A pecuária brasileira é baseada no uso de pastagens. Entretanto, o padrão estacional do clima determina a maior concentração da produção de forragem nos períodos de primavera e verão. O conhecimento das respostas produtivas e morfofisiológicas das principais espécies e cultivares forrageiros à temperatura e ao fotoperíodo, na ausência de déficit hídrico, permitem o desenvolvimento de modelos preditores de produção para o planejamento da atividade pecuária ao longo do ano. O presente estudo foi conduzido no Departamento de Zootecnia da ESALQ/USP em Piracicaba, SP, de outubro de 2005 a outubro de 2006, com o objetivo de gerar um banco de dados sobre as características produtivas e morfofisiológicas de capins do gênero Brachiaria e desenvolver modelos que descrevessem o acúmulo de forragem e as características do dossel em função de uma variável que combina os efeitos de temperatura e fotoperíodo (a Unidade Fototérmica – UF). Foram utilizados cinco cultivares de Brachiaria spp. (Basilisk, Marandu, Xaraés, Arapoty e Capiporã) cortados a cada 28 dias durante o verão e 42 dias durante o inverno, deixando-se um resíduo de 15 cm para todos os capins. As unidades experimentais (parcelas de 9 x 4 m) eram irrigadas para garantir ausência de déficit hídrico e adubadas com o equivalente a 120 kg ha-1 ano-1 de N e K2O . O delineamento experimental foi de blocos completos casualizados com quatro repetições. Em cada corte foi medido o acúmulo de forragem e, a partir daí, foram calculadas a produção anual de forragem, a produção de verão e a produção de inverno. Acompanharam-se semanalmente uma rebrotação de verão e a cada 10 dias uma rebrotação de inverno, quando foram avaliados a massa do resíduo, o acúmulo de forragem, a altura, o índice de área foliar (IAF), os ângulos foliares e a interceptação luminosa (IL) do dossel e as taxas de fotossíntese foliar dos capins, que foram utilizadas para a simulação da taxa de fotossíntese do dossel. O capim Capiporã produziu 18 Mg MS ha-1 ano-1, seguido dos capins Xaraés e Basilisk que acumularam 14,8 Mg MS ha-1 ano-1 e Arapoty e Marandu com 13,1 Mg MS ha-1 ano-1. Durante o verão o capim Capiporã foi o mais produtivo com acúmulo de 15,4 Mg MS ha-1. Durante o inverno o Arapoty foi o mais produtivo e acumulou 3,4 Mg MS ha-1, o que resultou na menor estacionalidade de produção do Arapoty, com 74% da produção concentrada no verão. Apesar dos capins terem produções diferentes, suas taxas de fotossíntese de dossel foram iguais (34,41 µmol CO2 m-2 s-1). O IAF médio dos cultivares foi de 2,53. A massa residual média passou de 4,48 Mg MS ha-1 no verão, para 5,71 Mg MS ha-1no inverno. Os modelos de acúmulo de forragem, acúmulo de folhas, IAF, IL e altura do dossel foram sempre diferentes entre cultivares e estações. Os modelos para acúmulo de forragem forneceram produções previstas próximas àquelas observadas, embora durante o inverno e início do verão tenham havido sub e super-estimativas, para todos os capins. O uso da UF mostrou-se efetivo na predição de características produtivas e no estudo de alguns atributos morfofisiológicos de gramíneas do gênero Brachiaria. Para que os modelos possam se tornar ferramentas práticas de planejamento de sistemas de produção, há, no entanto, a necessidade de que estudos complementares sejam realizados, aumentado o banco de dados disponível e a amplitude de condições de ambiente e manejo contempladas. / The brazilian livestock industry is based on the use of pastures. However, the seasonal pattern of climatic variations, concentrates most of the annual forage accumulation during the spring and summer. Thus, information on the yield potential, as well as on morphological and physiological responses of forages to temperature and daylength, in the absence of moisture constraints, may allow for the development of prediction models, which can be useful in planning forage-livestock systems. This study was carried out at the Departamento de Zootecnia of ESALQ-USP in Piracicaba, SP, from October 2005 through October 2006, with the purpose of generating a comprehensive dataset on the productive, morphological and physiological characteristics of Brachiaria spp. grasses, as well as to develop and evaluate models that describe and predict forage accumulation and sward characteristics as related to photothermal units (a climatic variable which integrates the effects of temperature and daylength). Five cultivars of Brachiaria spp. were used in the study: (Basilisk, Marandu, Xaraés Arapoty and Capiporã), harvested either every 28 days during the ‘summer’ and every 42 days during the ‘winter’, leaving a stubble of 15 cm. Plots (9 x 4 m) were irrigated to avoid water deficit and fertilized with 120 kg N and K2O ha-1 year-1. The experimental design was a randomized complete block with four replications. On each harvest, forage accumulation was measured in every plot and, from these data, annual and seasonal production for ‘summer’ and ‘winter’ were calculated. Additionally, one ‘summer’ regrowth and one ‘winter’ regrowth, were characterized weekly and every 10 days, respectively, where measurements were taken on stubble mass, forage accumulation, sward height, leaf area index (LAI), mean foliage angle, light interception (LI) and leaf photosynthesis, from which canopy photosynthesis was calculated. Capiporã was the most productive grass, with 18 Mg DM ha-1, followed by Xaraés and Basilisk, which yielded 14,8 Mg DM ha-1 year-1 on average, and Arapoty and Marandu with annual yield of 13,1 Mg DM ha-1. During the ‘summer’ Capiporã yielded 15,4 Mg DM ha-1. During the ‘winter’ Arapoty was the highest yielding (3,5 Mg DM ha-1), resulting in a less pronounced seasonal growth pattern (74% of total annual yield) than the other genotypes. Despite the contrasting yield potential, canopy photosynthesis did not differ (34,4 µmol CO2 m-2 s-1) across genotypes. The mean LAI of all swards was 2,53. Mean stubble mass increased from 4,48 Mg MS ha-1 in the ‘summer’ to 5,71 Mg MS ha-1 in the ‘winter’. Models of the evolution of forage mass and leaf mass above the stubble, as well as LAI, LI and sward height differed among genotypes and between seasons. The forage accumulation models gave yield predictions that were consistent with observed values, although under- and overestimations occurred for the all grasses, especially during the ‘winter’ and early ‘summer’. The use of models based on the photothermal unit proved effective for the prediction of yield and yield-related responses of Brachiaria grasses. In order for these models to become useful tools for systems planning and management, further study is needed in addition to richer, more comprehensive data libraries and a wider range of environments and management practices.

abstract: This thesis focuses on serial crystallography studies with X-ray free electron lasers (XFEL) with a special emphasis on data analysis to investigate important processes in bioenergy conversion and medicinal applications. First, the work on photosynthesis focuses on time-resolved femtosecond crystallography studies of Photosystem II (PSII). The structural-dynamic studies of the water splitting reaction centering on PSII is a current hot topic of interest in the field, the goal of which is to capture snapshots of the structural changes during the Kok cycle. This thesis presents results from time-resolved serial femtosecond (fs) crystallography experiments (TR-SFX) where data sets are collected at room temperature from a stream of crystals that intersect with the ultrashort femtosecond X-ray pulses at an XFEL with the goal to obtain structural information from the transient state (S4) state of the cycle where the O=O bond is formed, and oxygen is released. The most current techniques available in SFX/TR-SFX to handle hundreds of millions of raw diffraction patterns are discussed, including selection of the best diffraction patterns, allowing for their indexing and further data processing. The results include two 4.0 Å resolution structures of the ground S1 state and triple excited S4 transient state. Second, this thesis reports on the first international XFEL user experiments in South Korea at the Pohang Accelerator Laboratory (PAL-XFEL). The usability of this new XFEL in a proof-of-principle experiment for the study of microcrystals of human taspase1 (an important cancer target) by SFX has been tested. The descriptions of experiments and discussions of specific data evaluation challenges of this project in light of the taspase1 crystals’ high anisotropy, which limited the resolution to 4.5 Å, are included in this report In summary, this thesis examines current techniques that are available in the SFX/TR-SFX domain to study crystal structures from microcrystals damage-free, with the future potential of making movies of biological processes. / Dissertation/Thesis / Masters Thesis Chemistry 2020

Alors que les besoins en énergie de nos sociétés modernes ne cessent de croître, et que la prise en compte des enjeux environnementaux occupe une place de plus en plus importante dans le développement de nouvelles technologies, la mise au point de procédés de production d’hydrogène utilisant des ressources renouvelables, telles que le rayonnement solaire comme source d'énergie ou l’eau comme donneur d’électrons et de protons, est un enjeu de toute première nécessité. Ce travail de thèse s’inscrit dans la continuité des études menées au sein de l’équipe « SolHyCat » du Laboratoire de Chimie et Biologie des Métaux sur le développement de systèmes électro- et photocatalytiques pour la réduction des protons en H2. Dans cette perspective, nous avons ainsi contribué à l’élaboration d’une photocathode moléculaire de production de H2 à travers la compréhension de différents paramètres moléculaires relatifs à sa construction. Dans un premier temps, nous nous sommes intéressés à la construction d’un lien covalent entre un motif photosensibilisateur et un centre catalytique de réduction des protons via la synthèse de dyades modèles. En parallèle, nous avons développé une méthodologie de synthèse permettant l’introduction de groupements d’ancrage robustes dans la sphère de coordination de photosensibilisateurs métallo-organiques. Enfin, l'étude des propriétés électrochimiques d'une nouvelle série de catalyseurs de réduction des protons à sphère de coordination polypyridinique a été étudiée. / There is an urgent need to provide solutions for the energetic challenge our planet has to face. The production of “environmentally friendly” fuels such as dihydrogen H2 through sunlight-driven water splitting holds great promise. Hydrogen is indeed a carbon-free energy carrier that can be stored and used on request to produce electricity thanks to the mature fuel cell technology. Moreover, water and solar energy form the ideal couple for H2 production because they are both readily available and their use is considered to be safe for the environment. The design and study of molecular photocatalytic systems for H2 evolution from water has therefore been the subject of intensive research interest in the last decade and their implementation into functional dye-sensitized photoelectrocatalytic cells recently appeared in the literature. It is nevertheless necessary to optimize the efficiency of these molecular systems in order to reach the targeted solar-to-hydrogen conversion yield. In that context, this PhD thesis aimed at getting a better understanding of parameters relevant for the optimization of molecular H2-evolving photocathodes: first, different coupling strategies have been studied to covalently assemble a light-harvesting unit with a redox-active moiety; second, a synthetic methodology allowing introduction of robust anchoring groups in the coordination sphere of ruthenium photosensitizers has been widely studied and the first photoelectrodes characterized; finally, a new series of proton reduction catalysts featuring a polypyridinic ligand has been investigated, allowing important kinetic and mechanistic insights to be obtained.

We will combine hands-on science investigations with supporting literacy activities to help students build conceptual models of photosynthesis.

abstract: Chemical modification of (semi)conducting surfaces with soft-material coatings containing electrocatalysts provides a strategy for developing integrated constructs that capture, convert, and store solar energy as fuels. However, a lack of effective strategies for interfacing electrocatalysts with solid-state materials, and an incomplete understanding of performance limiting factors, inhibit further development. In this work, chemical modification of a nanostructured transparent conductive oxide, and the III-V semiconductor, gallium phosphide, is achieved by applying a thin-film polymer coating containing appropriate functional groups to direct, template, and assemble molecular cobalt catalysts for activating fuel-forming reactions. The heterogeneous-homogeneous conducting assemblies enable comparisons of the structural and electrochemical properties of these materials with their homogeneous electrocatalytic counterparts. For these hybrid constructs, rational design of the local soft-material environment yields a nearly one-volt span in the redox chemistry of the cobalt metal centers. Further, assessment of the interplay between light absorption, charge transfer, and catalytic activity in studies involving molecular-catalyst-modified semiconductors affords models to describe the rates of photoelectrosynthetic fuel production as a function of the steady-state concentration of catalysts present in their activated form. These models provide a conceptual framework for extracting kinetic and thermodynamic benchmarking parameters. Finally, investigation of molecular ‘proton wires’ inspired by the Tyrosine Z-Histidine 190 redox pair in Photosystem II, provides insight into fundamental principles governing proton-coupled electron transfer, a process essential to all fuel-forming reactions relevant to solar fuel generation. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2020

京都大学 / 0048 / 新制・課程博士 / 博士(農学) / 甲第21805号 / 農博第2318号 / 新制||農||1065(附属図書館) / 学位論文||H31||N5177(農学部図書室) / 京都大学大学院農学研究科農学専攻 / (主査)教授 白岩 立彦, 教授 奥本 裕, 教授 稲村 達也 / 学位規則第4条第1項該当 / Doctor of Agricultural Science / Kyoto University / DFAM

Philosophiae Doctor - PhD / Growth of symbiotic legume hosts is P limited, because of the high energetic requirements associated with N2 fixation. Attempts to overcome P deficiency in soils where legumes are grown involve addition of P-based fertilisers. However, these are produced from fmite, non-renewable resources that could be exhausted in the next 50-80 years. For this and other prudent reasons, viable alternatives are sought that include producing genetically enhanced plants with better P use efficiency (PUE). There exist some inter- and intraspecific genetic variation for associated traits of PUE in various legumes and these will have to be exploited to realize the development of P efficient cultivars. With the advent of sophisticated molecular tools, good progress has been made to understand the molecular response of some common physiological and morphological functions observed under LP. The research aims here were to investigate the energy costs and the alternative metabolic routes associated with C and N metabolism under LP in legumes, which is very scant in literature. We also investigated the recovery responses of nodulated roots upon P alleviation. Consequently, improvement strategies to produce legume varieties for better adaptation in poor P soils are envisaged. We have demonstrated varying degrees of sensitivity between the amide and ureide legume systems being investigated under short-term LP. The species-specific responses were ascribed to differences related to the agro-climatic origins, nodule morphologies and the type of N containing export product of the different legume types. These different responses also underscore possible different regulatory mechanisms under LP. Lupins were probed further, because of its apparent tolerance to P deficiency. Lupin nodules had between 3 to 5-fold higher Pj concentrations compared with soybeans under LP and HP, respectively. The maintenance of Pj levels, as oppose to a decline in the total P pool, is discussed in relation to its role in maintaining N2 fixation in lupins. Under LP, an effective Pj recycling mechanism in nodules is proposed to occur via the induction of the PEPc- MDH-ME route. This route also enhanced the capacity of root nodules to procure high malate concentrations that are used to fuel bacteroid respiration and N2 fixation. Two distinctly different cMDH proteins, one corresponding to HP and another corresponding to LP, were identified. The high malate concentrations reported here are speculated to have arisen through LP-induced cMDH. Metabolically available Pj decline developed gradually as P deficiency progressed. This coincided with a 15% decline in the %Ndfa. Moreover, under prolonged P deficiency the disproportionate synthesis of organic acids, most notably malate, that occurred at the expense of amino acids was proposed to account for this decline. The recovery in response to alleviation from LP involved alterations in the allocation of respiratory costs to growth and nutrient acquisition. Under LP, smaller nodules were formed and nodule metabolism revolved around accentuating PUE. Thus, there is considerable potential for improvement of P efficiency in legumes through manipulation of root: shoot partitioning.

Throughout time nature has been a prescribed healer of stress on the human condition. Its vital integration into our daily lives has been proven by scientific evidence. The majority of Americans spend approximately 1/3 of their life working, whatever that job may entail. Therefore, it makes sense that the environments that we spend so much of our life in for work at extremely important to our physical and mental health, however, current workplace models are not acknowledging that. Redefining the workplace to integrate nature would start to change work life in this country and how work itself is viewed. This thesis focuses on creating healthy and holistic production spaces for workers in areas like Western Massachusetts. Because this area shares urban and rural qualities while being primarily suburban of major Northeastern cities such as Boston and New York, it provides unique ground for a wide array of work occupations and work types. On a chosen site of Northampton, MA, a new work model will be developed around the exposure to nature integral to a collaborative co-op/ incubator space for small businesses in the area who need office and production spaces in a community setting. The progression of this research will lead to a developed incubation space with a positive environmental impact. Precedent research of archetypes such as Michael Singer’s Alterra Atria, and Miller Hull’s Bullitt Center exemplify sustainable criteria in relation to office architecture. By using concepts underlying an even further integration with the community ideas from John Dinkeloo & Kevin Roche’s Ford Foundation Headquarters, Candilis, Josic and Woods’ Freie University Berlin and BIG & Heatherwick’s New Google Headquarters. By creating a building that addresses the diverse makeup of the larger community of Northampton, MA, this co-op space would become an example of how a workplace can work with nature towards a greater impact on the environment post-occupancy.

Le but du projet était d'acquérir des connaissances plus approfondies sur les mécanismes impliqués dans l'absorption et l'accumulation des éléments traces métalliques (ETM) par les plantes et dans la résistance à leurs effets toxiques. Arabidopsis halleri, peut vivre sur des sols fortement pollués par des ETM et a la capacité d'accumuler des concentrations extraordinairement élevées de ces métaux dans les tissus aériens. A. halleri est utilisée comme espèce modèle pour étudier les mécanismes biologiques et moléculaires impliqués dans la tolérance et l'accumulation de fortes concentrations de métaux dans le sol. Cette espèce est caractérisée par une tolérance élevée à de fortes concentrations de Cd et Zn. Elle est également capable d'accumuler de très grandes quantités de Zn dans les pousses, appelées hyperaccumulateurs de Zn, et également de Cd dans certaines populations, appelées hyperaccumulateurs de Cd. Arabidopsis arenosa, une espèce étroitement liée à A. halleri, montre également une tolérance élevée aux deux métaux mais est signalée comme non-hyperaccumulatrice. Dans le sud de la Pologne, les deux espèces peuvent être observées sur les mêmes sites pollués. Les expériences ont été conçues pour étudier la variabilité inter et intra spécifique entre les populations métallisées (M) et non métallisées (NM) d'A. halleri et A. arenosa afin d'identifier les paramètres physiologiques et les gènes responsables du comportement contrasté de leur adaptation à la contamination métallique. L'accumulation des ETM, l'activité de l'appareil photosynthétique et le contenu pigmentaire ont été analysés dans les populations M et NM d'A. halleri et A. arenosa, poussant dans leur habitat naturel. Des expériences hydroponiques ont également été réalisées afin de déterminer les différences physiologiques entre les populations étudiées dans des conditions contrôlées. De plus, le niveau d'expression des gènes impliqués dans l'absorption des métaux, le transport radial, la translocation et la détoxification a également été étudié. Mes résultats montrent que la population M de A. arenosa est aussi tolérante aux métaux lourds que la population M de A. halleri. Il a été démontré que la population M de A. arenosa hyperaccumule le Cd et le Zn. De plus, les deux espèces diffèrent dans la réponse de l'appareil photosynthétique lorsqu'elles sont exposées à des concentrations élevées de Cd et de Zn, ce qui suggère des mécanismes différents impliqués dans l'homéostasie des métaux. Le niveau d'expression des gènes a montré des différences non seulement entre les espèces mais aussi entre les populations. Le type de traitement (Cd ou Zn) a également eu un effet différent sur toutes les populations en ce qui concerne certains gènes. Les résultats acquis dans ce projet comblent les lacunes dans les caractéristiques physiologiques des espèces d'A. halleri à la fois sur le terrain et dans des conditions hydroponiques contrôlées. La caractérisation physiologique approfondie de la population M de A. arenosa a identifié cette population comme un nouvel hyperaccumulateur de Cd et Zn. Ces résultats peuvent servir de base à de futures études sur le phénomène d'hyperaccumulation. De plus, l'observation de l'expression contrastée des gènes impliqués dans l'absorption, la translocation et la détoxification des métaux entre A. halleri et A. arenosa suggère que A. arenosa pourrait être un bon nouveau modèle pour étudier l'homéostasie et la tolérance aux métaux chez les plantes. / Doctorat en Sciences agronomiques et ingénierie biologique / info:eu-repo/semantics/nonPublished

L'oxydase terminale plastidiale (PTOX) est présente uniquement chez les organismesphotosynthétiques. PTOX oxyde le plastoquinol (PQH2) et réduit l'oxygène en eau.PTOX est impliquée dans la synthèse des caroténoïdes, dans le transportphotosynthétique d'électrons et dans la chlororespiration. De plus, son activité estconsidérée comme pouvant jouer un rôle en tant que soupape de sécurité, permettant de maintenir oxydé le pool de plastoquinones (PQ) et d'éviter la surréduction duchloroplaste et ainsi la photoinhibition. Chez la majorité des plantes testées, les niveaux de PTOX sont plus élevés dans des conditions de stress (une exposition à forte intensité lumineuse, par exemple). D'autre part, la surexpression de PTOX chez Arabidopsis thaliana n'a pas rendu les plantes moins sensibles à la photoinhibition. Par ailleurs, il semble que PTOX surexprimée chez Nicotiana tabacum a induit la génération des espèces réactives de l'oxygène (ERO) et une photoinhibition importante sous forte lumière.Le but de cette thèse était la caractérisation de l'activité enzymatique de PTOX enutilisant la protéine purifiée et de comprendre pourquoi PTOX protège du stressphotooxydant dans certaines conditions et pourquoi elle augmente ce stress quand elle est surexprimée in planta.L'analyse biochimique de PTOX recombinante purifiée a démontré que l'enzymeexiste principalement sous forme tétramérique. Cette forme se dissocie partiellement,principalement en dimères. Le turnover maximal de l'enzyme purifié correspond à 320électrons par seconde et par molécule de PTOX. Nous avons démontré que PTOXgénère des ERO dans une réaction secondaire dépendante de la concentration dusubstrat (PQH2) et du pH de la solution. À pH 8 (représentant le pH du stroma deschloroplastes actifs), PTOX a une activité antioxydante quand la concentration de PQH2 est basse et prooxydante quand cette concentration est élevée.En mesurant la fluorescence de la chlorophylle a, nous avons démontré quePTOX est active lorsqu'elle est ajoutée aux membranes enrichies en PSII.L'attachement aux membranes dépend du pH et de cations de la solution: lorsque le pHdiminue ou lorsque la solution est riche en cations monovalents, la quantité de PTOXattachée à la membrane diminue.L'activité de PTOX in planta et son effet sur le transport des électronsphotosynthétiques ont été analysés en utilisant Arabidopsis thaliana surexprimant laphytoène désaturase bactérienne (CRTI) et Nicotiana tabacum surexprimant PTOX1 deChlamydomonas reinhardtii. Arabidopsis thaliana surexprimant CRTI a un niveau plusimportant de PTOX et de production d'ERO et le transport cyclique des électrons estsupprimé chez les transformants. Cela implique que PTOX est en compétition avec letransfert cyclique pour les électrons du pool PQ et que PTOX joue un rôle importantdans le contrôle de l'état rédox de ce pool. En utilisant Nicotiana tabacum surexprimant PTOX1, nous avons démontré que PTOX fait concurrence au transfert linéaire d'électrons photosynthétique, mais que PTOX est inactivée quand le pH du stroma est neutre. Grâce aux résultats obtenus, nous proposons un modèle où l'association de PTOX avec la membrane est contrôlée par le pH du stroma. Quand le pH est neutre, PTOX est soluble et n'est pas active, ce qui évite l'interférence avec le transfert linéaire d'électrons. Quand le pH du stroma est alcalin et la chaîne des transporteurs photosynthétiques est surréduite (lors des conditions du stress), PTOX s'attache à la membrane, devient active et joue le rôle de soupape de sécurité. / The plastid terminal oxidase PTOX is encoded by higher plants, algae and some cyanobacteria. PTOX is a plastid-localized plastoquinol (PQH2) oxygen oxidoreductase. PTOX was shown to be implicated in plant carotenoid biosynthesis, photosynthetic electron transport and chlororespiration and may act as a safety valve protecting plants against photo-oxidative stress. PTOX protein levels increase during abiotic stress indicating a function in stress acclimation. But overexpression of PTOX in Arabidopsis did not attenuate the severity of photoinhibition or, when overexpressed in tobacco, even increased the production of reactive oxygen species (ROS) and exacerbated photoinhibition.Biochemical analysis of recombinant purified PTOX (PTOX from rice fused to the maltose-binding protein) showed that the enzyme exists mainly as a tetramer, which dissociated to a certain extent during electrophoresis, mainly into a dimeric form. The PTOX activity was 320 electrons s−1 PTOX−1. It was also shown that PTOX generates ROS in a side reaction in a substrate (decylPQH2) and pH-dependent manner when liposomes were used: at the basic stromal pH of photosynthetically active chloroplasts, PTOX was antioxidant at low decylPQH2 gaining prooxidant properties with increasing quinol concentrations. It is concluded that PTOX can act as a safety valve when the steady state [PQH2] is low while a certain amount of ROS is formed at high light intensities.It was shown by chlorophyll a fluorescence that recombinant purified PTOX is active when added to photosystem II (PSII)-enriched membrane fragments. PTOX attached tightly to the PSII-enriched membrane fragments. The amount of PTOX attaching to the membrane depended on pH and salts: an alkaline pH and monovalent compared to divalent cations increased PTOX attachment.PTOX activity in planta and its effect on photosynthetic electron transport were investigated using Arabidopsis expressing bacterial phytoene desaturase and tobacco expressing PTOX1 from Chlamydomonas. Arabidopsis expressing bacterial phytoene desaturase (CRTI lines) showed a higher PTOX content and increased PTOX related ROS generation. Furthermore, cyclic electron flow was suppressed in these lines. This implicates that PTOX competes efficiently with cyclic electron flow for PQH2 in the CRTI-expressing lines and that it plays a crucial role in the control of the reduction state of the plastoquinone pool. Using tobacco expressing PTOX1 from Chlamydomonas, it was shown that PTOX competes efficiently with photosynthetic electron flow, but gets inactive when the stromal pH is neutral. Based on the in vitro and in vivo results, a model is proposed, where the association of PTOX to the membrane is controlled by the stromal pH: When the stromal pH is neutral, PTOX exists as a soluble form and is enzymatically inactive avoiding the interference of PTOX with linear electron flow. When the stromal pH is alkaline and the photosynthetic electron chain is highly reduced under stress conditions as high light, PTOX binds to the membrane, gets enzymatically active and can serve as safety valve.