eram aplicados na dentina bovina e interagiam com os sistemas adesivos, sendo que o G. H. F. Dessensibilizador aumentou estatisticamente os valores de resistência de união do Single Bond, e do Prime & Bond 2.1, e o Oxagel não influenciou nos valores de resistência de união dos sistemas adesivos testados. / The objective of this work was it of evaluating in vitro comparatively , the shear bond strength of a resin composed to the bovine dentin in function of the combination of three different adhesive systems and of two different agents dessensitizers. The adhesive systems were used, to know: Single Bond, Master Bond and Prime & Bond 2.1 and the following agents dessensitizers: Oxagel and G. H. F. Desensitizer. For the making of the test bodies, 90 incisive bovine (n=10) they were used, which had its worn away vestibular surfaces, until the exhibition dentin, in such a way that, the area of standardized adhesion, had 4 diameter mm. After the accomplishment of the surface treatments, where the Groups control was just suitable for the application of the adhesive system, and the experimental Groups, for the application of the agents dessensitizers tested after the acid conditioning, cylinders of composed resin were made Z-100, with the aid of a bipartite matrix. The test bodies, they were stored then in water distilled for 37°C by 24 hours, and submitted soon after to the shear bond strength rehearsal, to a crosshead speed of 0,5 mm/min. The obtained results were submitted to the variance analysis, and to the test of Tukey, at the level of 5% of significance. It was observed that there was influence of the agents dessensitizers, when these were applied in the bovine dentin and interagiam with the adhesive systems, and G. H. F. Dessensitizer increased statistically the values of resistance of union of Single Bond, and of the it Prime & Bond 2.1, and Oxagel didn't influence in the values of resistance of union of the tested adhesive systems.

Este trabalho apresenta a preparação e caracterização de eletrodos de óxidos do tipo ânodos dimensionalmente estáveis (ADE) de composição nominal Ti/Ru0,3Ti0,7O2, modificados pela adição de metais em sua superfície. A introdução de um metal modulador permitiu alterar de forma significativa o comportamento eletroquímico de todo o recobrimento. A modificação do ADE foi realizada através de eletrodeposição de Pt e PbO2 e de deposição química de catalisadores nanodivididos (Pt e Pt-Ni). O objetivo desta preparação foi modular as propriedades dos ânodos visando melhorar as propriedades eletrocatalíticas e a resistência mecânica e com isto, introduzir uma nova classe de materiais para aplicá-los na eletrooxidação de formaldeído, acetaldeído e ácido oxálico. Para caracterizar os eletrodos foram utilizadas as técnicas de microscopia eletrônica de varredura (MEV), espectroscopia de energia dispersiva de raios X (EDX), voltametria cíclica (VC), eletrólises com acompanhamento por cromatografia líquida de alta eficiência (CLAE), espectrometria de massas eletroquímica diferencial (DEMS). Os experimentos com DEMS foram realizados na Universidade de Poitiers, França. As micrografias e as análises de EDX mostraram que o rutênio concentra-se em pequenas ?ilhas? na camada de óxidos e a modificação com platina, eletrodepositada ou depositada quimicamente, ocorre preferencialmente nas regiões ricas em rutênio. O eletrodepósito de PbO2 apresentou uma distribuição mais uniforme sobre toda a camada de óxido. As eletrólises foram realizadas aplicando um programa de pulsos de potenciais. Os resultados mostraram que a eficiência para formação do produto mais oxidado (CO2) está intimamente ligada à quantidade e distribuição da platina adicionada na superfície do ADE. Os eletrodos mais eficientes foram os de composições nominais: Ti/ADE modificado com eletrodeposição de Pt (100 ?g cm-2) e Ti/ADE modificado com deposição química do catalisador nanodividido de Pt-Ni (1:1) (60 ?g cm-2). Estes resultados foram confirmados pelos experimentos utilizando DEMS. O eletrodo Ti/ADE modificado com eletrodeposição de PbO2 (100 ?g cm-2) apresentou uma melhora na eficiência eletrocatalítica em relação ao ADE, porém inferior aos eletrodos modificados com platina. O desempenho do ADE na oxidação das pequenas moléculas orgânicas foi estudado em comparação com um eletrodo de platina eletrodepositada diretamente na placa de titânio. Os resultados de eletrólise mostraram que o ADE não funciona somente como um substrato inerte e sim auxilia na reação de oxidação, desempenhando um importante papel na eficiência catalítica total destes materiais. / This work presents the preparation and characterization of dimensionally stable anodes (DSA) of nominal composition Ti/Ru0.3Ti0.7O2, modified by the addition of metals to their surface. The introduction of the modulator metal allowed us to change the coating of the electrochemical behavior in a significant way. The DSAâ surface was modified by electrodeposition of Pt and PbO2, and chemical deposition of highly nanodispersive supported catalysts (Pt and Pt-Ni). The objective of this preparation was to modulate the anode properties in order to enhance the electrocatalytic properties and mechanical resistance of these materials. This new class of materials was applied in the electrooxidadtion of formaldehyde, acetaldehyde and oxalic acid. The electrode characterization was carried out by Scanning Electron Microscopy (SEM), Energy-dispersive X-ray Spectroscopy (EDS), cyclic voltammetry (CV), long-term electrolysis followed by high performance liquid chromatography (HPLC) and the differential electrochemical mass spectrometry (DEMS) which was performed at the University of Poitiers, France. The micrographs and EDS analyses showed that the ruthenium is concentrated in small ?islands? in the oxide layer, and the modification with platinum, either electrodeposited or chemically deposited, occurs preferable over the ruthenium-rich regions. The PbO2 electrodeposit was more uniformly distributed along the oxide layer. Bulk electrolysis experiments were carried out applying a potentiostatic program. The efficiency toward the formation of the more oxidized compound (CO2) is related with the amount and distribution of the platinum deposited on the DSAâ surface. The most efficient materials were: Ti/ADE modified with electrodeposited Pt (100 ?g cm-2) and Ti/ADE modified with chemically deposited Pt-Ni (1:1) (60 ?g cm-2). Through the DEMS experiments these results were confirmed. The DSAâ modified with PbO2 (100 ?g cm-2) was more efficient than the bare DSAâ. However, the amount of CO2 obtained with the former electrode is still lower than that obtained with the platinum-modified ones. The role of DSAâ on the direct activation of the oxidation of small organic molecules was studied applying an electrode containing the platinum directly electrodeposited over the titanium. The bulk electrolysis results showed that DSAâ is not just an inert substrate since it does play an important role in the overall efficiency of these materials.

The foremost aim for performing this study was to focus on the electrodeposition of mixed hydrogen molybdenum tungsten bronze films, which have the potential for e– transfer interactions carrying out the reduction of carbon dioxide. A yellow peroxymolybdic tungstate solution was prepared and used for the electrodeposition of hydrogen molybdenum tungsten bronze films on conductive carbon paper. Electrodeposition was carried out at -2.0 V from 20 - 120 minutes to determine the effect of deposition time on film thickness and CO2 reduction. These films were characterized by X-ray photoelectron spectroscopy. The deposited films served as a working electrode for CO2 electrochemical reduction utilizing 0.8 M NaHCO3 as the electrolyte. Carbon dioxide gas was bubbled into the cathode solution for an hour while bulk electrolysis was carried out at different applied potentials. Products were identified and evaluated using ion chromatography.

Diamonds are commonly present in an ore called kimberlite. Kimberlites vary extensively in mineralogy and therefore can complicate the processing and the extraction of diamonds. Weathering of kimberlite was investigated in this study and refers to the rapid physical breakdown of the kimberlite material. Kimberlite weathering by organic molecule adsorption was investigated utilising organic solutions; ethanol, formamide, n-hexane, oxalic acid, urea, pentanoic acid and acetic acid, was investigated. The extent of weathering was measured by XRD, TGA and FTIR analysis as well as by evaluating changes in the particle size distribution. The idea to predict the susceptibility of weathering of kimberlite based on TGA analysis was discredited as it was found that there was no correlation between the smectite content and the % weight loss from TGA analysis. In XRD analysis studies to relate the d-spacing to the extent of weathering, it was found that the technique was not sensitive enough to enable direct prediction of weathering behaviour. Organic solutions at 0.5 M generally did not significantly alter the d-spacing. In the case of utilising pure organic solutions, it was observed that switching from diluted to pure organic solutions did not increase the d-spacings but rather lowers the space. Cation pretreatment prior to organic weathering resulted in d-spacings up to 2 water layers (12.6 Å being equal to 1 water layer, 15.6 Å equal to 2 layers and 18.6 Å equal to 3 layer). The general trend was that the divalent cations Mg2+ and Ca2+ increased the d-spacings whereas monovalent cations Na+ and K+ decreased the spacing. In the case of pre-organic treatment prior to organic weathering, it was observed that the resulting d-spacing increased to 2 and 3 water layers. TGA analysis confirmed that the kimberlite structure did absorb organic reagents during weathering. It was observed that the amount of organic reagents adsorbed decrease in the following order: oxalic acid > urea > pentanoic acid > formamide > ethanol > n-hexane > acetic acid, with oxalic acid having been adsorbed the greatest. In addition, it was also shown that the extent of inorganic cation adsorption was more than that of organic solutions. The analysis also showed that organic pretreatments yielded promising results and the order followed was dimethyldioctadecylammonium chloride > cetylpyridium chloride > dodecylamine > pyridine in encouraging subsequent organic adsorption. FTIR analysis results indicated the mechanism by which organic molucules attached onto the mineral surface. It was observed that there was a disturbance of the bond between water in the interlayer and the silicate structure by a stronger H-bond gained through organic molecule attachment. Oxalic acid was observed to cause a structural change due to proton attack of the silicate structure. It was found that kimberlite weathering, in essence, occurred in the first 24 hours of contact with the solution. Oxalic acid was found to be the most effective weathering agent compared to the rest of the tested organic solutions. The effect of the tested solutions, at 0.5 M concentration, on kimberlite weathering followed the series Cu2+ > oxalic acid > ethanol > acetic acid > formamide > n-hexane > distilled water. This study therefore showed that the accelerated weathering of kimberlite utilising organic solutions was not as efficient as the Cu2+ solution. Comparing Cu2+ and oxalic acid treatments, results showed that 67 % of the particles passed 12 mm screen size due to Cu2+ weathering as opposed to 48 % in oxalic acid. In investigating the effects of variables, it was found that time of exposure had a small effect in the weathering of kimberlite. Increasing the organic concentration from 0.025 M to 0.5 M saw an increase in the amount of particles passing the 12 mm screen size by~20 %. There was however no improved weathering when the solution temperature was increased from approximately 25 °C to 40 °C. In the study of kinetics involved during organic weathering utilising oxalic acid and comparing with Cu2+ medium, a high mineral dissolution was observed in the first 24 hours of contact with the weathering solution. However, different weathering mechanisms were observed between Cu2+ and oxalic acid. Inorganic solutions foster weathering by interlayer cation exchange which results in the reduction of the surface energy and this encourages further crack propagation. Oxalic acid weathering mechanism was by proton attack of the structural cations which was then limited thereafter by oxalate salts precipitation. Al3+ and Mg2+ were the dominant cations in solution during weathering (reaching 40 mmol/L) at higher oxalic acid concentration. This indicated that the dissolution of the clay‟s octahedral structure occurred, making oxalic acid a more effective agent than the rest of the tested organic solutions. An alternative method to transform non-swelling clay minerals to swelling in kimberlites was highlighted. It was observed that cation exchange treatment in conjunction with acid and oxidation treatments on kimberlites containing non-swelling minerals: mica or forsterite that are at least 30 % in quantity may result in the presence of swelling clays after these treatments. Kimberlite that contained serpentine minerals was more resistant to alteration under these conditions. / Dissertation (MEng)--University of Pretoria, 2013. / gm2014 / Materials Science and Metallurgical Engineering / UPonly

This report describes a new group of anaerobic bacteria that degrade oxalic acid. The new genus and species, Oxalobacter formigenes, are inhabitants of the rumen and also of the large bowel of man and other animals where their actions in destruction of oxalic acid may be of considerable importance to the host. Isolates from the rumen of a sheep, the cecum of a pig, and from human feces were all similar Gram-negative, obligately anaerobic rods, but differences between isolates in cellular fatty acid composition and in serologic reaction were noted. Measurements made with type strain OxB indicated that 1 mol of protons was consumed per mol of oxalate degraded to produce approximately 1 mol of CO2 and 0.9 mol of formate. Substances that replaced oxalate as a growth substrate were not found.

This work includes an introduction and literature review of mycorrhizal enhancement of the mineral nutrition of host plants. Particular attention was paid to the role of mycorrhizae in acquiring P from inorganic phosphates. Two experiments were designed to study a mechanism by which the ectomycorrhizal fungus Hebeloma westraliensis can enhance the availability and absorption of P from Ca phosphate by <u>Eucalyptus diversicolor</u> in calcareous soil. Chapter 2 reports on a study on the growth and oxalic acid production of <u>H. westraliensis</u> in calcareous and noncalcareous liquid media. Oxalic acid is thought to chelate cations from phosphate precipitates allowing the release of phosphate into solution. L-threonine was added as an additional treatment to asses the ability of this amino acid to inhibit oxalic acid production by the fungus. Growth and oxalic acid production of <u>H. westraliensis</u> were increased in the calcareous systems. L-threonine had little effect on dry weight of mycelium but substantially reduced oxalic acid production in the calcareous media. Chapter 3 reports on a study of E. diversicolor X <u>H. westraliensis</u> mycorrhizal synthesis in model calcareous and noncalcareous systems. Oxalic acid production was measured in these systems as well as various measurements of solution and host plant nutrition. Solution P concentration decreased and solution Ca increased in the calcareous systems. Mycorrhization decreased the solution concentrations of both of these nutrient elements, due partly to sequestration of these elements in fungal and plant tissues. Plant tissue P concentration was decreased and tissue Ca concentration increased in the calcareous systems. Mycorrhization increased the concentrations of both of these elements in plant tissues. Mycorrhization also increased the height and dry weight of seedlings at harvest and ameliorated symptoms of nutrient deficiencies seen in nonmycorrhizal plants in the calcareous system. Oxalic acid production by the fungus and by the plant were increased in the calcareous system. Attempts to correlate oxalic acid production with solution and plant nutrition were unsuccessful. The relationships of these results to the mycorrhizal enhancement of plant nutrition in calcareous soil is discussed. / Master of Science

A reaeração superficial é um importante fenômeno físico responsável pela transferência de massa do oxigênio atmosférico para o curso de água, através da interface ar-água. Esse fenômeno é caracterizado quantitativamente pelo coeficiente de reaeração superficial (K2). O K2 pode ser influenciado por diversos fatores naturais, em especial, a turbulência, que é afetada pelas características físicas, hidrológicas e hidráulicas do escoamento da água. Este coeficiente é comumente utilizado em modelos ambientais de qualidade da água, o que implica na necessidade da sua correta estimativa. A estimativa do K2 pode ser realizada por meio de métodos e modelos de predição, entre os métodos destacam-se o traçador gasoso e a sonda solúvel flutuante. O objetivo principal desta pesquisa foi verificar a variabilidade espacial e temporal do K2 em cinco trechos de quatro riachos tropicais localizados no município de São Carlos-SP, em um ano de amostragem. Nesses trechos foram mensuradas as principais variáveis químicas, físicas, hidrológicas e hidráulicas do escoamento da água, além do K2 e a velocidade de dissolução de uma sonda solúvel flutuante (VS). A estimativa do K2 ocorreu pelo método do traçador gasoso, sendo o SF6 e o NaCl os traçadores utilizados. A estimativa do VS ocorreu pelo método da sonda solúvel flutuante, confeccionada com ácido oxálico dihidratado e no formato esférico. Análises estatísticas de correlação (Spearman, r) e variância (Kruskal-Wallis) foram realizadas, e modelos de regressão linear simples (RLS) e múltipla (RLM) foram desenvolvidos. Os trechos apresentaram profundidades e larguras médias inferiores a 0,42 m e 3,27 m, respectivamente. A vazão e a velocidade (U) média variaram de 0,001 m3.s-1 a 0,110 m3.s-1 e de 0,01 m.s-1 a 0,33 m.s-1, respectivamente. Os números de Reynolds e Froude (Fr) foram inferiores a 34070 e 0,36, respectivamente. Os K2 variaram de 16,90 dia-1 a 373,80 dia-1 e os VS, de 0,08 mm.min-1 a 0,77 mm.min-1. Os resultados demonstraram variação espacial significativa e temporal não significativa para as variáveis físicas, hidrológicas, hidráulicas, e também para o K2 e VS. U e Fr foram as principais variáveis que se correlacionaram com o K2 (r > 0,68). Um adequado modelo RLM de predição do K2 (R² = 0,75), baseado em todas as variáveis físicas, hidrológicas e hidráulicas e no VS, foi obtido. O modelo RLS, baseado apenas na relação entre K2 e VS, não foi adequado (R² = 0,43) para a predição do K2. A razão entre os fenômenos da reaeração superficial e dissolução da sonda solúvel flutuante não foi constante, e foi afetada pela turbulência na interface ar-água, o que indicou que o método da sonda solúvel flutuante ainda precisa ser aprimorado. Espera-se que, além de gerarem dados sobre um método (sonda solúvel flutuante) e tema (reaeração superficial) ainda relativamente pouco estudados em riachos tropicais, as informações apresentadas sobre as principais variáveis intervenientes no fenômeno de reaeração superficial e dissolução em riachos possam oferecer direcionamentos a estudos ambientais, especialmente àqueles que versam sobre a qualidade dos recursos hídricos. / Surface reaeration is an important physical phenomenon responsible for the transfer of atmospheric oxygen mass into the water course through the air-water interface. This phenomenon is quantitatively characterized by the surface reaeration coefficient (K2). K2 can be influenced by several natural factors, especially turbulence, which is affected by the physical, hydrologic and hydraulic characteristics of the watercourse. This coefficient is commonly used in environmental models of water quality, which implies the need for its correct estimation. The estimation of K2 can be performed by models of prediction and methods, among the methods the gas tracer and the floating soluble probe stand out. The aim of this research was to verify the spatial and temporal variability of K2 in five stretches of four tropical streams located in the city of São Carlos-SP, in a year of sampling. In these stretches the main chemical, physical, hydrological and hydraulic variables of the watercourse were measured, besides K2 and the dissolution rate of the floating soluble probe (VS). The estimation of K2 occurred by the gas tracer method, with SF6 and NaCl being the tracers used. The VS estimation was carried out by the floating probe method, made with oxalic acid dihydrate and in spherical format. Statistical analyzes of correlation (Spearman, r) and variance (Kruskal-Wallis) were performed, and simple linear regression (SLR) and multiple linear regression models (MLR) were developed. The stretches presented mean depths and widths of less than 0.42 m and 3.27 m, respectively. The flow rate and average velocity (U) ranged from 0.001 m3 s-1 to 0.110 m3s-1 and from 0.01 m.s-1 to 0.33 m.s-1, respectively. The Reynolds and Froude (Fr) numbers were less than 34070 and 0.36, respectively. The K2 ranged from 16.90 day-1 to 373.80 day-1 and the VS, from 0.08 mm.min-1 to 0.77 mm.min-1. The results showed significant spatial variation and a non-significant temporal variation for the physical, hydrological and hydraulic variables, as well as for K2 and VS. U and Fr were the main variables that correlated with K2 (r > 0.68). A suitable MLR prediction model of K2 (R² = 0.75), based on all physical, hydrological and hydraulic variables and VS, was obtained. The SLR model, based only on the relationship between K2 and VS, was not adequate (R² = 0.43) for K2 prediction. The ratio between the phenomena of surface reaeration and dissolution of the floating soluble probe was not constant and was affected by turbulence at the air-water interface, which indicated that the floating soluble probe method still needs to be improved. It is expected that, in addition to generating data on a method (floating soluble probe) and surface (surface reaeration) that are still relatively little studied in tropical streams, the information presented on the main variables involved in the phenomenon of surface reaeration and dissolution in streams may offer environmental studies, especially those dealing with the quality of water resources.