459 search hits
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Coordinated Tree Responses to Drought -Vulnerability and Sustainable Production: Hypotheses on Arid Ecosystem Adjustments to Limitations in Water Resources
(2004)
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Dennis Otieno
- Field and controlled greenhouse experiments were carried out to investigate tree responses to declining soil water content. Field experiments were conducted on naturally growing trees of Acacia tortilis and A. xanthophloea in the savanna region of Kenya and Quercus suber in the Mediterranean region of Portugal. The selected field sites were regions that experience regular drought periods during the year. Greenhouse experiments constituted two watering regimes. Seedlings of A. tortilis and A. xanthophloea grown from seeds initially obtained from the Kenya field site were raised and arranged on a greenhouse bench into two groups per species. The first set of plants were watered every other day (controls) while the second set were watered every seven days (water stress treatments). Field measurements included weather parameters, soil and plant water status, growth, sap flux density, leaf transpiration and stomatal conductance, tissue water relations and isotope labeling. Similar measurements were conducted on plants growing in the greenhouse. Also examined in the greenhouse were root biomass, root structure as well as whole plant biomass accumulation. A second set of experiments was carried out in the greenhouse by subjecting plants initially stressed and non-stressed to severe water stress by withholding water until plants were wilted overnight. The wilted plants were then re-watered regularly and their recovery after stress alleviation was monitored. Declining soil water content significantly affected plant water status in all the trees studied. Lowest psi pd recorded during the study period occurred in the month of June and were –2.0 and –1.1 MPa for A. xanthophloea and A. tortilis respectively. The same species subjected to repeated water stress in the greenhouse attained mean minimum psi pd of –2.4 and –1.2 MPa for A. xanthophloea and A. tortilis respectively at the end of the drying cycle. Mean minimum psi pd recorded for Q. suber during summer was –1.8 MPa and occurred in September. There were however, significant differences among trees. Decline in psi associated with increasing soil drought led to decline in leaf initiation and leaf expansion and both processes ceased at higher water stress levels. For the Acacia species, even leaf shedding occurred at higher stress levels. There was also a decline in stomatal conductance (gs) during water stress, leading to decrease in transpiration rates (E). Maximum stomatal conductance of 340 mmol m-2 s-1 were observed during rainy seasons for the Acacia trees while mean maximum values of 300 mmol m-2 s-1 were recorded for Q. suber when soil moisture conditions were favorable. Stomatal conductance declined by 31%, 67% and 67% in A. tortilis and A. xanthophloea in the savanna and Q. suber in the Mediterranean regions respectively. Daily tree water use (Qtree) as well as leaf transpiration reflected changes in psi and gs. Root to leaf hydraulic conductance equally declined with increasing soil drought. Q. suber trees adjusted osmotically by a magnitude of 0.7 MPa, while bulk modulus of elasticity (epsilon) increased by a magnitude of 17 MPa. Osmotic adjustment of 0.48 MPa was observed in greenhouse plants of A. tortilis while epsilon declined by a magnitude of 7 MPa in A. xanthophloea. A. tortilis plants in the greenhouse showed increased absolute root growth, root depth and root:shoot (r:s) ratio. The dimorphic rooting pattern in Q. suber resulted into hydraulic lift and this could as well occur in A. tortilis because of similarity in their rooting patterns. Most plant responses were reactionary and were aimed at enhancing soil water uptake and reducing transpiration water loss when soil water content was declining. Similar responses were observed for both greenhouse and naturally growing field plants of the same species. Decline in leaf initiation and leaf expansion as well as leaf senescence reduced tree crown size hence potential tree transpiration. This however, had negative impact on plant productivity. Increased root growth as well as osmotic adjustment increased tree water uptake from the soil. The balance between root water uptake and leaf transpiration through growth and stomatal regulation was aimed at protecting xylem integrity. The overall results showed that soil characteristics, root activities and root distribution patterns are the main factors determining tree functioning and productivity in drylands, while the coordinated interaction between the aboveground shoot and belowground root activities ensures survival during drought. Maintained production and survival will ensure distribution and success in the arid environments. Repeated water stress imparted water stress resistance qualities on seedlings enabling them to survive longer during severe stress. The study emphasizes the role of soil resource base as well as species interactions in the functioning and balance of dryland ecosystems.
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Biogeochemistry of Organotin and Organolead compounds in a Forested Catchment in NE-Bavaria, Germany
(2004)
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Jen-How Huang
- Organotin-compunds (OTC) and Trimethyllead (TML) have a higher toxicity than their corresponding inorganic forms and may affect the functioning of ecosystems. Little is known about their behaviour and fate in the terrestrial environment. The goal of this thesis was to investigate the biogeochemistry of OTC (methyltin, butyltin and octyltin compounds) and TML in a forested catchment, especially their input and output budget. The occurrence of OTC, TML, Sntotal and Pbtotal in the atmosphere, soils, precipitation, and runoff in a forested ecosystem in NE-Bavaria, Germany were investigatedand the inputs and outputs in the solute phase determined. In addition, their ad-desorption and transformation (degradation) in forest soils was studied using batch experiments and long term incubations, respectively. OTC and TML concentrations in the gas phase during April to June 2003 was on average 110 pg Sn m–3 and 0,34 pg Sn m–3. Tri-, di-substituted and octyl species were the dominant OTC in the gas phase. In aerosols, only butyltin compounds, dimethyltin and monomethyltin (<500 pg Sn m–3) were found and mono-substituted OTC predominated. For OTC, the washout factors were in the order: mono- >= di- > tri-substituted OTC, and the gas / particle partition coefficients were mono- >> di- >> tri-substituted OTC. Aerosol particles serve as a sink for OTC in the atmosphere, especially for monomethyltin and monobutyltin. From August 2001 to August 2002, the concentrations of tin and lead compounds in precipitation were in the order: fog > throughfall > bulk precipitation. Average concentrations of OTCtotal ranged from 57 ng Sn l–1 in fog to 5.8 ng Sn l–1 in bulk precipitation. The concentrations of Sntotal were between 490 ng Sn l–1 in fog and 140 ng Sn l–1 in bulk precipitation, on average. Average concentrations of TML were 1 ng Pb l–1 in fog and 0.1 ng Pb l–1 in bulk precipitation and 9.6 and 0.76 micro-g Pb l–1 in case of Pbtotal. The annual total deposition from the atmosphere, estimated as throughfall + litterfall fluxes, amounted to 3.7 mg Pb ha-1 yr-1 for TML and 52 g Pb ha-1 yr-1 for Pbtotal. The contribution of litterfall was 1.5% and 32%, respectively. The annual total deposition of OTCtotal was 172 mg Sn ha–1 yr–1, with 26% represented by litterfall. The total deposition of Sntotal was 4.9 g Sn ha–1 yr–1, of which 4.1% was litterfall. The annual flux with runoff from the catchment was 0.5 mg Pb ha-1 yr-1 for TML and 2.8 g Pb ha-1 yr-1 for Pbtotal. The annual runoff of OTCtotal and Sntotal amounted to 25 mg Sn ha–1 yr–1 and 2.4 g Sn ha–1 yr–1, respectively. The mass balance showed high retention of tin and lead compounds in the catchment. The total soil storage of OTCtotal and Sntotal in the catchment were 6.7 g Sn ha–1 and 1.9 kg Sn ha–1. Total soil storage in the catchment was 11.6 mg Pb ha-1 for TML and 222 kg Pb ha-1 for Pbtotal. The dominance of mono-substituted compounds in precipitation is well reflected in the concentrations and storages of OTC in both upland and wetland soils. More than 90% of the soil storage of TML was found in the wetland soils of the catchment representing only 30 % of the area. Most Pbtotal (>90%) was found in the upland soils. In upland soils, TML was only detectable in the forest floor. OTC degraded slowly in soils with half-lives (t½) estimated from 0.5 to 15 years. The degradation rates in soils were generally in the order mono- >= di- > tri-substituted OTC. Decomposition rates of OTC in the forest floor were higher than in wetland and mineral soils. TML degraded rapidly in the forest floor (Oa) with a t½ of 0.09 years. The degradation of TML in Fen (t½ = 1.2 years) and in the mineral soil (Bw-C, t½ = 1.7 years) was much slower. Emission of tetramethyltin and tetramethyllead from wetland soils was not observed, suggesting little Sn and Pb methylation in the wetland soils. The adsorption and desorption isotherms for all species and soils were linear over the concentration range of 10–100 ng Sn, Pb ml–1. The strength of OTC adsorption correlated well with the carbon content and cation exchange capacity of the soil and was in the order mono- > di- > tri-substituted OTC and butyltin > methyltin compounds. The adsorption and desorption showed a pronounced hysteresis. The ratio of total soil storages in the catchment to the present annual input was 3.6 years for TML, reflecting the rapid degradation of TML in forest soils. The ratios of OTC soil storages to their present annual deposition ranged from 4.3 to 400 years. These high ratios reflect probably the high stability, low mobility of OTC in soils and the variation of the annual total deposition.
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Nanostructure formation in thin polymer films
(2004)
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Nicolaus Rehse
- In the first part of this thesis an improved process is presented to prepare laterally structured substrates via hierarchical self organization. A miscut silicon surface annealed at 1400 K under ultra high vacuum conditions is used. The resulting facets are stable against oxidation and form a topographic pattern which can be further modified to a chemical pattern via evaporation of gold on every other facet. By controlling the time of annealing, we create structures with a reproducible mean width ranging from 40 to 400 nm. Despite the rather complex ultra high vacuum treatment and an additional evaporation step, we are able to produce substrates in a relatively short time (36 h). These substrates show a nanometer sized structure over an area of 0.5 cm². The automation of the cleaning process and a controlled heating during the annealing increases the yield of high-quality, stepped substrates. These structures allowed us to study the behavior of ultra-thin polystyrene films on topographically structured substrates. The film thickness of some nanometers is comparable to the radius of gyration of the polymers. The substrate corrugation causes a regular variation of the film thickness. We start with a homogeneous film, which is annealed above the glass transition temperature. During annealing the films are stable or form long polymer nanochannels, which lie in the grooves of the substrate structure. The balance of the radius of gyration and the film thickness controls the stability of the polymer film, while the corrugation only triggers the dewetting. The same behavior is found for films on flat substrates. Here small contaminations nucleate the formation of holes. Evaporation of gold stripes and their modification with self assembled monolayers leads to chemical patterned substrates. This expands the possibilities to manipulate the substrate wettability on the nanometer scale. The second part of the thesis describes the formation of ordered structures in block copolymer films. ABC triblock copolymers show a large variety of morphologies in thin films. We have shown that surface reconstructions play an important role in the structure formation process of these structures. In very thin films, where the film thickness is smaller than the long period of the polymer's micro domains, confinement effects overlap with the surface effects. The component with the lowest surface energy is accumulated at the free surface. It needs a subtle balance between the different surface energies (external fields) and the interaction of the three polymer blocks (internal fields) to create a surface reconstruction. This was shown by variation of the chemistry of the end block and by changing the sequence of blocks in the experiment. To analyze the surface reconstruction we used selective staining along with scanning electron microscopy, selective etching in oxygen plasma in combination with scanning probe microscopy, as well as quantitative TappingMode atomic force microscopy. Surface reconstructions of block copolymers show remarkable similarities with reconstructions of single crystal surfaces. In both cases the driving force for a rearrangement is the decrease in surface free energy of the ideal surface. A second analogy between the lamella forming SBM triblock copolymer and Si(100) is the fact that two non-equivalent layers of matter aligned parallel to the free surface lead to two different terminations at the surface. This shows that the phenomenon of surface reconstructions is not limited to classic crystals. The results of this thesis give new insights in the behavior of polymers at surfaces and in thin films. This gives the opportunity to create or manipulate nanometer sized structures accurately via self assembly, external stimuli, or a combination of both.
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Keteneylidenetriphenylphosphorane as a 'C2O building block' in the synthesis of highly functionalised tetramic and tetronic acids
(2004)
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Claire Melanophy
- Naturally occurring 4-hydroxy-pyrrol-2(5H)-ones and 4-hydroxy-furan-2(5H)-ones are known to possess a wide range of biological activities such as anti-viral and tumour inhibition. For this reason, the synthesis of a number of these compounds was attempted, namely Tenuazonic Acid, Reutericyclin and Carlosic acid. A general synthesis of 4-hydroxy-pyrrol-2-ones was established by reaction of a phosphorus ylide (Ph3PCCO) with a variety of amino esters. A number of derivatives were prepared with varying substituents at the 3- and 5-positions of the nitrogen heterocycle. A general method for the preparation of highly functionalised furan-2-ones from simple alpha-hydroxy esters was also developed. Progress has been made in the synthesis of N-substituted pyrrol-2-ones where simple amide esters were reacted with a phosphorus ylide (Ph3PCCO) in the construction of highly functionalised nitrogen heterocycles. A new acylation procedure was developed in order to selectively introduce an acetyl residue to pyrrolidine-2-ones and furan-2-ones. A phosphorus ylide (Ph3PCCO) and its solid supported variant were used as acylating agents under relatively mild, basic conditions. Complex heterocycles were prepared using the Diels-Alder methodology and from reaction of Ph3PCCO with relatively simple molecules.
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Equilibrium and Dynamic Phase Behavior in Thin Films of Cylinder-Forming Block Copolymers
(2004)
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Armin Knoll
- The equilibrium and dynamic phase behavior in thin films of cylinder-forming block copolymers has been studied. The results range from an extension of an advanced strong segregation theory to the treatment of cylindrical microdomains in a thin film, over the detailed analysis of the phase behavior and the microdomain dimensions of a cylinder forming model system, to the first in-situ measurements of phase transitions and microdomain dynamics in a thin block copolymer film. The first result concerns the characterization of thin films of the model system, a polystyrene-b-polybutadiene-b-polystyrene (SBS) triblock copolymer, by tapping mode scanning force microscopy (TM-SFM). We present a procedure to establish reliably with TM-SFM the true surface topography of a soft polymeric sample. The measurement of an array of amplitude phase distance (APD) curves enables us to distinguish quantitatively between the ¡°real¡± surface topography and lateral differences in tip indentation. We find that conventional TM-SFM height images are not necessarily reflecting the surface topography of the sample. In the case of SBS we find that the surface is flat and that conventional TM-SFM height images only reflect lateral differences of tip indention. A theoretical treatment of the phase behavior in thin films of cylinder forming block copolymers within an advanced strong segregation theory (SST) framework of Olmsted and Milner is developed. Although the theory is strictly valid only for highly stretched chains and does not account for minority domain structures other than cylinders, the predicted results are in good agreement to the experimental results in this thesis. The microdomain structures under consideration are in plane oriented cylinders terminating with either the matrix component CP or a half cylinder CH at the surface and cylinders oriented perpendicular to the film CS. The thin film phase behavior of concentrated solutions of SBS in chloroform is studied experimentally. The stable phases are mapped as a function of film thickness and polymer concentration phi. The variation of phi can be interpreted as a variation of the molecular interactions between the two polymer components and between the components and the boundary surfaces. The preferential attraction of the majority component to the surface, the surface field, causes the cylinders to align parallel to the plane of the film, whenever the thickness fits an integer multiple of cylinder layers. At intermediate thickness the cylinders align perpendicular to the film plane. At higher polymer concentration, i.e., at stronger surface fields, a perforated lamella (PL) of polystyrene forms. The surface field needed for PL formation increases with increasing film thickness. A wetting layer exists underneath all films, which either consists of pinned molecules or of a half lamella. Furthermore the principal microdomain spacings of the thin film microdomain structure are investigated in detail and compared to the SST results. A new image analysis algorithm provides the recognition and the localization of the different structures in the SFM phase contrast images. The microdomain spacings are discussed as a function of various parameters like the film thickness, the polymer concentration and the local curvature of the structure. An examination of the dependence of the spacing in the C¨U structure from the local curvature of the cylinders yields an effect of second order, which can be explained in terms of density conservation of the polymer blocks. The spacing of the CP structure also depends on the local thickness of the thin film. The CP phase is stable in a certain thickness range close to its preferred thickness. Within this range it has to adapt the cylinder "height" to be commensurable with the film thickness. Minimization of the interface between the blocks leads to a smaller or larger lateral cylinder spacing if the cylinder has to stretch or shrink its dimension perpendicular to the plane of the film, respectively. Finally the first in-situ observation of phase transitions in thin block copolymer films is presented. During annealing with a controlled atmosphere of chloroform vapor the film develops islands and holes. The development of the microdomain structure and its spacial fluctuations are captured. The rather high polymer concentration in the thin film results in a time scale of the fluctuations in the order of seconds to minutes. The decreasing film thickness inside of the holes triggers phase transitions from CS to CP to PL according to the phase diagram of the system.
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The complex foraging strategy of the specialised gallfly Urophora cardui (Diptera: Tephritidae) for host plants (Cirsium arvense, Asteraceae)
(2004)
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Wiltrud Daniels
- Nearly all herbivorous insects in terrestrial ecosystems depend on plants for their survival and reproduction. They dominate terrestrial ecosystems due to species and individual abundance. Interactions between these two groups are thus of a high significance for the analysis and the understanding of complex interactions in terrestrial ecosystems. Foraging for host plants by herbivorous insects is of a central importance. In the present thesis the foraging strategy of the specialised gall fly Urophora cardui on the creeping thistle, Cirsium arvense, was investigated as an example for the foraging of a specialised herbivorous insect. Males and females of U. cardui use the larval host plant as rendezvous place. The males establish territories on the plant, which they defend against conspecifics. The females lay eggs into axillary buds in order to initiate gall development. Male and female body size, measured as weight at eclosure or capsule width, was not correlated with male respectively female longevity. Males lived shorter than females. Capsule width of males and females was not significantly different, while weight at eclosure and fresh weight at death was. Females weighed more, which may be due to their higher need of energy during adult life. Both sexes lost body weight during life. All behaviours, which are performed by males and females on the host plant were defined, recorded and analysed. Females spent most of the time on resting, probing axillary buds, running on the plant and grooming. Males spent most of the time on copulation and patrolling their territory. The behaviour of both sexes was highly variable between individuals. Concerning the movement pattern on an already chosen host plant, females concentrate on the upper parts of the host plant. They were mainly occupied with extensive probing of various axillary buds, which occurred in a suitable developmental stage at the top of the plant. In contrast males patrolled the whole plant, although only the upper leaves became marked. If they encountered another male threatening and fighting were inevitable. Fights lasted several hours interrupted by threatening periods. Mating of males and females usually followed oviposition. Neither females nor males accepted modified host plants or models of thistles. Their behaviour on modified thistles was reduced mainly to running around the plant and grooming. These results indicate a rigid host plant template using the input of several senses, the flies always recognise models and modified plants as a non-host plant. Both sexes were able to discriminate host plants and non-host plants from a distance of 0.8-2m. The time male and female flies needed until they selected one of the host plants in a particular host plant stand depended on the number of non-host plants, host plants and the number of suitable hosts. The decision-time became shorter, if there were not too many suitable host plants. This may be due to decreasing sampling time of the host plants present. U. cardui females did not prefer plants of a certain height. In contrast the branching level, which indicates the number of axillary buds, and the number of flower buds played a significant role during foraging for host plants. Plants with an intermediate branching level were preferred, while those with many flower buds were avoided. Males of U. cardui were able to select their territorial plant on olfactory cues or on visual cues likewise. In contrast, emales were not able to recognise their host plant on olfactorial cues alone. But, if male-marked and unmarked host plants were available they significantly preferred the marked thistles. The differentiation between marked and unmarked C. arvense was made according to olfactorial cues, since plants did not differ in their height, nor in their branching level, nor in the number of flower buds. Thus, female selection of larval host plants depended on male choice for territories. This result is remarkable, especially in evolutionary terms, since males select the larval host for the offspring of their predecessor. Interestingly males preferred plants marked by conspecifics also. On the tip of the females’ ovipositor there is a receptor field with several morphological different receptor types. Mainly they seem to have mechanoreceptive as well as chemosensory functions. These receptors enable the females to measure and analyse the inner structure of the plant tissue at the axillary bud. The width of the apical meristem of the axillary buds was proven to influence female choice of the oviposition site. Axillary buds with an apical meristem-diameter above 0.62mm had a higher probability to became chosen by the U. cardui females. The clutch size was adjusted to the diameter of the apical meristem, indicating, that females were able to estimate the quality of the respective axillary bud influencing larval performance. These results were summarized in a model of the foraging strategy of U. cardui.
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Pattern Formation in Rotating Fluid Systems under the Influence of Magnetic Fields
(2004)
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Erol Kurt
- Patterns are observed in many different systems in nature. They are seen in the cloud streets, in sand ripples, in the morphology of plants and animals, on weather maps, in chemical reactions. In all these cases one deals with open, continuous dissipative systems which are driven out of equilibrium by an external stress. If this stress is larger than a certain threshold value, the symmetry of the temporally and spatially homogeneous ground state is spontaneously broken. The resulting patterns show then periodicity in space and/or in time. One of the best studied examples is the convection instability when a fluid layer is subjected to a temperature gradient. For instance, in a horizontal fluid layer heated from below and cooled from above a striped patterns of convection rolls develop. This scenario describes the famous Rayleigh- Benard convection (RBC), as a standard paradigm of pattern formation. Many concepts and mathematical tools to analyze the patterns have been developed and tested for this case. This thesis deals with two different pattern forming systems, namely a particular example of a convection instability and the case of a shear flow driven instability. In the first part of the thesis, a variation of the standard RBC is investigated. We consider the problem of convection induced by radial buoyancy in an electrically conducting fluid contained in a rotating (angular frequency, Omega) cylindrical annulus which is cooled at the inner surface and heated from outside. In addition, an azimuthal magnetic field (B) is applied for instance by an electrical current through the cylinder axis. The motivation of this study has come originally from the geophysical context. This setup is hoped to capture some important features of convection patterns in rotating stars and planets near the equatorial regions. The problem is also of considerable interest from a more general point of view in that it is concerned with formations of patterns in the presence of two competing directional effects, in this case rotation and the magnetic field. The second part of the thesis is devoted to the the pattern formation by a shear flow between two rotating and infinitely electrically conducting plates with a magnetic field perpendicular to the plates. This geometry is called the magnetic Ekman-Couette layer and has been a basic model for magnetic activities at the boundary of the Earth's liquid core or at the tachocline in the Sun below the convection zone for a few decades. To analyze the forementioned problems, various codes and computational tools had to be developed, for instance, we were able to describe complex spatio-temporal patterns by the direct simulations of the underlying hydrodynamic equations for our problems. The discussion of the physical details of the systems are postponed to the introductory sections of the corresponding parts of the thesis. In Chapter 1, a general formulation of the linear and nonlinear analysis, methods, which are applicable to both pattern forming systems in this work will be presented. The investigation of thermal convection in a plane layer which is a geometry equivalent to the cylindrical annulus will be discussed in Chapter 2. The next chapter (Chapter 3) covers both the linear and nonlinear analyses in the case of magnetic Ekman-Couette layer problem. Finally, in Chapter 4, we will present the general conclusions on both of the systems.
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The Allatoregulatory Neuropeptides and their Genes in the fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae)
(2004)
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Mohatmed Abdel-latief
- The genes encoding the S. frugiperda allatotropin (Spofr-AT), allatostatin (Spofr-AST), allatostatin type-A (Spofr-AST A) peptide family and allatotropin 2 (Spofr-AT 2) peptides were isolated from S. frugiperda brain cDNA. The Spofr-AT gene is expressed in three mRNA isoforms with 134, 171, and 200 amino acids, respectively, differing from each other by alternative splicing. The Spofr-AST cDNA encodes 125 amino acid residues including one copy of the Manse-AST mature peptide (type-C allatostatin). The deduced precursor sequence of Spofr-AST A gene contains 231 amino acids and allowed unambiguous identification of nine (or ten) peptides of YFXFGL-a peptide family, which are tandemly arranged in three blocks. A cDNA that encodes 53 amino acids was cloned from S. frugiperda brain cDNA, including one copy of a non-amidated decapeptide (Arg-Val-Arg-Gly-Asn-Pro-Ile-Ser-Cys-Phe-OH). This peptide strongly stimulates the synthesis and release of juvenile hormone (JH) in vitro by the corpora allata (CA) of S. frugiperda adult females and was code-named Spofr-AT 2. The primary structure of Spofr-AT 2 is identical at its C-terminus (-NPISCF) with the M. sexta type-C allatostatin (Manse-AST). One-step RT-PCR for semi-quantification of the gene expression, it is demonstrated that both genes (Spofr-AT and Spofr-AST) are expressed in brain, digestive tract, and reproductive organs of larvae, pupae, and adults of S. frugiperda in a time-, tissue-, and sex specific manner. The tissue-specific localization of the prohormone expression, as demonstrated by whole-mount in situ hybridization, confirms the overall tissue distribution of the prohormones as shown by RT-PCR and supports the pleiotropic functions of the peptides. Spofr-AST type-A gene is expressed in the brain, midgut, and reproductive organs of S. frugiperda larvae and adults in a time- and tissue-specific manner. Data confirm the nature of the allatostatin type-A peptides as brain/gut myoregulatory hormones. Northern blotting and RT-PCR analyses revealed that the Spofr-AT 2 gene is expressed as one transcript in the brain, midgut, and ovary in a tissue- and developmental-specific manner. Treating the CA with the synthetic peptide caused an up to tenfold increase in the release of JH. The stimulation of JH release was dose-dependent with an apparent EC50 of ca. 10-7 M. CA that were activated with Spofr-AT 2 could be inhibited by the addition of synthetic Manse-AST. In conclusion, the presented date strengthen the hypothesis that “allatoregulating” neuropeptides are diverse in structure, widely distributed and exhibit multiple functions. The functions may be tissue-specific as well as specific to particular developmental stages of insects. Knowledge of the various peptide precursor sequences has opened the way for synthesis of these peptides for detailed physiological and functional studies. Further quantitative experiments formulated in context of the life history of the animals will certainly yield a more detailed understanding of the mode of action of these peptides in S. frugiperda. Other major challenges in the future will be to clone the receptors for these peptides and to study the receptor distribution in the fall armyworm.
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Synthesis, characterisation and application of low molecular weight and polymeric 1,3-di-2-thienylbenzo[c]thiophenes
(2004)
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Roman Kisselev
- The synthesis and characterisation of the new class of compounds, dithienylisothianaphthene phenyldiamines (DTITNPDs) is described. These bifunctional hole transport dyes combine well-known hole-transport property of triaryl amines and thiophenes as well as low band gap nature of isothianaphthene (ITN) moiety. The synthetic strategy is chosen in such a way to obtain low molecular weight and polymeric DTITNPDs. Low molecular weight DTITNPDs are synthesised by Pd-catalysed amination of dibromo dithienylisothianaphthene with secondary amines. On the other hand, poly(DTITNPD)s are obtained via polycondensation of diiodo dithienylisothianaphthene and bis(secondary amine)s using a modified Ullmann reaction. The multi-step syntheses of dibromo dithienylisothianaphthene and novel diiodo dithienylisothianaphthene are optimised. The preparation of new bis(secondary amine)s are also described. Moreover, the influence of substituents on optical, electro-chemical and thermal properties of DTITNPDs and poly(DTITNPD)s is also investigated. The low molar mass (monomers) and polymeric DTITNs are also obtained. These compounds also possess hole transport property of thiophene and low band gap nature of ITN. Poly(DTITN)s are synthesised from corresponding monomers using FeCl3 oxidative polymerisation. Multi-step syntheses of DTITN monomers are also presented here. The main highlight of this work is the realisation of solution processable and film-forming and air-stable poly(DTITN) and poly(DTITNPD)s in addition to the model compounds DTITNs and DTITNPDs. The model compounds, polymeric DTITNPDs as well as poly(DTITN)s are characterised by means of 1H-NMR-, FT-IR-, MS- and UV-Vis- spectroscopy. Their thermal and electro-chemical behaviour is studied using TGA, DSC and CV measurements. All intermediates, synthesised in this work are also fully characterised by spectroscopic methods discussed above, except UV-Vis spectroscopy. Novel DTITNs and DTITNPDs show good thermal and electro-chemical stability as well as ability to form smooth thin films. DTITNPDs show lower band gap, solubility in common organic solvents and better thermal stability compared to DTITNs. For the application in organic electro-optical devices materials with improved optical and charge transport properties are required. Moreover, these hole transport dyes should match the energy levels (HOMO/LUMO) of the electron transport partner for efficient charge transfer/injection. In this respect, the main attention is placed on variation of energy levels in synthesised molecules by structure modification. The structure modification in DTITN usually changes the LUMO level in the molecule. In contrast to the DTITN, the introduction of different substituents into diphenylamine allows manipulation of HOMO level in DTITNPDs. Thus, the combination of DTITN and triarylamines leads to DTITNPD, where the values of both energy levels can be varied. Novel DTITNPDs exhibit smaller band gap compared to DTITNs. The better delocalised HOMO level in the DTITNPDs compared to DTITNs leads to novel hole transport dyes with Eg less than 1.8 eV. The low molar mass DTITNPDs are tested in plastic solar cells and multi-layer solar cells in combination with electron transport perylene bisimide derivatives and fullerene (C60). The poly(DTITN) is used in plastic solar cell in combination with a soluble fullerene derivative, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). All of these compounds show good performance in solar cells. External quantum efficiency (IPCE spectrum) for the last solar cell shows a maximum of 40 % at 350 nm and a maximum of 15 % at 540 nm, at maximum wavelength of absorption. A promising result is obtained using low molar mass DTITNPD as red-emitter in OLEDs. When the red emitter doped in Alq3 at concentration of 1 % is used as emitting layer in OLED, the pure red electroluminescence with maximum brightness of 13830 cd/m2 at operating voltage of 12 V is observed. This device exhibits a high efficiency of 3.8 cd/A at 6 V bias, emitting bright red electroluminescence with CIE coordinates of x = 0.66 and y = 0.34, closely resembling the desired standard red colour (NTSC standard: x = 0.67 and y = 0.34) set for RGB displays.
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Complex nanostructures in triblock terpolymer thin films
(2004)
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Sabine Ludwigs
- The thin film phase behavior of poly(styrene)-block-poly(2-vinylpyridine)-block-poly(tert-butyl methacrylate) (PS-b-P2VP-b-PtBMA) triblock terpolymers with volume fractions f(PS) : f(P2VP) : f(PtBMA) = 1 : 1.2 : x, with x ranging from 3.05 to 4, is studied with a combinatorial gradient approach. Gradients in film thickness are prepared via thin film flow coating of dilute solutions in chloroform. Upon controlled annealing in nearly saturated solvent vapor the films form terraces of well-defined step height. The dependence between morphology and film thickness is studied with optical microscopy, tapping mode SFM, and SEM. Though showing different morphologies in the bulk, the same sequence of surface structures is found with increasing film thickness for the whole range of compositions: a disordered phase in the thinnest regions, a liquid-like distribution of upright standing cylinders, cylinders oriented parallel to the film, and finally a hexagonally ordered perforated lamella structure (PL) on the first terrace with a thickness of d = (37+3) nm. Higher terraces also exhibit PL as surface structures. Due to the chemical nature of the block components and the particular stoichiometry of the polymer a wetting layer with a PtBMA-rich top layer is formed next to the substrate. By imposing an additional gradient in substrate surface energy, orthogonal to the gradually increasing film thickness, the perforated lamella is shown to be a stable phase, regardless of the chemical nature of the substrate, which makes this structure and methodology robust for application in nanotechnology. The complex phase behavior observed in thin films is supported by mesoscale computer simulations based on dynamic density functional theory. Thin films of the above mentioned triblock terpolymers are modeled as a melt of A3B4C12 Gaussian chains which is confined in a slit with film thickness H. By adjusting the interaction parameters between the polymer components and the surfaces, the experimentally observed sequence of surface structures as function of the film thickness can be successfully modeled. At well-defined film thickness the perforated lamella structure is formed. In analogy with earlier work on a two-component system these structures are identified as surface reconstructions of the bulk structure. In particular, the core-shell PL can be seen as analogue to the PL surface reconstruction of cylinder-forming AB and ABA systems. The influence of film thickness, surface field, and the interaction parameters between the different polymer components on the phase behavior is also explored. A large spectrum of surface structures is observed in analogy to the experiments. Further attention has been given to the perforated lamella structure. This structure can be visualized as P2VP/PS/P2VP lamellae which are perforated by channels of PtBMA interconnecting between two outer layers of PtBMA. A highly ordered PL structure could be prepared with a very small number of defects over an area of about 12 x 4 µm2. Because of the special functionalities of the triblock terpolymer a rather versatile nanostructure was produced. By selective UV-depolymerization of the PtBMA matrix phase, the PL phase might potentially be used for lithographic applications similar to the case of perpendicularly oriented poly(methyl methacrylate) (PMMA) cylinders in PS-b-PMMA block copolymer thin films. Furthermore, a responsive membrane can be created by selective removal of the matrix phase. The remaining PL has a P2VP shell which might be either switched via the pH-value or loaded with metal components. A polymer-analogous reaction of the matrix phase of the PL to poly(methacrylic acid) via acid-catalyzed hydrolysis leads to a pH-responsive nanostructure without altering the overall structure. With SFM in aqueous environment structural changes of the PL phase are studied as function of the pH-value. Upon changing the pH of the surrounding medium a strong swelling of the original film thickness is observed at pH-values > 6 to a maximum degree of 7.5-fold swelling. This swelling is explained with a conformational change of the matrix phase poly(methacrylic acid). The hexagonal arrangement of the pattern is not affected. The first two blocks PS and P2VP act as skeleton of the PL phase which withstands the mechanical forces exerted on the strongly swollen PMAA. In contrast to the PL phase core-shell cylinders oriented parallel to the interfaces cannot withstand these forces and are solubilised at high pH-values.
