87 search hits
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Polymer Melts Investigated by Field Cycling NMR Relaxometry: From Simple Liquid to Reptation Dynamics
(2012)
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Axel Herrmann
- The focus of this thesis is the investigation of linear polymer melts by applying Field Cycling Nuclear Magnetic Resonance (FC NMR) relaxometry. The objective is to understand their microscopic dynamics and its dependence on the molecular mass (M) of the polymer chains.
With the commercial availability of FC NMR relaxometers, the method gained attraction for studying dynamics of soft condensed matter due to its ability to detect both the structural or alpha-relaxation (identified with the segmental dynamics) and slower collective dynamics. In the case of polymer melts the latter is described most often by the Rouse model for non-entangled chains and the Doi/Edwards tube-reptation model for entangled polymers. Since 2004 a commercial relaxometer by Stelar has been operated in the Rössler group. Its capability to rapidly switch between different magnetic fields allows to measure the spin-lattice relaxation time in the proton frequency range from 10 kHz to 20 MHz. In previous works by the Rössler group the pioneering works by Kimmich and co-workers have been extended in order to combine the results of a broad temperature range: Frequency-temperature superposition is applied to construct master curves in the susceptibility representation. The key benefits are: the susceptibility is scaled by time constant of segmental dynamics and an "isofrictional" representation is achieved; the accessible frequency range is significantly increased; the time constants are provided and compared with those obtained by other techniques; the regimes of glassy and polymer dynamics can be easily distinguished; finally, the dipolar correlation function is obtained directly by Fourier transform.
In this thesis by employing the above approach, the dipolar correlation function of polybutadienes (PB) melts is presented and comprises - depending on M - glassy, Rouse and entanglement dynamics. The latter two relaxation regimes can be described by different power-laws, which are compared to the predictions of the tube-reptation model. A good agreement is found for the Rouse regime (I). For the constrained Rouse regime (II) at long times, a highly protracted crossover to completely established reptation dynamics is discovered. That is, the exponent depends on M and reaches 0.32 only at M=441000, which is in accord with Double Quantum (DQ) 1H NMR results by Saalwächter and co-workers and very close to 0.25 predicted for regime II of the tube-reptation model. This is only achieved by additional relaxation experiments in cooperation with the Fujara group at TU Darmstadt, since their home-built FC NMR relaxometer is equipped with an active stray field compensation, which allows to reach extremely low frequencies down to 200 Hz. Consequently, the frequency range is extended by two decades toward lower frequencies with respect to the commercial spectrometer and the obtained correlation function stretches over 10 decades in time and 8 in amplitude for molecular masses up to 220 Me. This establishes FC 1H NMR also at long times as competitive with DQ 1H NMR.
The analyses of the dipolar correlation function appear to support the applicability of the tube-reptation model. However, intramolecular and intermolecular relaxation contributions have to be discriminated and up to now the dominance of the first has been assumed implicitly. Therefore, isotopic blends of high-M protonated and deuterated PB are investigated, which allows to decompose the 1H master curves into intramolecular and intermolecular relaxation contributions. They reflect reorientational and translational dynamics, respectively. It is demonstrated that at long times or low frequencies the intermolecular contribution dominates. Consequently, the reorientational correlation function obtained from the intramolecular part exhibits a faster decay with the long-time exponent 0.49. This is ascertained by the FC 2H NMR relaxation of completely deuterated PB, which detects reorientational dynamics only. The observed exponent is significantly larger than 0.25 of regime II of the tube reptation model. Concomitantly, the segmental mean square displacement is attained from the intermolecular part following an approach by Kimmich and Fatkullin. The predicted power-laws of the tube-reptation model for the Rouse and constrained Rouse regimes are identified for the first time by FC NMR: a transition between the power-laws t^ 0.49 and t^0.19 is revealed, respectively. Thus, NMR relaxometry is designated as a method comparable to neutron scattering to study subdiffusion in polymer melts. In conclusion, the power-law predictions of the tube-reptation model are disclosed by the segmental mean square displacement, yet not by the reorientational correlation function. Thus, the simple tube-reptation model does not completely describe the microscopic dynamics of polymer melts.
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Neue Materialien auf Basis arylsubstituierter 1,3,5-Triazine für blau phosphoreszierende organische Leuchtdioden
(2012)
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Andrea Jahreis
- Als Beleuchtungstechnologie der Zukunft stellen organische Leuchtdioden (OLEDs) eine effiziente Alternative zur Glühbirne dar und eröffnen darüber hinaus aufgrund ihrer Eigenschaft als Flächenstrahler gänzlich neue Design- und Anwendungsmöglichkeiten. Weißes Licht in OLEDs wird meist durch Kombination von Emittern der drei Primärfarben rot, grün und blau erzeugt. Für rot und grün sind bereits zahlreiche stabile Matrix-Emitter-Systeme sowie Transport- und Blockermaterialien bekannt. Stabile blaue Emitter und zu diesen kompatible Materialien sind dagegen rar und stehen deshalb im Fokus der Materialentwicklung. Diese Arbeit befasst sich mit der Synthese und Charakterisierung neuer niedermolekularer Verbindungen auf Basis arylsubstituierter 1,3,5-Triazine als Elektronenleiter, Lochblocker oder Matrixmaterial für OLEDs mit blauen Phosphoreszenzemittern. Für den Einsatz in diesen müssen die Materialien ein komplexes Anforderungsprofil erfüllen: neben der thermischen und chemischen Stabilität ist bei Verwendung von blauen Phosphoreszenzemittern vor allem ein ausreichend hohes Triplettniveau eine wichtige Voraussetzung. Die im Rahmen dieser Arbeit hergestellten Aryl-1,3,5-triazine können aufgrund der Verknüpfung von Diphenyltriazin als Grundeinheit mit elektronisch unterschiedlichen Substituenten über verschiedene Brückeneinheiten in drei Materialklassen eingeteilt werden. Die symmetrisch aufgebaute Verbindungsklasse der arylsubstituierten Bis-1,3,5-triazine wurde durch die Verknüpfung zweier Diphenyltriazineinheiten über verschiedene aromatische Brücken erzielt und basiert auf der Struktur des literaturbekannten Elektronenleiters 4,4‘-Bis-[2-(4,6-diphenyl-1,3,5-triazinyl)]-1,1‘-biphenyl, der eine Biphenylbrücke - und damit ein für blau nicht ausreichendes Triplettniveau - besitzt. Durch Einführung neuer Brückeneinheiten sollte eine Verringerung der Konjugation und somit eine Anhebung des Triplettniveaus erreicht werden, um die Bis-1,3,5-triazine auch in Kombination mit blauen Phosphoreszenzemittern verwenden zu können. Hierfür wurden drei verschiedene Konzepte verfolgt: Eine Verdrillung des Biphenyls durch Methylgruppen in 2- und 2‘-Position, eine meta-Anknüpfung über einen Phenylring sowie die Verwendung von Dibenzofuran als Heteroaromaten mit geringerer Konjugation als Biphenyl. Die Synthese der Bistriazine erfolgte in zwei Schritten. Zunächst wurde in einer Ringschlussreaktion aus den jeweiligen Disäurechloriden der Brückeneinheit und dem gewünschten Benzonitrilderivat das 3,5-Diaza-pyryliumsalz gebildet. Die anschließende Ammonolyse lieferte schließlich das entsprechende Bistriazin. Durch Alkylsubstitution der äußeren Phenylringe konnte die Löslichkeit der Materialien stark verbessert werden. Alle Verbindungen zeichnen sich durch hohe thermische Stabilität und eine für den späteren Aufdampfprozess vorteilhafte Sublimationsneigung aus. Durch die Substitution mit tert-Butylgruppen gelang es, die Kristallisationsneigung zu unterdrücken und amorphe Filme mit hohen Glasübergangstemperaturen bis zu 182 °C zu erhalten. Die größte Verringerung der Konjugation konnte für die Derivate mit meta-Anknüpfung erreicht wird. Die dafür maßgeblichen optischen Bandlücken lagen zwischen 3,9 eV und 4,1 eV. Die Konjugation bestimmt ebenso das Triplettniveau, das für einen effizienten Betrieb der OLEDs höher liegen muss als das Triplettniveau des Emitters. Die Bistriazine erreichten Triplettenergien bis zu 2,84 eV und sind damit ausreichend für die Verwendung mit blauen Phosphoreszenzemittern. Die Stabilität der Materialien unter dem Einfluss von Elektronen wurde sowohl im Cyclovoltammetrie-Experiment als auch im „Single-Carrier“-Bauteil nachgewiesen. Für die Klasse der silylsubstituierte Phenyl-1,3,5-triazine wurde zur Verbesserung der morphologischen Eigenschaften und der Löslichkeit eine der Diphenyltriazineinheiten der Bistriazine durch eine Triphenylsilylgruppe ausgetauscht. Im Vergleich zu den Bistriazinen konnte die Löslichkeit der Materialien deutlich verbessert und die Kristallisationsneigung gesenkt werden. Die Verbindungen bilden stabile amorphe Filme und weisen Glasübergangstemperaturen bis 115 °C auf. Ihre Triplettenergien von bis zu 2,91 eV liegen über denen der Bistriazine und sind daher auch in Kombination mit tiefer blauen Phosphoreszenzemittern verwendbar. Durch wiederholte Reduktionen im Cyclovoltammetrie-Experiment konnte gezeigt werden, dass es sich bei dieser Klasse ebenfalls um stabile Elektronenleiter handelt. Die donorsubstituierten Phenyl-1,3,5-triazine bilden durch Kombination von Diphenyltriazin als Akzeptor und Phenylcarbazol als Donor ein bipolares Matrixmaterial. Die beiden Einheiten sind dabei nicht konjugativ über eine Etherbindung verknüpft, um ein Absinken der Triplettenergie zu vermeiden. Die Materialien dieser Klasse zeigen von allen untersuchten Verbindungen in dieser Arbeit die geringste Neigung zur Kristallisation. Sie bilden stabile amorphe Filme mit Glasübergangstemperaturen bis 90 °C aus. Die erfolgreiche Trennung des Ladungstransports auf separierten Molekülteilen wurde in der Cyclovoltammetrie und in quantenmechanischen Rechnungen bestätigt. In fluoreszenzspektroskopischen Messungen konnte das Vorliegen von Carbazolexcimeren nachgewiesen werden. Daher konnten die Materialien nicht als Matrix in blau phosphoreszierenden organischen Leuchtdioden eingesetzt werden. Abschließend wurde ein Material aus der Klasse der Bistriazine als Elektronenleiter und Blocker für Löcher und Exzitonen in OLEDs mit blauen Phosphoreszenzemittern eingesetzt. Für das beste Bauteil mit optimierter Ladungsträgerbalance wurden eine externe Quantenausbeute von 14,7 % (bei 300 cd/m²) und eine maximale Leuchtdichte von 113.000 cd/m² erzielt.
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Impact of extreme hydrological conditions on belowground carbon cycling and redox dynamics in peat soils from a northern temperate fen
(2012)
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Cristian Estop Aragonés
- Peatlands have an important role in the global carbon cycle and constitute the largest pool of carbon stored in terrestrial ecosystems due to their disproportionally high areal soil carbon density. This globally relevant carbon stock is the result of a process mostly initiated after the last glaciation period. A key factor for this long term carbon accumulation is the relative low decomposition of organic matter in these predominantly water logged ecosystems. Hydrological conditions play thus a fundamental role in peatlands and the feedback of carbon cycling in these ecosystems in response to climate change is under debate. Peatlands are important CO2 sinks but also constitute global sources of CH4. The atmospheric exchange and production rates of these greenhouse gases are strongly influenced by the hydrological regime. An increased frequency of extreme meteorological conditions resulting in drying and flooding events is predicted to occur in the future.
The major issue regarding the climate change debate at the global scale is how rapid these greenhouse gases are being released to the atmosphere. Despite the general consensus regarding the broad effects of drying and flooding on CO2 and CH4 exchange, belowground processes producing such greenhouse gases and their response to water table dynamics is underrepresented and usually simplified or overgeneralized. Temperature, moisture, oxygen content and nutrient content are among the major environmental controls for organic matter decomposition rates in peat soils. Another important and intrinsic control is peat quality or humification degree of organic matter. The interrelation and relevance of all these factors vary among sites and with hydrological condition in a temporal and spatial scale.
This work presents investigations focusing on belowground redox processes aiming to witness the dynamic interrelation of soil physical and chemical (soil gas and pore water chemistry) variables, and evaluates the relevance of some controls of organic matter decomposition during a wide range of hydrological conditions. Most of this work shows information under in situ conditions and complementary laboratory experiments were performed minding the in situ observations. The findings contribute to general knowledge by providing raw data in fen peats under fluctuating and contrasting water table conditions in a relatively high spatiotemporal resolved scale. Dryings led to increased air filled porosity, O2 intrusion, CO2 degassing, inhibition of methanogenesis and renewal of electron acceptors. The opposite trend occurred upon rewetting with pulses of iron and sulphate reduction and delayed methane production to a variable extent. Upon flooding, continued anaerobic conditions stimulated the accumulation of reduced products, methanogenic precursors (acetate and hydrogen) and CH4.
The general assumption that the water table directly controls the oxygen content in peat was relativized. This work shows that such relation is greatly influenced by peat physical properties, which partially control the changes in moisture. Based on these findings, the mineral content and the degree of compaction in organic soils can be implemented to more accurately represent the dynamics of aeration in peats upon water table changes. Another general assumption is that drying events, i.e. temporary decline of water table below mean position, lead to increased CO2 production and emission from peat soils to the atmosphere. Such statement was also relativized and must account for the depth distribution of respiration rates in relation to the mean water table of the peat deposit. Based on these findings, the high relative contribution of upper peat layers already exposed above the water table mask the effects of increased CO2 production in deeper peat upon water table drop. Additionally, the role of moisture was shown to be little for aerobic respiration. This work also evaluates the importance of drought severity by accounting for the post drought effects on methane production. More intense and prolonged drying events led to a greater regeneration of electron acceptors in peat soil, which broadly suppressed or limited methane production upon rewetting. This relation was not simple and several factors such as water table position, post drought water table fluctuations, temperature and organic matter content contributed to the recovery of methane production after drying. The provision of substrates by fermentation processes limited peat respiration during shallow water table and drying. In contrast, accumulation of acetate and hydrogen was observed during flooding indicating a decoupling of fermentation from terminal metabolism and favouring the co-occurrence of iron reduction, sulphate reduction and methanogenesis.
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Monte Carlo Simulation Methods for Studying the Thermodynamics of Ligand Binding & Transfer Processes in Biomolecules
(2012)
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R. Thomas Ullmann
- The binding and transfer of ligands is of central
importance for the function of many biomolecular
systems. The main topic of this thesis is the
development and application of Monte Carlo (MC)
simulation methods for studying complex ligand
binding equilibria which can also involve
conformational changes. The simulated systems
were described by microstates within a continuum
electrostatics/molecular mechanics (CE/MM) model
of the receptor-ligand system. The CE/MM modeling
methodology was improved. The improvements led to
more detailed molecular models that enable a more
realistic reproduction of system properties and
environmental conditions. The developed simulation
methods were applied to biomolecular systems whose
function involves aspects that are important for
the understanding of bioenergetic energy
transduction. The results of this thesis are
presented in five articles that are published in
peer reviewed scientific journals.
Manuscript A presents the Monte Carlo simulation
software GMCT which was largely developed in this
thesis. The software offers a variety of different
simulation methods that allow the user to harness
the full potential of CE/MM models in the simulation
of complex receptor systems.
Manuscript B presents a novel theoretical framework
for free energy calculations with the free energy
perturbation method. The novel framework is more
broadly applicable and can lead to more efficient
simulations than previous formulations. The
derivation of the formalism also led to interesting
insights into general statistical mechanics. The
formalism was implemented in GMCT and could already
be used fruitfully for the free energy calculations
presented in Manuscripts C and D.
Manuscript C demonstrates the application of free
energy measures of cooperativity to study the
coupling of protonation, reduction and conformational
change in azurin from Pseudomonas aeruginosa (PaAz).
Such a coupling is prototypic for bioenergetic systems
because it forms the thermodynamic basis of their
energy transducing function. PaAz is an experimentally
well characterized, small electron transport protein.
For this reason, PaAz was used here as model system
to demonstrate the usefulness of cooperativity free
energies in detecting and quantifying thermodynamic
coupling between events in complex biomolecular
systems. The results of this study led to new insight
that could help to determine the still enigmatic
physiological role of PaAz.
In Manuscript D, free energy calculations were
applied to study the thermodynamics of transport
through the ammonium transporter Amt-1 from
Archaeoglobus fulgidus (AfAmt-1). Ammonium is the most
directly utilizable nitrogen source for plants and
microorganisms. AfAmt-1 and its homologues facilitate
the transport of ammonia/ammonium across biological
membranes in living beings from all domains of life.
It is intensely debated how these proteins perform
their function and whether ammonia or its protonated
form ammonium is actually transported. The study
extended upon previous theoretical studies by
including the effects of substrate concentration,
electrochemical transmembrane gradients,
proton-coupled binding equilibria and competitive
binding of different ligand species. It was found
that the transported species is most likely the
ammonium ion. An ammonia/proton symport mechanism
that involves a pair of coplanar histidine residues
at the center of the transmembrane pore as transient
proton acceptor is made plausible by the high
genetic conservation of these residues.
Manuscript E presents a first application of the
microstate description within a CE/MM model to the
simulation of the non-equilibrium dynamics of a
molecular system. We simulated the re-reduction
kinetics of the primary electron donor in the
photocycle of the bacterial photosynthetic reaction
center from Blastochloris viridis. The simulation
results are in very good agreement with
experimentally measured data.
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Budget and fluxes of nitrogen in mountainous agroecosystems in a summer monsoonal climate under intensive land use
(2012)
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Janine Kettering
- A balanced nitrogen (N) cycle in intensively managed ecosystems is necessary as it underpins other ecosystem services. This study evaluated the agricultural practices in a typical mountainous catchment in South Korea in respect to N dynamics and their potential effect on water quality with the aim to develop options for a more sustainable catchment management.
In the 1st study, we used two approaches to calculate N budgets for the 5 key crops of the basin at the field scale. The gross and net N budgets for all crop types were found to be positive. Based on the small differences between the results of the two approaches we identified fertilizer N as well as soil Nmin as the dominant N input sources. As fertilizer N application was the major N input source (>50%), its reduction is the major scope of action for N savings at the field scale. A closely linked action is the synchronization of fertilizer N with soil Nmin. The large amount of fertilizer that is applied prior to planting (>60%) at the beginning of the monsoon season revealed that split applications could help reducing the fertilizer N additions and increase the low N use efficiencies (NUE). Based on the significant differences between gross and net N surplus for rice and bean fields, we identified the high amount of plant residues remaining after harvesting (>100 kg N ha-1) as a further factor for potential N savings. The 5 main crops accounted for over 80% of the total catchment N surplus (>400 Mg), even though their contribution to the area was only around 20%. A land use shift to perennial crops with lower N inputs was therefore found to be a possible but spatially limited chance to reduce N surpluses at the catchment scale. The comparison of catchment N surplus with stream N export revealed that 73-86% of the agricultural N surpluses were transported to water bodies in the catchment by either leaching or surface runoff.
In the 2nd study, we followed the fate of fertilizer N in a ridge and furrow (R/F) cultivation with polyethylene (PE) mulch by using 15N tracer. N leaching was simulated with Hydrus 2D. The comparison of 4 N fertilization levels (0, 150, 250 and 350 kg N ha-1) revealed that already 150 kg N ha-1 is sufficient to reach the maximal yield of radishes. Based on the low results of fertilizer N use efficiency (FNUE), we recommend two applications during the first 25 days of growth and a further application around day 50. These split applications adjusted to the plants’ needs increase the FNUE of the radish and decrease the fertilizer N losses during the growing season. However, split applications might be impractical in plastic covered R/F cultivations because mechanical equipment to apply fertilizer under the PE mulch is required. Based on the finding that 15N retention in soil and nitrate concentration in seepage water decreased similarly for ridges and furrows during the entire growing season, we conclude that the PE mulch had no significant effect on 15N retention in soil and on nitrate concentration in seepage water and did therefore not effectively protect the fertilizer in the ridges from percolation. Based on the simulation results, we found that the ridges and furrows contributed approximately an equal amount of leached N to the total amount. We therefore conclude that the PE mulch provided little protection for the fertilizer N in the ridges during heavy rainfall. N leaching amounts were further found to increase linearly with an increase in N addition rate as it is well known for R/F cultivations without PE mulch. The PE mulch did therefore not prevent the linear increase in leaching with an increase in fertilizer N addition. We summarize that without the use of additional measures such as split applications of fertilizer, the application of PE mulch in a summer monsoon climate with heavy rainfall events does not positively influence the N leaching rates.
In the 3rd study, we monitored soil water dynamics in the field and used this data set to simulate the influence of PE mulch on water fluxes with Hydrus 2D. We simulated soil water dynamics in 1) conventional flat tillage (CT); 2) R/F cultivation without PE mulched ridges (RT); and 3) R/F cultivation with PE mulched ridges (RTpm). The comparison of the simulated pressure heads during dry and wet periods revealed that the PE mulch induced significant soil moisture patterns only during the dry periods. During monsoon events, the effect of the PE mulch was dependent on the soil texture and the hydraulic conductivity. Summarizing the advantages and disadvantages of the R/F cultivation with PE mulch on sloped fields, the practice was observed to have the lowest amount of drainage water, the lowest evaporation rates but also the highest surface runoff rates. Hence, PE mulching might be assessed as a tool to reduce percolating water, but it concurrently increases water contribution to the river network by surface runoff.
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The Integrated Size and Price Optimization problem
(2012)
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Miriam Kießling
Sascha Kurz
Jörg Rambau
- We present the Integrated Size and Price Optimization Problem (ISPO)
for a fashion discounter with many branches. Based on a two-stage
stochastic programming model with recourse, we develop an exact
algorithm and a production-compliant heuristic that produces small
optimality gaps. In a field study we show that a distribution of
supply over branches and sizes based on ISPO solutions is
significantly better than a one-stage optimization of the
distribution ignoring the possibility of optimal pricing.
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Shape Calculus Applied to Elliptic Optimal Control Problems
(2012)
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Michael Frey
- This thesis is devoted to the analysis of a very simple, pointwisely state-constrained optimal control problem of an elliptic partial differential equation. The transfer of an idea from the field of optimal control of ordinary differential equations, which proved fruitful with respect to both theoretical treatment and design of algorithms, is the starting point. On this, the state inequality constraint, which is regarded as an equation inside the active set, is differentiated in order to obtain a control law.
A geometrical splitting of the constraints is necessary to carry over this approach to the chosen model problem. The associated assertions are rigorously ensured. The subsequent derivation of a control law in the sense of the abovementioned idea yields an equivalent reformulation of the model problem. The active set appears as an independent and equal optimization variable in this new formulation. Thereby a new class of optimization problem is established, which forms a hybrid of optimal control and shape-/topology optimization: set optimal control. This class is integrated into the very abstract framework of optimization on vector bundles; for that purpose some important notions from the field of calculus on manifolds are introduced and related with shape calculus.
First order necessary conditions of the set optimal control problem are derived by means of two different approaches: on the one hand a reduced approach via the elimination of the state variable, which uses a formulation as bilevel optimization problem, is pursued, and on the other hand a formal Lagrange principle is presented.
A comparison of the newly obtained optimality conditions with those known form literature yields relations between the Lagrange multipliers; in particular, it becomes apparent that the new approach involves higher regularity. The comparison is embedded to the theory of partial differential-algebraic equations, and it is shown that the new approach yields a reduction of the differential index.
Upon investigation of the gradient and the second covariant derivative of the objective functional different Newton- and trial algorithms are presented and discussed in detail. By means of a comparison with the well-established primal-dual active set method different benefits of the new approach become apparent. In particular, the new algorithms can be formulated in function space without any regularization. Some numerical tests illustrate that an efficient and competitive solution of state-constrained optimal control problems is achieved.
The whole work gives numerous references to different mathematical disciplines and encourages further investigations. All in all, it should be regarded as a first step towards a more comprehensive perspective on state-constrained optimal control of partial differential equations.
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Beyond productivity- Effects of extreme weather events on ecosystem processes and biotic interactions
(2012)
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Julia Walter
- Under global climate change, extreme weather events, such as heat waves, drought or heavy rain spells, are projected to increase in magnitude and frequency. As these may affect vegetation and ecosystems more than gradual shifts in mean climatic parameters, investigating the consequences of extreme weather events recently became an important issue in climate change research. The main focus of most experiments investigating effects of extreme weather events on vegetation is on primary productivity. In our experiment in artificially planted communities, even an extreme drought of 1000-year recurrence did not have effects on above- or below-ground biomass production from 2005-2010.
Thus, the main objectives of this thesis were (1) to investigate if extreme weather events have an effect on ecosystem functions beyond productivity, (2) to test if such a high resistance or resilience in response to drought regarding productivity also exists in more naturally grown plant communities and (3) to further elucidate possible mechanisms of the surprisingly large stability of the plant communities.
To investigate these objectives, several experimental studies were conducted in artificially planted, as well as in naturally grown grassland communities and consequences of extreme weather events for ecosystem processes, such as decomposition and herbivory were investigated. In a pot experiment, it was studied, if grass plants react improved towards repeated drought when compared to a first drought and thus reveal a kind of drought memory. Such a memory might be one possible, but up until now widely neglected mechanism of resilience.
Even though biomass production remained stable in our experiment in artificially planted communities, biomass quality was severely affected by extreme drought, thereby strongly affecting the development of a herbivore caterpillar feeding on drought-exposed leaves. Further, plant compounds of the host plant depended on the composition of the plant community it was grown in. This in turn resulted in strong effects on the larval mortality of herbivores feeding on such plants.
In contrast to the study in artificially planted communities, aboveground net primary productivity (ANPP) was reduced in naturally composed grassland in response to extreme rainfall variability, including an extreme drought followed by heavy rainfall. Forage quality was altered by drought. Furthermore, mowing frequency strongly altered forage quality and biomass production, but did not interact with rainfall variability and thus did neither buffer, nor amplify effects of extreme rainfall variability. Despite effects of rainfall variability on ANPP, grassland showed high resilience after drought followed by heavy rain, as effects were large shortly after the extreme event, but did not persist until a second harvest later in the year.
In natural grassland, rainfall variability and drought also affected ecosystem processes, here litter decomposition, beyond productivity. Drought followed by heavy rain pulses decreased decomposition rates. Decomposition in more frequently mown meadows was more vulnerable towards drought exposure. Winter warming and additional winter rain had no long-term effect on decomposition. To conclude, projected increases in drought frequency under climate change may inhibit decomposition and alter nutrient and carbon cycling along with soil quality in temperate grassland, whereas a reduction of snow cover leading to more variable soil surface temperatures may counteract increased decomposition under winter warming.
In this thesis, an ecological stress memory as one possible mechanism of resilience is defined as any response of a single plant after a stress experience that improves the reaction of the plant towards future stress experience and which is assessed on a whole plant level. This thesis further provides evidence of a drought memory in grass plants: Plants repeatedly subjected to drought showed improved photo-protection and a higher rate of living biomass when compared to plants faced with their first drought. Similarly, tree seedlings exposed to drought in summer revealed higher frost resistance during winter, providing evidence of a long-lasting “cross-stress-memory” .
To sum up, the thesis shows that extreme weather events, even though neither severely affecting biomass production in artificially composed, nor in naturally growing communities in the long-term, exert strong influence on physiological or biogeochemical parameters, such as plant compounds or soil biotic activity. These changes in turn modify ecosystem functions beyond productivity, for example herbivory or decomposition, possibly altering biotic interactions and nutrient cycling. Furthermore, the findings imply that plants exhibit a stress memory after stress exposure, which may be one mechanisms leading to a high stability and resilience upon frequent stress.
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Jahresbericht 2011 - Universität Bayreuth Rechenzentrum
(2012)
- Jahresbericht
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The peptidyl-prolyl isomerase Pin1 is required for maintenance of the spindle assembly checkpoint
(2012)
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Andreas Brown
- Chromosomes are replicated during S-phase and segregated during M-phase of the eukaryotic cell cycle. The two sister chromatids of each duplicated chromosome are topologically entrapped and, thus, paired by the ring-shaped protein complex cohesin. They are separated in anaphase of mitosis when cohesin is endoproteolytically cleaved by separase. Activation of this giant protease requires the degradation of its two inhibitors, securin and cyclin B1, which is mediated by the anaphase promoting complex or cyclosome (APC/C), a multisubunit ubiquitin ligase, in conjunction with its essential co-activator Cdc20.
The spindle assembly checkpoint (SAC) is a surveillance mechanism that monitors the chromosomes' interactions with the microtubules of the mitotic spindle apparatus. In response to even one erroneous attachment the affected kinetochore emits a "wait anaphase" signal, which is amplified and culminates in the quantitative sequestration of Cdc20 by the SAC components Mad2 and BubR1. The consequent inactivation of the APC/C causes a metaphase arrest and gives the cell time to correct the error. Given its great importance for chromosome segregation fidelity, it comes at no surprise that loss of the SAC causes cell death while its curtailing is associated with tumour formation.
Pin1 is a peptidyl-prolyl-isomerase with strong preference for phosphorylated Ser-Pro or Thr-Pro motives within its protein substrates. In the present thesis, evidence for the involvement of Pin1 in the maintenance of a robust SAC response is presented.
Antibodies against Pin1 were raised and used to establish the effective immunodepletion of Pin1 from extracts of Xenopus laevis eggs. While the SAC could readily be activated in mock-treated samples of this cell free system, securin was degraded despite the presence of unattached kinetochores when Pin1 had previously been removed. Proving the specificity of this effect, a SAC mediated arrest could be rescued by adding back recombinant Pin1 to depleted extracts. Similarly, addition of dominant negative but not of wild-type Pin1 to SAC-arrested extracts resulted in a checkpoint override.
Chemical inhibition of human Pin1 with two different molecules in two different cancer cells lines invariably forced the cells to exit mitosis in the absence of spindles. This resulted in the premature disappearance of securin, cyclin B1 and a mitosis-specific phosphorylation on Ser10 of histone H3. Thus, Pin1's role as a checkpoint component is conserved in mammals.
In search for the relevant target, Cdc20 was identified as a novel interaction partner of vertebrate Pin1. This association requires phosphorylation of Cdc20 on Ser-Pro/Thr-Pro sites and occurs only during mitosis. Importantly, the Pin1-Cdc20 interaction is direct and not bridged via another checkpoint component or a subunit of the core APC/C. The experimental data suggest that Pin1-dependent isomerization of Cdc20 might bias it to preferentially associate with Mad2 and BubR1 instead of APC/C.
Taken together, these findings contribute to a better understanding of the molecular mechanisms involved in SAC signalling and unravel a previously unappreciated role of Pin1 for genome integrity.
