This master thesis deals with the contrast agents and their application in the ultrasound perfusion analysis. It is focused on Bolus & Burst method which, as a combination of two approaches that have been used so far, allows an absolute quantification of perfusion parameters in the region of interest. Contrast agent concentration time sequence is modeled as a convolution of the parametrically defined arterial input function and the tissue residual funkction. Thesis discusses different mathematical models of these functions as well as the methods of the parameters estimation. The methods functionality is validated on simulated and also preclinical data.

v III. Abstract (EN) According to the Disposable soma theory, senescence is mainly influenced by adaptive allocation of resources among the physical growth, reproduction and self-maintaining processes, which depend on expected survival of an individual. The amount of allocated resources may be influenced by external environmental factors such as heavy metal pollution or by inner factor e.g. reproduction. The aim of my diploma thesis was to clarify the relationships between senescence, selected haematological traits, reproduction and efficiency of innate immunity mechanism. Moreover, we are interested in the role of heavy metal pollution in ageing and haematology. The heterophiles to lymphocytes ratio (H/L) was used as a marker of long-term stress caused by these physiological expenses. This study summarizes partial data collected within the long-term research of great tits (Parus major) in Prague, Czech Republic, I participated in. We have more than 300 samples of repeatedly captured individuals at our disposal. The structure of this unique dataset allows us to observe changes in the traits of our interest during an avian lifespan. Our results show that during aging, males experience a noticeable increase in the H/L ratio compared to females. Although we have not found the effect of age on the brood size,...

Due to growing transmission speed burst-forming errors tend to occur still more frequently not exclusively in data transmission. The presented paper concentrates on the search for alternative burst error correction solutions complementing the existing methods in use. Its objective is an elaboration of a detailed analysis of the issue of convolution codes for error burst correction which can be used in individual anti-error systems and thus an achievement of better results than those attained by mass application of the existing solutions. First the methods implemented to remove or suppress burst errors are briefly characterized. This part is followed by a detailed description of the individual systematic convolution codes by means of mathematical tools which extend the set of possible evaluative criteria of anti-error systems which can be applied while assessing proposals for individual solutions. The acquired code properties are compared with convolution codes as well as with other versions of proposals for message protection against an error burst. The processed convolution codes are subject to testing by means of Matlab mathematical programme simulation in order to validate the correctness of the derived mathematical tools. This is because simulation represents the principal method applied to verify and present an already proposed security process and enables the acquisition of a better overview of the issue at hand. The feasibility of the individual anti-error systems is then confirmed by way of creating a circuit behaviour description in the VHDL language. Its high portability presents a big advantage when drafting individual systems of the actual implementation.

Indiana University-Purdue University Indianapolis (IUPUI) / Tobacco smoking is considered a major modifiable risk factor for periodontal disease. Tobacco contains about 6700 compounds and almost 4000 compounds of these have been identified in tobacco smoke. Nicotine is the addictive ingredient in tobacco and has been shown to affect multiple cellular processes. Cigarette smoke condensate (CSC) is the particulate matter of smoke. It is believed to be a powerful inducer of inflammatory responses. Neutrophils are the first line of host defense and are critical cells in the maintenance of periodontal health through their role in the control of bacteria, but they can also contribute to the progression of periodontal disease by the production and release of reactive oxygen species (ROS). Virulence factors from periodontal pathogens, such as Porphyromonas gingivalis (P. gingivalis), stimulate the respiratory burst of neutrophils. In this dissertation, three studies aimed at understanding the oxidative activity of neutrophils when stimulated with either nicotine, cigarette smoke condensate (CSC) or four other components of tobacco smoke (2-naphthylamine, hydroquinone, acrolein, and acetaldehyde) with or without P. gingivalis supernatant. The release of matrix metalloproteinase-9 (MMP-9) was also examined. ROS production increased significantly when the neutrophils were stimulated with nicotine. P. gingivalis induced the maximum ROS production when compared to all the other components examined. The combination of nicotine and P. gingivalis did not have an additive effect on ROS production. Nicotine significantly increased the MMP-9 release from the neutrophils. On the contrary, CSC inhibited ROS production at all the concentrations examined. The combination of CSC and P. gingivalis resulted in the inhibition of ROS production. MMP-9 release was also increased from the CSC-treated neutrophils. The four other tobacco smoke components examined affected ROS production and MMP-9 release differently. These projects demonstrated that CSC inhibited the ROS production from neutrophils, which can be attributed to several components in tobacco smoke that may include acrolein and hydroquinone. More research is needed to determine the mechanisms of inhibition and if other tobacco components are involved in ROS inhibition

Swimming performance strongly influences fitness in aquatic organisms and is closely tied to external body morphology. Although this connection has been closely examined at the individual and species level, few studies have focused on this relationship as it pertains to functional group assemblages. Using functional groups based on similarities in habitat use and morphology, I tested the hypothesis that swimming performance can be reliably predicted by functional group composition. I measured swimming performance as burst speed using a simulated predator attack and as prolonged speed using a step-endurance test in a laboratory flume. I measured morphology using geometric morphometric techniques. A difference in swimming behavior in four of the seven species was observed in the step-endurance test. Benthic species exhibited bracing behavior as an alternative to body-caudal fin (BCF) propulsion in the prolonged speed trials. Swimming performance exhibited a weak relationship with functional groups based on habitat or morphology. Rather a species-based model was the best predictor of swimming performance. Although species exhibited variation in swimming performance, body size was the strongest predictor of absolute swimming performance across all models. Relative swimming performance (measured in body lengths/sec) was negatively related to body size. The results of this study suggest that functional groups are not always reliable predictors of performance and they necessitate empirical testing to validate their effectiveness. This study also provides critical swimming performance data for previously unstudied Great Basin fishes which could be valuable for predicting fish passage through culverts, weirs and fish ladders.

Innovations in optical communication are expected to transform the landscape of global communications, internet and datacenter networks. This dissertation investigates several important issues in optical communication such as fairness, throughput, blocking probability and differentiated quality of service (QoS). Novel algorithms and new approaches have been presented to improve the performance of optical circuit switching (OCS) and optical burst switching (OBS) for long haul, and datacenter networks. Extensive simulations tests have been conducted to evaluate the effectiveness of the proposed algorithms. These simulation tests were performed over a number of network topologies such as ring, mesh and U.S. Long-Haul, some high processing computing (HPC) topologies such as 2D and 6D mesh torus topologies and modern datacenter topologies such as FatTree and BCube. Two new schemes are proposed for long haul networks to improve throughput and hop count fairness in OBS networks. The idea is motivated by the observation that providing a slightly more priority to longer bursts over short bursts can significantly improve the throughput of the OBS networks without adversely affecting hop-count fairness. The results of extensive performance tests have shown that proposed schemes improve the throughput of optical OBS networks and enhance the hop-count fairness. Another contribution of this dissertation is the research work on developing routing and wavelength assignment schemes in multimode fiber networks. Two additional schemes for long haul networks are presented and evaluated over multimode fiber networks. First for alleviating the fairness problem in OBS networks using wavelength-division multiplexing as well as mode-division multiplexing while the second scheme for achieving higher throughput without sacrificing hop count fairness. We have also shown the significant benefits of using both mode division multiplexing and wavelength division multiplexing in real-life short-distance optical networks such as the optical circuit switching networks used in the hybrid electronic-optical switching architectures for datacenters. We evaluated four mode and wavelength assignment heuristics and compared their throughput performance. We also included preliminary results of impact of the cascaded mode conversion constraint on network throughput. Datacenter and high performance computing networks share a number of common performance goals. Another highly efficient adaptive mode wavelength- routing algorithm is presented over OBS networks to improve throughput of these networks. The effectiveness of the proposed model has been validated by extensive simulation results. In order to optimize bandwidth and maximize throughput of datacenters, an extension of TCP called multipath-TCP (MPTCP) has been evaluated over an OBS network using dense interconnect datacenter topologies. We have proposed a service differentiation scheme using MPTCP over OBS for datacenter traffic. The scheme is evaluated over mixed workload traffic model of datacenters and is shown to provide tangible service differentiation between flows of different priority levels. An adaptive QoS differentiation architecture is proposed for software defined optical datacenter networks using MPTCP over OBS. This scheme prioritizes flows based on current network state.

[ES] La distribución de contenidos multimedia, tanto a través de Internet como de redes gestionadas dedicadas exclusivamente a este fin, está teniendo una mayor cuota del tráfico total y crece cada día, especialmente el servicio de vídeo streaming. Por ello, ha surgido el interés de investigar para mantener y mejorar la Calidad de Servicio (QoS) en redes de acceso y backbone, y ofrecer Calidad de Experiencia (QoE) a los usuarios finales; mediante el ajuste de parámetros, configuraciones, protocolos y algoritmos a los requerimientos de la red en cuanto a la alta demanda de tráfico de vídeo. Lo anterior aprovechando al máximo los recursos de la red, tanto en el núcleo y las redes de acceso, como el ancho de banda y la capacidad total instalada, y al mismo tiempo, proponiendo alternativas de transporte eficientes para el tráfico de vídeo agregado sin incrementar los costos de los servicios; y alternativas para reducir los costos operativos y gastos de mantenimiento de manera eficiente, como por ejemplo la reducción del consumo de energía. En primer lugar, esta tesis aborda el estudio de propuestas de conmutación óptica en redes troncales y la distribución de contenidos multimedia, particularmente la propuesta de Red de Conmutación Óptica de Ráfagas (OBS) desde el punto de vista de la Ingeniería Telemática. Por lo tanto, se estudia la arquitectura, protocolos y algoritmos de estas redes para hacer aportes a la evaluación del tráfico de vídeo y alternativas para ofrecer QoS y QoE para el tráfico multimedia. En este sentido, este trabajo contribuye con propuestas para la evaluación de la calidad mediante simulación utilizando trazas de transmisiones de vídeo reales, y propuestas de algoritmos de scheduling y ensamblaje de ráfagas ópticas para ofrecer QoS y QoE al tráfico de vídeo en las redes OBS. En segundo lugar, se estudian los patrones de tráfico de vídeo a nivel de aplicación y de red para caracterizarlos y realizar estudios de eficiencia energética y ahorro de consumo energético en las tarjetas de red utilizadas por servidores de vídeo en streaming y clientes que implementan el estándar IEEE 802.3az Energy Efficient Ethernet (EEE), tanto en redes gestionadas, como IPTV que implementa transmisión de vídeo RTP, como en redes no gestionadas, como Internet, que implementa el estándar Dynamic Adaptive Streaming over HTTP (DASH) para servicios Over The Top (OTT). Para alcanzar este objetivo, se implementan testbeds y se llevan a cabo experimentos para concluir cómo el tráfico de vídeo podría beneficiarse de la eficiencia energética y lograr una reducción a gran escala en el consumo de energía si el estándar EEE se implementa a nivel mundial en las tarjetas Ethernet de los dispositivos involucrados en la distribución de vídeo. El tema integrador que se aborda en esta tesis es la distribución de vídeo digital. Por lo tanto, este trabajo también presenta un estudio sobre la codificación y adaptación de transmisión de vídeo en redes IP, presentando aspectos relacionados con la evaluación de la calidad de vídeo y la evaluación de la calidad de transmisión de la red. Por ello, se proponen e implementan metodologías y bancos de pruebas que involucran la programación y ejecución de simuladores, scripts, algoritmos, software y, en general, frameworks para realizar estudios de transmisiones de vídeo en redes de telecomunicaciones, cubriendo el amplio espectro de las Comunicaciones Multimedia. Este estudio podría ser de utilidad para emprender otros estudios necesarios ante la rápida evolución tanto de la demanda de servicios multimedia como de las propuestas de nuevas técnicas de distribución de contenidos. / [CAT] La distribució de continguts multimèdia, a través d'Internet i a través de xarxes gestionades dedicades exclusivament a aquesta finalitat, està tenint una part més gran del trànsit total i augmenta cada dia, especialment el servei de streaming de vídeo. Per tant, ha sorgit l'interés de la recerca per mantenir i millorar la Qualitat de Servei (QoS) en les xarxes d'accés i troncals, i per oferir Qualitat d'Experiència (QoE) als usuaris finals, ajustant paràmetres, configuracions, protocols i algoritmes als requeriments de la xarxa pel que fa a l'alta demanda de trànsit de vídeo. Aquest últim, aprofitant al màxim els recursos de xarxa al nucli i a les xarxes d'accés, com ara l'amplada de banda i la capacitat instal·lada total, i alhora, proposant alternatives de transport eficients per al trànsit agregat de vídeo sense augmentar els costos dels serveis i alternatives per reduir les despeses operatives i de manteniment d'una manera eficient, com ara reduir el consum d'energia. En primer lloc, aquesta tesi tracta l'estudi de les propostes de commutació òptica en xarxes troncals pel que fa a la distribució de continguts multimèdia, especialment l'estudi de la proposta de xarxes de commutació de ràfegues òptiques (OBS) des del punt de vista de l'Enginyeria Telemàtica. Per tant, s'estudia l'arquitectura, els protocols i els algoritmes d'aquestes xarxes per fer contribucions a l'avaluació del trànsit de vídeo i alternatives per oferir QoS i QoE per al trànsit multimèdia. En aquest sentit, aquest treball contribueix amb propostes per a l'avaluació de la qualitat mitjançant la simulació utilitzant traces de transmissions de vídeo reals, i propostes d'algoritmes de programació i muntatge de ràfegues òptiques per oferir QoS i QoE al trànsit de vídeo a les xarxes OBS. En segon lloc, s'estudien els patrons de trànsit de vídeo a nivell d'aplicació i xarxa per caracteritzar-los i realitzar estudis sobre eficiència energètica i estalvi energètic en les targetes de xarxa utilitzades pels servidors i clients de vídeo en streaming que implementen l'IEEE 802.3az Energy Efficient Ethernet (EEE) estàndard, tant en xarxes gestionades com IPTV, que implementa la transmissió de vídeo RTP, com en xarxes no gestionades com Internet, que implementa l'estàndard Dynamic Adaptive Streaming sobre HTTP (DASH) per als serveis Over The Top (OTT). Per assolir aquest objectiu, s'implementen bancs de proves i es realitzen experiments per concloure com el trànsit de vídeo podria beneficiar l'eficiència energètica i aconseguir una reducció a gran escala del consum d'energia si s'implementa l'estàndard EEE a les targetes Ethernet dels dispositius implicats en la distribución de vídeo. El tema integrador que s'aborda en aquesta tesi és la distribució de vídeo digital, per tant, aquest treball també presenta un estudi sobre la codificació i adaptació de la transmissió de vídeo a les xarxes IP, presentant aspectes relacionats amb l'avaluació de la qualitat del vídeo i l'avaluació de la qualitat de la transmissió de la xarxa. Per això, es proposen i s'implementen metodologies i bancs de proves que impliquen la programació i execució de simuladors, scripts, algoritmes, programari i, en general, frameworks per a la realització d'estudis de transmissions de vídeo en xarxes de telecomunicacions, abastant l'ampli espectre de les Comunicacions Multimèdia. Aquest estudi podria ser útil per emprendre altres estudis necessaris en la ràpida evolució tant de la demanda de serveis multimèdia com de les noves propostes de tècniques de distribució de continguts. / [EN] Multimedia content distribution, both over the Internet and over managed networks dedicated exclusively for this purpose, is having a greater share of total traffic and it is raising every day, especially the video streaming service. Therefore, research interest has arisen to maintain and improve the Quality of Service (QoS) in access and backbone networks, and to offer Quality of Experience (QoE) to end users, by adjusting parameters, configurations, protocols and algorithms to the network requirements regarding the video traffic high demand. The latter is carried out taking full advantage of the network resources at core and access networks such as bandwidth and total installed capacity, and at the same time, proposing efficient transportation alternatives for the aggregated video traffic without increasing services costs and alternatives to reduce operational and maintenance expenditures in an efficient manner, such as reducing the energy consumption. In the first place, this thesis deals with the study of optical switching proposals in backbone networks regarding multimedia content distribution, particularly studying the Optical Bursts Switching (OBS) Networks proposal from the Telematics Engineering point of view. Therefore, architecture, protocols and algorithms of these networks are studied to make contributions to the video traffic evaluation and alternatives to offer QoS and QoE for multimedia traffic. In this sense, this work contributes with proposals for the evaluation of quality through simulation using traces of real video transmissions, and optical burst assembly and scheduling algorithms proposals to offer QoS and QoE to video traffic in the OBS networks. Secondly, video traffic patterns at the application and network level are studied to characterize them and carry out studies on energy efficiency and energy consumption savings in the network cards used by streaming video servers and clients that implement the IEEE 802.3az Energy Efficient Ethernet (EEE) standard, both in managed networks, such as IPTV that implements RTP video streaming, and unmanaged networks, such as Internet which implements Dynamic Adaptive Streaming over HTTP (DASH) standard for Over The Top (OTT) services. To reach this goal, testbeds are implemented, and experiments are carried out to conclude how video traffic could benefit from the energy efficiency and achieve a large-scale reduction in energy consumption if the EEE standard is implemented word wide in the Ethernet cards of devices involved in video distribution. The integrating subject that is addressed in this thesis is the distribution of digital video. Therefore, this work also presents a study on the IP networks video transmission encoding and adaptation, presenting aspects regard the video quality evaluation and the network transmission quality assessment. Therefore, methodologies and testbeds are proposed and implemented that involves programming and execution of simulators, scripts, algorithms, software and, in general, frameworks to carry out studies for video transmissions in telecommunications networks, covering the broad spectrum of Multimedia Communications. This study could be useful to undertake other necessary studies in the rapid evolution of both, multimedia services demand and new content distribution techniques proposals. / Vargas Hernández, TR. (2023). A Contribution to Video Streaming Quality and Energy Efficiency in Optical and Ethernet Networks [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/192760

Les réseaux optiques à commutation de rafales (OBS) sont des candidats pour jouer un rôle important dans le cadre des réseaux optiques de nouvelle génération. Dans cette thèse, nous nous intéressons au routage adaptatif et au provisionnement de la qualité de service dans ce type de réseaux. Dans une première partie de la thèse, nous nous intéressons à la capacité du routage multi-chemins et du routage alternatif (par déflection) à améliorer les performances des réseaux OBS, pro-activement pour le premier et ré-activement pour le second. Dans ce contexte, nous proposons une approche basée sur l’apprentissage par renforcement où des agents placés dans tous les nœuds du réseau coopèrent pour apprendre, continuellement, les chemins du routage et les chemins alternatifs optimaux selon l’état actuel du réseau. Les résultats numériques montrent que cette approche améliore les performances des réseaux OBS comparativement aux solutions proposées dans la littérature. Dans la deuxième partie de cette thèse, nous nous intéressons au provisionnement absolu de la qualité de service où les performances pire-cas des classes de trafic de priorité élevée sont garanties quantitativement. Plus spécifiquement, notre objectif est de garantir la transmission sans pertes des rafales de priorité élevée à l’intérieur du réseau OBS tout en préservant le multiplexage statistique et l’utilisation efficace des ressources qui caractérisent les réseaux OBS. Aussi, nous considérons l’amélioration des performances du trafic best effort. Ainsi, nous proposons deux approches : une approche basée sur les nœuds et une approche basée sur les chemins. Dans l’approche basée sur les nœuds, un ensemble de longueurs d’onde est assigné à chaque nœud du bord du réseau OBS pour qu’il puisse envoyer son trafic garanti. Cette assignation prend en considération les distances physiques entre les nœuds du bord. En outre, nous proposons un algorithme de sélection des longueurs d’onde pour améliorer les performances des rafales best effort. Dans l’approche basée sur les chemins, le provisionnement absolu de la qualité de service est fourni au niveau des chemins entre les nœuds du bord du réseau OBS. À cette fin, nous proposons une approche de routage et d’assignation des longueurs d’onde qui a pour but la réduction du nombre requis de longueurs d’onde pour établir des chemins sans contentions. Néanmoins, si cet objectif ne peut pas être atteint à cause du nombre limité de longueurs d’onde, nous proposons de synchroniser les chemins en conflit sans le besoin pour des équipements additionnels. Là aussi, nous proposons un algorithme de sélection des longueurs d’onde pour les rafales best effort. Les résultats numériques montrent que l’approche basée sur les nœuds et l’approche basée sur les chemins fournissent le provisionnement absolu de la qualité de service pour le trafic garanti et améliorent les performances du trafic best effort. En outre, quand le nombre de longueurs d’ondes est suffisant, l’approche basée sur les chemins peut accommoder plus de trafic garanti et améliorer les performances du trafic best effort par rapport à l’approche basée sur les nœuds. / Optical Burst Switching (OBS) networks are candidates to play an important role in the context of next generation optical networks. In this thesis, we are interested in adaptive routing and quality of service provisioning for these networks. In the first part of the thesis, we study the capability of multi-path routing and alternative routing (deflection routing) to improve the performance of the OBS network proactively for the former and reactively for the latter. In this context, we propose a reinforcement learning-based approach where learning agents, placed in each OBS node, cooperate to learn, continuously, optimal routing paths and alternative paths according to the current state of the network. Numerical results show that the proposed approach improves the performance of the OBS network compared to existing solutions in the literature. In the second part of the thesis, we consider the problem of absolute quality of service provisioning for OBS networks where worst-case performance of high priority traffic is guaranteed quantitatively. Particularly, we are interested in the loss-free transmission, inside the OBS network, of high priority bursts, while preserving statistical multiplexing gain and high resources utilization of the OBS network. Also, we aim to improve the performance of best effort traffic. Hence, we propose two approaches: (a) the node-based approach; and (b) the path-based approach. In the node-based approach, we propose to assign a set of wavelengths to each OBS edge node that it can use to send its guaranteed traffic. This assignment takes into consideration physical distances between edge nodes. Furthermore, we propose a wavelength selection algorithm to improve the performance of best effort bursts. In the path-based approach, absolute quality of service provisioning is offered at end-to-end path level. To do this, we propose a routing and wavelength assignment approach which aims to reduce the number of wavelengths required to establish contention free paths. Nevertheless, if this objective cannot be reached because of the limited number of wavelengths in each fiber link, we propose an approach to synchronize overlapping paths without the need for additional equipments for synchronization. Here again, we propose a wavelength selection algorithm for best effort bursts. Numerical results show that both the node-based and the path-based approaches successfully provide absolute quality of service provisioning for guaranteed traffic and improve the performance of best effort traffic. Also, path-based approach could accommodate more guaranteed traffic and improve the performance of best effort traffic compared to node-based approach when the number of wavelengths is sufficient.

The theoretical part of the thesis deals with the explanation of the actions that occur during the stimulation of tissues with the electric current. A significant analogy with electrical circuits is used to describe the phenomena at the molecular and cellular level. The models of membrane and cell are necessary for understanding the behaviour of more complex structures, such as tissues and organs. A considerable attention is paid to the conditions of electrical stimulation which bring about response in the stimulated area. Next, the cumulative effect of the subthreshold stimulation is analysed. The mechanisms of common treatment effects of the electrotherapeutic methods are outlined. The research results in the practical part of the thesis – the design for a microstimulator. Properties of the microstimulator and compliance with standard requirements are verified by testing the electromagnetic compatibility and electrical safety, conducted by the Institute for testing and certification, JSC. The effects of microstimulation on living organisms are experimentally investigated on horses, in collaboration with the Veterinary and Pharmaceutical University. For the first time, thermodynamic sensors are used for the objective assessment of the microstimulation therapeutic effect. These miniature sensors are placed on the horse´s front legs and monitor the changes in thermal activity while only one limb is really stimulated and the other is just considered as a reference. Comparison and statistical evaluation of the measured signals could provide a more detailed view of the thermal changes within the stimulated area, which is significantly related to blood circulation in limbs, and with the support of the reduction of edema. The course of the experiment which deals with the effect of microstimulation on edema of the horse´s legs caused by minor injuries (tendinitis, sprains, etc.), is documented in photographs or videos that are significant for possible evaluation of the effectiveness of the stimulation in this application.

A acidez do solo é um dos principais fatores limitantes à produção vegetal. Apesar da toxicidade por alumínio ter sido extensamente investigada, pouca atenção tem sido dada ao estresse causado pelo baixo pH em si. Existem diferenças marcantes entre células quanto à sensibilidade ao pH baixo que dependem do seu estado de crescimento e desenvolvimento celular e que devem ser exploradas para se entender o que determina a sensibilidade e tolerância a pH baixo. Em alguns casos, a suscetibilidade a pH baixo está relacionada a desarranjos na parede de células em crescimento, chegando a causar o rompimento da célula, como já foi demonstrado em pêlos radiculares em expansão. Por outro lado, o metabolismo oxidativo e a geração de espécies reativas de oxigênio (ROS) na parede podem influenciar neste processo por romper ou criar ligações dentro ou entre cadeias de polissacarídeos, modulando assim a extensibilidade da parede celular. Em células de tabaco (Nicotiana tabacum) cv. BY-2, há um aumento acentuado na sensibilidade ao pH baixo no final da fase lag da cultura, que ocorrre entre 12 e 24 h de cultivo. Os objetivos deste trabalho foram: a) Investigar se a mudança na sensibilidade pH baixo ocorre durante a retomada do ciclo celular e determinar, com o uso de inibidores do ciclo celular, o período do ciclo em que isto ocorre; b) verificar se o aumento da sensibilidade a pH baixo está relacionado com a expansão celular ou com alterações no potencial osmótico da célula; c) examinar o efeito da aplicação de H2O2 ou ascorbato sobre a resposta de células sensíveis a pH baixo; d) testar a hipótese de que a sensibilidade a pH baixo pode ser revertida por meio de um choque hipo-osmótico prévio; e) avaliar o possível papel da modulação oxidativa da parede celular na reversão de sensibilidade das células a pH baixo expostas ao choque hipo-osmótico. A retomada do ciclo celular é necessária para que ocorra a alteração de sensibilidade a pH baixo, pois a remoção de auxina (2,4-D) ou a adição de bloqueadores de canais de K+ impediu ou atrasou, respectivamente, a alteração na sensibilidade a pH baixo. O uso de inibidores do ciclo celular demonstrou que as células de BY-2 se tornam mais sensíveis a pH baixo durante o final da fase G1 mas antes do ponto de checagem da transição G1/S do ciclo celular. A aplicação de H2O2, diminuiu a suscetibilidade das células a pH baixo, ao contrário da aplicação de ascorbato. Foi demonstrado que a aplicação prévia de tratamento hipo-osmótico por 60 min reverteu a sensibilidade de células a pH baixo. A aplicação de inibidores de NAPDH oxidase da membrana plasmática e de peroxidases revelou a participação destas enzimas na reversão de sensibilidade das células a pH baixo, indicando a possibilidade de geração de ROS e de modulação oxidativa da parede. Embora já tenha sido descrito que ocorre uma explosão oxidativa com choque hipo-osmótico, ainda não havia sido demonstrado a conseqüência disto. Este trabalho fornece indícios de que uma explosão oxidativa poderia modificar a parede tornando-a mais resistente e a célula menos suscetível a pH baixo / Soil acidity is a major factor limiting plant growth worldwide. Although aluminum toxicity, which occurs only at low pH, has been extensively studied, little attention has been given to stress caused by low pH. There are marked differences in the sensitivity of cells to low pH which are contingent on the growth and developmental stage of the cells. These differences should be explored to further the understanding of the factors governing sensitivity and tolerance to low pH. In at least some cases, the susceptibility of cells to low pH is related to derangements in the wall of growing cells, which can cause ruptures or bursting of the cells, as has been clearly demonstrated in expanding root hairs. On the other hand, the oxidative metabolism and generation of reactive oxygen species (ROS) can modulate cell wall extensibility by breaking or making bonds within and between cell wall polymers. In tobacco (Nicotiana tabacum) cv. BY-2 cells, there is a sharp increase in sensitivity to low pH at the end of the lag phase of the cell culture, which occurs between 12 and 24 h of subculture. The objectives of this study were: a) determine if the changes in sensitivity to low pH occurred during the restart of the cell cycle and, by employing cell cycle inhibitors, at which points of the cycle does this occur; b) examine if the changes in sensitivity to low pH are related to cell expansion or changes in osmotic potential of the cell; c) examine how the application of H2O2 or ascorbate affects the response of cells to low pH; d) test the hypothesis that sensitivity of cells to low pH can be reverted by the previous application of a hypo-osmotic shock; e) evaluate the possible role of oxidative modulation of the cell wall in hypo-osmotic-induced reversal of the sensitivity of cells to low pH. The restart of the cell cycle was shown to be necessary for the change in sensitivity to low pH occur, since the absence of auxin (2,4-D) or the addition of K+ channel blockers prevented or delayed this change, respectively. The use of cell cycle inhibitors demonstrated that BY-2 cells become sensitive to low pH at the end of G1 but before the G1/S transition restriction point of the cell cycle. Exogenous H2O2, but not ascorbate, reduced the effect of low pH on sensitive cells. Sensitive cells submitted to 60 min hypo-osmotic treatment became insensitive to low pH. This reversal of sensitivity depended on the activity of plasma membrane NADPH oxidase and peroxidase, as evidenced by the use of DPI and SHAM, inhibitors of these enzymes, respectively. This suggests that ROS is generated and that oxidative modifications of the cell wall occur. Although hypo-osmotic treatments have been shown to generate an oxidative burst, its purpose or implication has not yet been shown. This study provides evidence that an oxidative burst might modify and strengthen the cell wall, making cells less susceptible to low pH