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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
61

En utredning av meddelande-orienterade lager för Twingly

Säll, Robert January 2013 (has links)
Att flera datorer används för att gemensamt lösa problem är inte någonting nytt. Det finns många distribuerade system i bruk och många olika lösningar för hur dessa ska kommunicera med varandra. Vissa använder sig av meddelande-orienterade lager för kommunikation vilket det finns väldigt många implementationer av. RabbitMQ är ett exempel där att kommunikation går genom en (eller ett kluster av) central nod och kommunicerar med hjälp av protokollet Advanced Message Queue Protocol, AMQP. I en helt annan kategori finns ZeroMQ som inte definierar någon central nod för all kommunikation att passera utan peer to peer är istället möjlig vilket innebär snabbare responstider men försvårar hur olika klienter hittar till varandra. Det bloggindexerande företaget Twingly kör idag med ett distribuerat system som använder flera olika kösystem för att koordinera ut arbete till de olika datorerna. De vill kolla närmare på hur de kan bygga sitt system med hjälp av meddelande-orienterade lager. Resultatet av arbetet är att RabbitMQ innebär mindre komponenter att hålla reda på vilket innebär att koden blir mindre komplex. Det som kommer gratis med att använda RabbitMQ är just att klienterna inte behöver känna till varandra utan endast behöver känna till RabbitMQ-servern. Nackdelen är att RabbitMQ-servern kommer bli en flaskhals för systemet. ZeroMQ är däremot friare att implementera den funktionalitet man själv behöver vilket är till fördel i de fall tid och pengar finns för att skapa ett eget system byggt ovanpå ZeroMQ. För Twingly som vill ha ett system inom en snar framtid är RabbitMQ ett bättre val av dessa två alternativ.
62

Hantering av stokastiska variationer : Val av säkerhetskapacitetsbuffert baserat på kundorderpunktens position

Christensson, Rebecka, Gustavsson, Rebecka January 2018 (has links)
Syfte – Studien syftar till att kartlägga hur kundorderpunktens position förhåller sig till företags val av säkerhetskapacitetsbuffertar för att hantera stokastiska variationer i kapacitetsbehov. Därmed är studiens frågeställningar: Vilka är de huvudsakliga säkerhetskapacitetsbuffertarna som används för att hantera stokastiska variationer i kapacitetsbehov? Hur förhåller sig kundorderpunktens position till val av säkerhetskapacitetsbuffertar för att hantera stokastiska variationer i kapacitetsbehov? Metod – För att besvara studiens frågeställningar har en fallstudie genomförts och inkluderar en litteraturstudie, intervjuer och dokumentstudier. Intervjuer genomfördes med sju respondenter från sex företag och empirin som framkom från intervjuerna och dokumentstudier analyserades tillsammans med teori från litteraturstudien för att få fram studiens resultat. Resultat – Studien har identifierat tio tillvägagångssätt för att hantera stokastiska variationer, varav två som inte tidigare nämnts av studiens teoretiska ramverk. De tio tillvägagångssätten har därefter kategoriserats in i sex säkerhetskapacitetsbuffertar baserat på likheter. Efter identifiering av kundorderpunktens position har möjliga kopplingar mellan kundorderpunktens position och val av säkerhetskapacitetsbuffert analyserats för att undersöka eventuella samband. Tre tendenser till kopplingar har identifierats men vidare studier krävs för att säkerställa kopplingarna. Implikationer – Området kring hantering av variationer i kapacitetsbehov med en uppdelning i en kapacitetsbuffert och en säkerhetskapacitetsbuffert är relativt outforskat. Att undersöka eventuella kopplingar mellan val av säkerhetskapacitetsbuffertar och kundorderpunktens position kan fylla ett kunskapsgap och kan teoretiskt bidra till vidare studier inom området säkerhetskapacitetsbuffertar. Genom att lära av andras erfarenheter ges en ökad förståelse vilket kan hjälpa företag att fatta mer välgrundade beslut vilket kan leda till förbättrad prestation, vilket är studiens praktiska bidrag. Begränsningar – Studiens begränsning ligger i att endast sju respondenter från sex företag ingick i studien, där endast en intervju gjordes med respektive respondent. Antalet använda tillvägagångssätt och deras kopplingar till kundorderpunktens position hade kunnat stärkas ifall ett större antal respondenter från olika typer av företag deltagit i studien. Därigenom hade kopplingarnas generaliserbarhet kunnat stärkas. Dessutom hade troligtvis fler tillvägagångssätt identifierats ifall en större bredd studerats. I studien har den totala kapaciteten hanteras som två separata delar för hantering av förväntade och oväntade behov. Infallsvinkeln är relativt ny och är inte etablerad vilket kan försvåra och begränsa förutsättningen för verksamheter att applicera resultatet. / Purpose – The study aims to map how the position of the order penetration point is related to companies’ choice to handle stochastic variations in capacity requirements through the use of safety capacity buffers. Hence, the research questions are: 1. Which are the main safety capacity buffers used to handle stochastic variations in capacity requirements? 2. How is the position of the order penetration point related to the choice of safety capacity buffers to handle stochastic variations? Method – A case study has been conducted in order to answer the research questions and thus includes a literature study, interviews and document studies. The interviews were conducted with seven respondents from six companies and the empirical data that emerged from the interviews and document studies were analysed together with theory from the literature study to reach the result of the study. Findings – The study has identified ten approaches to handle stochastic variations, two of which was not previously mentioned in the theoretical framework. The ten approaches have then been categorized into six safety capacity buffers based on similarities. After identifying the position of the order penetration point possible connections between the position of the order penetration point and choice of safety capacity buffers have been analysed to explore possible relationships. Three tendencies to connections have been identified but further studies are required to ensure the connections. Implications – The research area of handling variations in capacity requirements with a division in one capacity buffer and one safety capacity buffer is relatively unexplored. Exploring possible connections between the choice of safety capacity buffers and the position of the order penetration point could fill a gap of knowledge and theoretically contribute to further research within the area of safety capacity buffers. Learning of others’ experiences gives an increased understanding which can help companies to make more informed decisions that can lead to better performance, which is the practical contribution of the study. Limitations – The studies limitations lies in that only seven respondents from six companies were included in the study, were only one interview was conducted with each respondent. The number of used approaches and their connections to the order penetrations point could have been strengthened in case an increased number of respondents from different kind of companies had participated in the study. Thereby the connections’ generalizability could have been strengthened. Additionally more approaches probably would have been identified in case a greater width had been studied. In the study the total capacity have been handled as two separated parts to handle expected and unexpected requirements. The approach is relatively new and not established which can complicate and limit the ability for companies to apply the results.
63

Filmes finos de carbono depositados por meio da técnica de magnetron sputtering usando cobalto, cobre e níquel como buffer-layers / Carbon thin films deposited by the magnetron sputtering technique using cobalt, copper and nickel as buffer-layers

COSTA e SILVA, DANILO L. 08 October 2015 (has links)
Submitted by Maria Eneide de Souza Araujo (mearaujo@ipen.br) on 2015-10-08T12:18:50Z No. of bitstreams: 0 / Made available in DSpace on 2015-10-08T12:18:50Z (GMT). No. of bitstreams: 0 / Dissertação (Mestrado em Tecnologia Nuclear) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP
64

Filmes finos de carbono depositados por meio da técnica de magnetron sputtering usando cobalto, cobre e níquel como buffer-layers / Carbon thin films deposited by the magnetron sputtering technique using cobalt, copper and nickel as buffer-layers

COSTA e SILVA, DANILO L. 08 October 2015 (has links)
Submitted by Maria Eneide de Souza Araujo (mearaujo@ipen.br) on 2015-10-08T12:18:50Z No. of bitstreams: 0 / Made available in DSpace on 2015-10-08T12:18:50Z (GMT). No. of bitstreams: 0 / Neste trabalho, foram produzidos filmes finos de carbono pela técnica de magnetron sputtering usando substratos monocristalinos de alumina com plano-c orientado em (0001) e substratos de Si (111) e Si (100), empregando Co, Ni e Cu como filmes intermediários (buffer-layers). As deposições foram conduzidas em três etapas, sendo primeiramente realizadas com buffer-layers de cobalto em substratos de alumina, onde somente após a produção de grande número de amostras, foram então realizadas as deposições usando buffer-layer de cobre em substratos de Si. Em seguida foram realizadas as deposições com buffer-layers de níquel em substratos de alumina. A cristalinidade dos filmes de carbono foi avaliada por meio da técnica de espectroscopia Raman e complementarmente por difração de raios X (DRX). A caracterização morfológica dos filmes foi feita por meio da microscopia eletrônica de varredura (MEV E FEG-SEM) e microscopia eletrônica de transmissão de alta resolução (HRTEM). Picos de DRX referentes aos filmes de carbono foram observados apenas nos resultados das amostras com buffer-layers de cobalto e de níquel. A espectroscopia Raman mostrou que os filmes de carbono com maior grau de cristalinidade foram os produzidos com substratos de Si (111) e buffers de Cu, e com substratos de alumina com buffer-layers de Ni e Co, tendo este último uma amostra com o maior grau de cristalinidade de todas as produzidas no trabalho. Foi observado que o cobalto possui menor recobrimento sobre os substratos de alumina quando comparado ao níquel. Foram realizados testes de absorção de íons de Ce pelos filmes de carbono em duas amostras e foi observado que a absorção não ocorreu devido, provavelmente, ao baixo grau de cristalinidade dos filmes de carbono em ambas amostras. / Dissertação (Mestrado em Tecnologia Nuclear) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP
65

Monensina sódica, bicarbonato de sódio e duas fontes de calcário em rações com alto teor de concentrado na terminação de cordeiros Santa Inês / Monensin, sodium bicarbonate and limestone sources in rations with high concentrated for feedlot lambs

Adilson Luis Gastaldello Junior 08 February 2008 (has links)
Dois experimentos foram realizados para avaliar o desempenho, características da carcaça, digestibilidade e parâmetros ruminais em cordeiros confinados recebendo rações com 90% de concentrado (contendo principalmente milho e farelo de soja) e 10% de feno de "coastcross" como volumoso, contendo calcário calcítico, calcário calcítico tipo "Filler" e bicarbonato de sódio (NaHCO3), com a adição ou não de monensina. Os tratamentos foram: CC= 1,3% Calcário calcítico; CCF= 1,3% Calcário calcítico tipo "Filler": CC + BS= 1% Bicarbonato de sódio + 1,3% Calcário calcítico, com e sem adição de 30 mg/kg de MS de monensina. O delineamento utilizado foi o de blocos completos casulalizados, em fatorial 2 x 3 de acordo com idade e peso inicial nos dois experimentos. No Experimento 1: foram utilizados 42 cordeiros não castrados da raça Santa Inês divididos em 7 blocos e 6 tratamentos, com peso inicial médio de 21 ± 3 kg e a idade de 88 ± 5 dias, não sendo observado diferença (P > 0,05) entre os tratamentos nas variáveis consumo de matéria seca (CMS) e ganho de peso médio diário (GMD), as quais apresentaram médias de 0,955 e 0,290 kg/dia respectivamente. Porém, verificou-se melhor conversão alimentar (P < 0,05) nos tratamentos com monensina (3,32 kg MS/kg de ganho) comparado aos tratamentos sem monensina (3,58 kg MS/kg de ganho). Não houve efeito nos parâmetros de carcaça sendo a média de 50,38%; 2,42%; 13,94 cm2 e 1,87 mm, para rendimento de carcaça quente, quebra por resfriamento, área de olho de lombo e espessura de gordura respectivamente. No Experimento 2, 24 cordeiros foram alocados por peso em 4 blocos, e mantidos em gaiolas para ensaio de metabolismo para determinar a digestibilidade aparente das rações no trato digestório, metabolismo de nitrogênio e parâmetros ruminais (pH, AGCC e N-NH3). A digestibilidade da matéria seca, matéria orgânica e proteína bruta foram maiores no tratamento com CCF (P < 0,05), comparado aos outros tamponantes. Da mesma forma, a digestibilidade da fração fibra detergente neutro foi maior nos tratamentos contendo monensina em relação aos tratamentos sem adição de monensina. Não houve diferença (P > 0,05) entre os tratamentos, no metabolismo de nitrogênio, pH, consumo de água, relação acetato:propionato e concentração de butirato. Porém houve interação (P<0,05) entre os tratamentos CCF e adição de monensina para concentração de N-NH3, AGCC totais, acetato e propionato, sendo as concentrações destes menores neste tratamento quando o ionóforo foi adicionado à ração. Adicionalmente, houve interação entre os tratamentos CC + BS e monensina sobre a concentração de AGCC totais, acetato e propionato, sendo as concentrações menores nos tratamentos sem a monensina. O uso de CCF não interferiu no desempenho e parâmetros da carcaça, porém apresentou melhor digestibilidade da MS e efeito associativo negativo com a monensina. A adição de bicarbonato de sódio, não alterou os resultados de desempenho e carcaça e houve efeito associativo positivo com a monensina. A presença de monensina sódica melhorou a conversão alimentar. / Two trials were conducted to evaluate performance, carcass characteristics, apparent digestibility and ruminal parameters in feedlot lambs fed a 90% concentrate (based on corn and soybean meal) and 10% of "coastcross" hay ration, with two sources of calcarium and sodium bicarbonate (NaHCO3), with or without monensin. Treatments were: CC = 1.3% limestone; CCF= 1.3% "Filler" Calcitic limestone; CC + BS= 1% Sodium bicarbonate + 1.3% Calcitic limestone, with or without monensin (30 mg/kg DM basis). Lambs were assigned to a complete randomized blocks with a 2x3 factorial design, according to body weight and age at beginning of the trial. In experiment 1: Forty two ram Santa Ines lambs were distributed in 7 blocks and 6 treatments, with initial average body weight of 21.2 ± 3 kg and 88 ± 5 days old. There were no differences in DMI and ADG between treatments (P > 0.05), which averaged 0.955 and 0.290 kg/day, respectively. However, there was a better feed conversion (P < 0.05) of the lambs fed diets containing monensin (3.32 kg DM/kg of gain) compared to diets without monensin (3.58 kg DM/kg of gain). There were no differences in carcass characteristics, which averaged 50.38%, 2.42%, 13.94 cm2 and 1.87 mm of hot carcass weight, chilling losses, longissimus muscle area and fat thickness, respectively. In experiment 2: Twenty four lambs were distributed according to the body weight in 4 blocks. They were placed in metabolism crates to evaluate apparent digestibility of the diets in total digestive tract, nitrogen metabolism and ruminal parameters (pH, total volatile fatty acids and N-NH3). Dry matter, organic matter and crude protein digestibilities were greater for treatments with CCF (P < 0.05), compared to other buffers. Neutral detergent fiber digestibility was higher for treatments with the inclusion of monensin compared to treatments without monensin. There were no differences (P > 0.05) in nitrogen metabolism, pH, water intake, acetate:propionate relation and butirate concentration. There was interaction between treatment CCF and monensin inclusion for N-NH3, total volatile fatty acids, acetate and propionate concentrations, which were lower when the ionophore was included in CCF treatment. There was also interaction between treatments CC + BS and monensin for total volatile fatty acid, acetate and propionate, which were in lower concentrations in the CC + BS treament without monensin. Filler" Calcitic limestone inclusion in the diet had no detrimental effect on lamb performance and carcass characteristics, but DM apparent digestibility was improved and there was a negative associative effect with monensin. Sodium bicarbonate inclusion didn´t change performance and carcass characteristics of the lambs and there was a postitive associative effect with monensin for total volatile fatty acids. In addition monensin inclusion in the diet improved feed conversion.
66

Queueing Network Models for Performance Evaluation of Dynamic Multi-Product Manufacturing Systems

January 2020 (has links)
abstract: Modern manufacturing systems are part of a complex supply chain where customer preferences are constantly evolving. The rapidly evolving market demands manufacturing organizations to be increasingly agile and flexible. Medium term capacity planning for manufacturing systems employ queueing network models based on stationary demand assumptions. However, these stationary demand assumptions are not very practical for rapidly evolving supply chains. Nonstationary demand processes provide a reasonable framework to capture the time-varying nature of modern markets. The analysis of queues and queueing networks with time-varying parameters is mathematically intractable. In this dissertation, heuristics which draw upon existing steady state queueing results are proposed to provide computationally efficient approximations for dynamic multi-product manufacturing systems modeled as time-varying queueing networks with multiple customer classes (product types). This dissertation addresses the problem of performance evaluation of such manufacturing systems. This dissertation considers the two key aspects of dynamic multi-product manufacturing systems - namely, performance evaluation and optimal server resource allocation. First, the performance evaluation of systems with infinite queueing room and a first-come first-serve service paradigm is considered. Second, systems with finite queueing room and priorities between product types are considered. Finally, the optimal server allocation problem is addressed in the context of dynamic multi-product manufacturing systems. The performance estimates developed in the earlier part of the dissertation are leveraged in a simulated annealing algorithm framework to obtain server resource allocations. / Dissertation/Thesis / Doctoral Dissertation Industrial Engineering 2020
67

Enhance Inter-service Communication in Supersonic K-Native REST-based Java Microservice Architectures

Buono, Vincenzo, Petrovic, Petar January 2021 (has links)
The accelerating progress in network speeds and computing power permitted the architectural design paradigm to shift from monolithic applications to microservices. The industry moved from single-core and multi-threads, code-heavy applications, running on giant machines 24/7 to smaller machines, multi-cores single threads where computing power and memory consumption are managed very critically. With the advent of this novel approach to designing systems, traditional multi-tier applications have been broken down into hundreds of microservices that can be easily moved around, start, and stop quickly. In this context, scaling assumed a new meaning, rather than scaling up by adding more resources or computing power, now systems are scaled dynamically by adding more microservices instances. This contribution proposes a theoretical study and a practical experiment to investigate, compare and outline the performance improvements aid by the implementation of Protocol Buffers, Google's language-neutral, binary-based representational data interchange format over traditional text-based serialization formats in a modern, Cloud-Native, REST-based Java Microservice architecture. Findings are presented showing promising results regarding the implementation of Protobuf, with a significant reduction in response time (25.1% faster in the best-case scenario) and smaller payload size (72.28% better in the best-case scenario) when compared to traditional textual serialization formats while literature revealed out-of-the-box mechanisms for message versioning with backward compatibility.
68

Perceptions and utilization of riparian forest buffers by farming interest located in the Big Sunflower Watershed

Roberts, Hall Royal 06 August 2011 (has links)
The field of Landscape Architecture can further develop a niche for the design of sustainable productive landscapes. This study attempts to understand a major stakeholders’ perceptions and use of riparian buffers and other conservation practices for water quality in an agricultural watershed of Mississippi. A survey was distributed to agricultural producers in the Big Sunflower Watershed of the MS Delta. The survey informs the interested parties of producers’ perceptions and uses of riparian forested buffers, perceptions and uses of conservation practices that restore water quality, perceptions of their environment, perceptions of surface water quality, enrollment of governmental incentive programs, and utilization of digital technology. Analysis of this data could lead to a better understanding of the knowledge and attitudes farmers have of the riparian systems and watershed processes at work within the region and factors that influence the farmers’ decisions of implementing conservation plans.
69

Improving Energy Efficiency of Network-on-Chips Using Emerging Wireless Technology and Router Optimizations

DiTomaso, Dominic F. 25 July 2012 (has links)
No description available.
70

Respostas à acidez em células de tabaco (Nicotiana tabacum) cv. BY-2 em diferentes estados de sensibilidade / The response of tobacco (Nicotina tabacum) cv. BY-2 cells to low pH at different stages of sensitivity

Stefanuto, Vanderlei Antonio 26 September 2006 (has links)
Os solos ácidos recobrem, cerca de 40% da área terrestre, constituindo-se em um dos principais fatores limitantes à população à produção vegetal do planeta. No Brasil, os solos ácidos abrangem cerca de dois terços do território nacional. De um modo geral, a acidez do subsolo reduz a profundidade do sistema radicular, aumentando a susceptibilidade à seca e diminuindo o uso de nutrientes pelas plantas. Além da alta atividade de íon hidrogênio (H+), os solos ácidos geralmente apresentam níveis tóxicos de alumínio, sendo que a toxicidade por AI tem sido intensivamente investigada nos últimos anos. No entanto, a toxicidade por AI ocorre apenas a pH baixo e em condições onde a toxicidade por prótons geralmente também se manifesta. Apesar disto, trabalhos envolvendo a toxicidade por prótons são escassos. A sensibilidade de células a pH baixo depende da fase de crescimento e desenvolvimento em que estas se encontram. Em raízes, as células mais sensíveis são as da região de alongamento e em suspensões celulares as células na fase log de crescimento são mais sensíveis do que as células na fase estacionária. Este trabalho faz parte de uma linha de pesquisa que procura explorar as diferenças que existem entre células quanto à sensibilidade a pH baixo para melhor entender a toxicidade e tolerância a prótons. Foram utilizadas células da cultura de tabaco cv. BY-2, um sistema modelo que apresenta diversas vantagens sobre o uso de raízes para a realização deste trabalho. Os objetivos deste estudo foram de a) determinar condições apropriadas para a exposição destas células a acidez; b) caracterizar a resposta destas células a pH baixo, sob influência de diferentes fatores ambientais, quando se encontram em estados distintos de sensibilidade ao pH baixo; e c) verificar se mudanças na sensibilidade das células a pH baixo podem ser decorrentes de alterações na composição da membrana plasmática ou na atividade das ATPases. Vários testes iniciais foram realizados com o intuito de se definir algumas condições experimentais - o tempo de exposição, a composição do meio e o tampão a ser empregado. Optou-se por lavar as células e depois incubá-las por 1h em meio simples com pH desejado e contendo apenas Ca \'Cl IND. 2\' 2mM, KCl 10mM. E o tampão MES ou biftalato (10 mM). O biftalato foi testado porque o tampão MES, usado normalmente no meio de cultura completo, não é eficiente na faixa de pH abaixo de 5,0. O biftalato (pKa = 4,1) praticamente não afetou a viabilidade celular avaliada pela permeabilidade a trypan blue, mas inibiu o crescimento celular no meio de cultura completo. Mesmo assim, os dois tampões foram utilizados em paralelo em diversos experimentos e verificou-se que os resultados foram semelhantes. A viabilidade das células na fase log (2 dias) foi reduzida quando se abaixou o pH de 5,6 a 3,8, sendo que caiu mais acentuadamente até o pH de 4,8. As células da fase estacionaria (7d) foram insensíveis ao baixo pH. A um pH fixo de 4,2 aumentando-se a concentração de Ca \'Cl IND. 2\' para cerca de 8 a 16 mM praticamente aboliu o efeito deletério do pH baixo. Para se ter o mesmo efeito com a adição de KCl, foi necessário adicionar entre 80 e 160 mM. A adição de sacarose também amenizou os efeitos do pH\'sendo praticamente revertido a uma concentração de 100 Mm. Os resultados indicam a importância da força iônica e da osmolaridade da solução, mas o efeito de Ca não parece depender apenas destas duas propriedades. A inibição de ATPases, pelo uso de DCCD, não pareceu ter qualquer relação com a sensibilidade a pH baixo. Tanto células de tabaco na fase log quanto estacionárias foram sensíveis à aplicação de ortovanadato de sódio. Em células da fase estacionária, este efeito mais acentuado a pH 4,2, sugerindo que nestas células, as H+ ATPases do tipo P da membrana plasmática podem ter algum papel na tolerância destas células ao baixo pH. Encontrou-se diferenças no perfil protéico de frações enriquecidas em membrana plasmática entre células da fase log e estacionárias e entre células tratadas ou não a pH baixo. Estas diferenças precisam ser melhor estudadas. / Acid soils account for about 40 % of the surface area of the world and are one of the major factors limiting plant productivity. In Brazil, these soils comprise roughly two-thirds of its total territory. In general, soil acidity is detrimental because it limits the depth of the root system, increasing susceptibility to drought and decreasing the use of nutrients. In addition to the high levels of hydrogen ion activity, acid soils usually have Al toxicity hazards, a topic which has been intensely studied in the past years. However, Al toxicity only occurs at low pH, under conditions in which proton toxicity is also a problem. Despite this, studies of proton toxicity are lacking. The sensitivity of cells to low pH depends on their growth and developmental status. In roots, the most sensitive cells are those of the elongation zone and in cell cultures, cells in the log phase are more sensitive than those of the stationary phase. This study is part of a larger attempt to explore the differences that exist between cells with respect to their sensitivity to low pH to further understand toxicity and tolerance to protons. Cells of tobacco BY-2, a plant cell model system which has several advantages over the use of roots, was used in this study. The objectives were a) to determine the appropriate conditions to expose these cells to low pH; b) characterize the response of these cells to low pH, when different environmental factors are varied and at different stages of cellular sensitivity to acidity; and c) examine if changes in the sensitivity of cells to low pH are due to changes in the composition of plasma membranes or in the activity of ATPases. Several preliminary tests were performed to define the experimental conditions to be used ? the duration of exposure to low pH, the composition of the medium and the buffer to be used. A simple solution containing only CaCl2 2mM, KCl 10 mM and MES or phthalate buffer (10 mM) was chosen to wash and incubate the cells at the pH of interest. Phthalate was tested because MES, the buffer normally used in the culture medium, is not effective at pH values below 5.0. Phthalate (pKa = 4,1) had very little effects on cell viability as evaluated by membrane permeability to trypan blue, but it severely inhibited the growth of the cell culture in complete medium. Nevertheless, both buffers were used in several subsequent experiments, the results of which were found to be similar between both buffers. The viability of log-phase cells (2 day-old) was reduced when the pH was lowered from pH 5,6 to 3.8, but this was sharper down to pH 4,8. Cells in stationary phase (7 day-old) were insensitive to low pH. At a fixed pH of 4,2, increases in the concentration of CaCl2 up to 8 or 16 mM abolished most of the deleterious effects of low pH. To generate the same effect, KCl had to be added at concentrations between 80 and 160 mM. The addition of sucrose also alleviated the effects of low pH. The results suggest the importance of solution ionic strength and osmolarity on sensitivity to low pH, but the effects of Ca do not appear to depend only on these two properties. The inhibition of ATPases, by DCCD, did not appear to bear any relation to cellular sensitivity to low pH. Both log-phase and stationary-phase cells were affected by the addition of sodium orthophosphate. In stationary-phase cells, this effect was more pronounced at pH 4,2, suggesting that in these cells, P-type H+-ATPases of the plasma membrane may play a role in the tolerance of these cells to lo pH. Differences were found in the protein profile of enriched plasma membrane fractions both between log-phase and stationary-phase cells and between cells treated or not with low pH. These differences, however, need to be better examined.

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