Showing papers by "Leibniz University of Hanover published in 1999"

Impacts of aluminum on the cytoskeleton of the maize root apex. short-term effects on the distal part of the transition zone

Abstract: Using monoclonal tubulin and actin antibodies, Al-mediated alterations to microtubules (MTs) and actin microfilaments (MFs) were shown to be most prominent in cells of the distal part of the transition zone (DTZ) of an Al-sensitive maize (Zea mays L.) cultivar. An early response to Al (1 h, 90 μM) was the depletion of MTs in cells of the DTZ, specifically in the outermost cortical cell file. However, no prominent changes to the MT cytoskeleton were found in elongating cells treated with Al for 1 h in spite of severe inhibition of root elongation. Al-induced early alterations to actin MFs were less dramatic and consisted of increased actin fluorescence of partially disintegrated MF arrays in cells of the DTZ. These tissue- and development-specific alterations to the cytoskeleton were preceded by and/or coincided with Al-induced depolarization of the plasma membrane and with callose formation, particularly in the outer cortex cells of the DTZ. Longer Al supplies (>6 h) led to progressive enhancements of lesions to the MT cytoskeleton in the epidermis and two to three outer cortex cell files. Our data show that the cytoskeleton in the cells of the DTZ is especially sensitive to Al, consistent with the recently proposed specific Al sensitivity of this unique, apical maize root zone.

Effects of f O2 and H2O on andesite phase relations between 2 and 4 kbar

Abstract: Experimental phase equilibria have been investigated on three medium-K silicic andesite (60–61 wt % SiO2) samples from Mount Pelee at 2–4 kbar, 850–1040°C, under both vapor-saturated CO2-free and vapor-saturated CO2-bearing conditions. Most experiments were crystallization experiments using dry glasses prepared from the natural rocks. Both normal- and rapid quench experiments were performed. Two ranges of oxygen fugacity (fO2) were investigated: NNO (Ni-NiO buffer) to NNO + 1 and NNO + 2 to NNO + 3. At 2 kbar for moderately oxidizing conditions, plagioclase (pl) and magnetite (mt) are the liquidus phases, followed by low-Ca pyroxene (opx); these three phases coexist over a large temperature (T)-H2O range (875–950°C and 5–7 wt % H2O in melt). Amphibole (am) is stable under near vapor-saturated CO2-free conditions at 876°C. At 900°C, ilmenite (ilm) is found only in experiments less than or equal to NNO. Upon increasing pressure (P) under vapor-saturated CO2-free conditions, pl + mt is replaced by am + mt on the liquidus above 3.5 kbar. For highly oxidizing conditions, mt is the sole liquidus phase at 2 kbar, followed by pl and opx, except in the most H2O-rich part of the diagram at 930°C, where opx is replaced by Ca-rich pyroxene (cpx) and am. Compositions of ferromagnesian phases systematically correlate with changingfO2 Experimental glasses range from andesitic through dacitic to rhyolitic, showing systematic compositional variations with pl + opx + mt fractionation (increase of SiO2 and K2O, decrease of Al2O3, CaO, FeOt, and MgO). FeO*/MgO moderately increases with increasing SiO2. For fO2 conditions typical of calk-alkaline magmatism (approximately NNO + 1), magnetite is either a liquidus or a near-liquidus phase in hydrous silicic andesite magmas, and this should stimulate reexamination for the mechanisms of generation of andesites by fractionation from basaltic parents.

Numerical computation of multivariate t-probabilities with application to power calculation of multiple contrasts

Abstract: A new method to calculate the multivariate t-distribution is introduced. We provide a series of substitutions, which transform the starting q-variate integral into one over the (q—1)-dimensional hypercube. In this situation standard numerical integration methods can be applied. Three algorithms are discussed in detail. As an application we derive an expression to calculate the power of multiple contrast tests assuming normally distributed data.

Does aluminium affect root growth of maize through interaction with the cell wall – plasma membrane – cytoskeleton continuum?

Abstract: The mechanism of aluminium-induced inhibition of root elongation is still not well understood It is a matter of debate whether the primary lesions of Al toxicity are apoplastic or symplastic The present paper summarises experimental evidence which offers new avenues in the understanding of Al toxicity and resistance in maize Application of Al for 1 h to individual 1 mm sections of the root apex only inhibited root elongation if applied to the first 3 apical mm The most Al-sensitive apical root zone appeared to be the 1–2 mm segment Aluminium-induced prominent alterations in both the microtubular (disintegration) and the actin cytoskeleton (altered polymerisation patterns) were found especially in the apical 1–2 mm zone using monoclonal antibodies Since accumulation of Al in the root apoplast is dependent on the properties of the pectic matrix, we investigated whether Al uptake and toxicity could be modulated by changing the pectin content of the cell walls through pre-treatment of intact maize plants with 150 mM NaCl for 5 days NaCl-adapted plants with higher pectin content accumulated more Al in their root apices and they were more Al-sensitive as indicated by more severe inhibition of root elongation and enhanced callose induction by Al This special role of the pectic matrix of the cell walls in the modulation of Al toxicity is also indicated by a close positive correlation between pectin, Al, and Al-induced callose contents of 1 mm root segments along the 5 mm root apex On the basis of the presented data we suggest that the rapid disorganisation of the cytoskeleton leading to root growth inhibition may be mediated by interaction of Al with the apoplastic side of the cell wall – plasma membrane – cytoskeleton continuum

Bovine dentin as a substitute for human dentin in shear bond strength measurements.

Abstract: PURPOSE: To evaluate the suitability of bovine coronal and root dentin as substitutes for human primary and permanent dentin in shear bond strength tests for dentin adhesives. MATERIALS AND METHODS: 30 bovine permanent central incisors, 30 human primary and 30 human third molars were cut mesiodistally. The pulpal and buccal dentin surfaces of the buccal cuts were milled with a bur to a thickness of 1.0 mm. A dentin adhesive and a hybrid composite were applied exactly according to the instructions given by the manufacturer on each buccal and pulpal surface, except in primary teeth where only the buccal surface was used. Shear bond strength was determined after 24 hours of storage in an aqueous solution. The results were statistically analyzed using the Wilcoxon- or the Mann-Whitney-U-test. RESULTS: There were no differences in shear bond strengths between human permanent dentin and bovine coronal dentin (13.3 +/- 6.1 vs. 15.2 +/- 7.6), and between each pulpal and buccal aspect. Significant differences were found between bovine root dentin and human primary dentin (17.4 +/- 8.3 vs. 7.7 +/- 5.0, P < 0.001). Significant differences were also found between bovine root dentin and human permanent dentin (17.4 +/- 8.3 vs. 13.3 +/- 6.1, P < 0.05), and bovine root and coronal dentin (17.4 +/- 8.3 vs. 15.2 +/- 7.6, P < 0.05). Furthermore, significant differences were recorded between human primary and human permanent dentin (7.7 +/- 5.0 vs. 13.3 +/- 6.1, P < 0.001) and bovine coronal dentin (7.7 +/- 5.0 vs. 15.2 +/- 7.6, P < 0.001).

Investigation of the LCST of polyacrylamides as a function of molecular parameters and the solvent composition

Abstract: The paper investigates the thermoprecipitation of two macromolecule structures, poly(N-isopropylacrylamide) (poly-NIPA) and poly(N,N-diethylacrylamide) (poly-DEA) from aqueous solution. The majority of the data are collected for small (M w < 5000 g/mol) homogeneous (D < 1.3) molecules of the indicated type synthesized by anionic, group transfer, and radical polymerization in the presence of a chain transfer agent. Conventional radical polymers (M w < 200,000 g/mol) are also synthesized and used for comparison. Turbidity curves (photometry) and transition enthalpies (high sensitivity differential scanning calorimetry) are measured to investigate the phase transition as a function of the molecular size and the tacticity as well as the concentration of certain solution additives (simple salts, glucose, and the surfactant tetrabutylammonium acetate) and mixtures thereof. Where applicable, the results are interpreted on the basis of a two-state model to gain insight in the cooperativity of the transition.

Fluctuation-Induced Transitions in a Bistable Surface Reaction: Catalytic CO Oxidation on a Pt Field Emitter Tip

Abstract: Fluctuations which arise in catalytic CO oxidation on a Pt field emitter tip have been studied with field electron microscopy as the imaging method. Fluctuation-driven transitions between the active and the inactive branch of the reaction are found to occur sufficiently close to the bifurcation point, terminating the bistable range. The experimental results are modeled with Monte Carlo simulations of a lattice-gas reaction model incorporating rapid CO diffusion. {copyright} {ital 1999} {ital The American Physical Society}

Semantics of (disjunctive) logic programs based on partial evaluation

Abstract: We present a new and general approach for defining, understanding, and computing logic programming semantics. We consider disjunctive programs for generality, but our results are still interesting if specialized to normal programs. Our framework consists of two parts: (a) a semantical , where semantics are defined in an abstract way as the weakest semantics satisfying certain properties, and (b) a procedural , namely a bottom-up query evaluation method based on operators working on conditional facts . As to (a), we concentrate in this paper on a particular set of abstract properties (the most important being the unfolding or partial evaluation property GPPE) and define a new semantics D-WFS, which extends WFS and GCWA. We also mention that various other semantics, like Fitting's comp 3 , Schipf's WFS c , Gelfond and lifschitz' STABLE and Ross and Topor's WGCWA (also introduced independently by Rajasekar et al. (A. Rajasekar, J. Lobo, J. Minker, Journal of Automated Reasoning 5 (1989) 293–307)), can be captured in our framework. In (b) we compute for any program P a residual program res(P) , and show that res(P) is equivalent to the original program under very general conditions on the semantics (which are satisfied, e.g., by the well-founded, stable, stationary, and static semantics). Many queries with respect to these semantics can already be answered on the basis of the residual program. In fact, res(P) is complete for D-WFS, WFS and GCWA.

Aqueous extracts from dentin adhesives contain cytotoxic chemicals

Abstract: It was the aim of our study to investigate the composition and cytotoxicity of aqueous elutes from five dentin adhesives currently used in clinical practice: Solobond Plus™, Solist™, Scotchbond Multipurpose™, Syntac SC™, and Prime & Bond™ 2.1. Water extracts were analyzed by gas chromatography/mass spectrometry (GC/MS) and relative quantities of identified compounds were compared by means of an internal caffeine standard [%CF]. The in vitro cytotoxic effects of substances released into DMEM were determined using immortalized 3T3-fibroblast cultures. In addition, the cytotoxicity of ethylene glycol (EG), which was identified in the extracts of Syntac SC, was evaluated. All dentin adhesives tested released various chemical components, like comonomers (mainly ethylene glycol compounds), HEMA, and initiating substances (e.g., camphorquinone). Elutes of Solobond Plus, which contained very high amounts of TEGDMA, were extremely cytotoxic. Two bonding agents (Scotchbond Multi-purpose, Syntac SC), which released significant quantities of HEMA, induced severe cytotoxic effects. In contrast, extracts from Solist and Prime & Bond 2.1 had very small effects on cell proliferation; these elutes contained small amounts of released chemical compounds. EG, a product of HEMA hydrolysis, in concentrations ranging from 0.025–25 mM was not cytotoxic. In summary, these results provide evidence that all dentin adhesives tested in the present study release in aqueous media chemical compounds some of which (for example, TEGDMA and HEMA) are cytotoxic. © 1999 John Wiley & Sons, Inc. J Biomed Mater Res (Appl Biomater) 48: 772–777, 1999

Long-term results after placement of dental implants: longitudinal study of 1,964 implants over 16 years.

Abstract: In a retrospective study, Kaplan-Meier implant survival analyses were conducted on 883 patients with 1,964 implants of various systems placed, followed up, documented, and statistically evaluated at an oral surgery and dentistry practice between January 1981 and January 1997. The goal of this study was to evaluate the success of osseointegrated implants of the Branemark, Frialit-1 (Tubinger Implant), Frialit-2, and IMZ systems and Linkow blade implants. For all systems, mandibular implants were generally more successful than maxillary implants. The preprosthetic loss rate was 1.9%, and 4.3% of implants were lost after prosthetic treatment. The lowest loss rates were seen with implants in intermediate and distal extension spaces and with single-tooth replacements using IMZ, Frialit-2, and Branemark implants. In edentulous arches, implants of the IMZ and Branemark systems had the lowest failure rates.

Morphological productivity across speech and writing

Abstract: Claims about the productivity of a given affix are generally made without differentiating productivity according to type of discourse, although it is commonly assumed that certain kinds of derivational suffixes are more pertinent in certain kinds of texts than in others. Conversely, studies in register variation have paid very little attention to the role derivational morphology may play in register variation.This paper explores the relation between register variation and derivational morphology through a quantitative investigation of the productivity of a number of English derivational suffixes across three types of discourse in the British National Corpus (written language, context-governed spoken language, and everyday conversations). Three main points emerge from the analysis. First, within a single register, different suffixes may differ enormously in their productivity, even if structurally they are constrained to a similar extent. Second, across the three registers under investigation a given suffix may display vast differences in productivity. Third, the register variation of suffixes is not uniform, i.e. there are suffixes that show differences in productivity across registers while other suffixes do not, or do so to a lesser extent. We offer some tentative explanations for these findings and discuss the implications for morphological theory.

Quantification of bacterial polyhydroxyalkanoic acids by Nile red staining.

Abstract: The fluorescence properties of one chemically and seven biologically produced polyhydroxyalkanoic acids were investigated as film castings and in living cells respectively after staining with Nile red. All these polyesters show a similar fluorescence behaviour, revealing a clear fluorescence maximum at an excitation wavelength between 540 nm and 560 nm and an emission wavelength between 570 nm and 605 nm. This could be shown by the use of two-dimensional fluorescence spectroscopy and flow cytometry. The examination of native poly(3-hydroxybutyric acid), poly(3HB), granules isolated from cells of Ralstonia eutropha H16 showed that the addition of 6.0 μg Nile red is necessary for total staining of 1.0 mg granules. The fluorescence intensity at an excitation wavelength of 550 nm and an emission wavelength of 600 nm showed high correlation to the poly(3HB) concentration of grana suspensions at different grana concentrations. These results and the staining of cell suspensions during cultivation experiments revealed that Nile red has a high potential for the quantitative determination of hydrophobic bacterial polyhydroxyalkanoic acids.

How many molecules form a liquid

Abstract: Broad-band dielectric spectroscopy is employed to study the molecular dynamics of (dielectrically active) glass-forming liquids which are confined to (dielectrically inactive) zeolites and nanoporous glasses. For the H-bond-forming liquid ethylene glycol (EG) embedded in zeolites of different sizes and topologies one observes a sharp transition from a single-molecule dynamics (with an Arrhenius-type temperature dependence) to that of a liquid (with a temperature dependence of the mean relaxation rate following a Vogel-Fulcher-Tammann (VFT) law): while EG in silicalite (showing a single-molecule relaxation) has a coordination number of four, EG in zeolite beta or -5 has a coordination number of five and behaves like a bulk liquid. For the H-bonded liquid propylene glycol confined to (uncoated and silanized) nanopores (pore sizes: 2.5 nm, 5.0 nm and 7.5 nm), a molecular dynamics is observed which is comparable to that of the bulk liquid. Due to surface effects in uncoated nanopores, the relaxation time distribution is broadened on the long-term side and the mean relaxation rate is decreased by about half a decade. This effect can be counterbalanced by lubricating the inner surfaces of the pores. That causes the molecular dynamics of the molecules inside the pores to decouple from the solid walls and the resulting relaxation rate becomes slightly faster compared to that for the bulk liquid. For the `quasi'-van der Waals liquid salol confined to silanized nanopores, the molecular dynamics is completely different to that for the H-bonded systems: over a wide temperature range, the dynamics of the confined system is identical to that of the bulk liquid. But with decreasing temperature, a sharp pore-size-dependent transition is found from a VFT-type to an Arrhenius-type temperature dependence. This reflects the inherent length scale of cooperativity of the dynamic glass transition.

Interfacial area density in bubbly flow

Abstract: The interfacial area per unit volume is one of the key parameters in bubbly flow. Momentum, mass and energy transfer occur through the interface between the phases. The functionality of two phase reactors with bubbly flow depends mainly on these three transfer processes. Thus, the design process of a reactor requires the prediction of interfacial area density. In the present work a simple equation for the interfacial area density is derived from the population balance, taking into account the events of coalescence and bubble break-up for each bubble fraction. The system of partial integro-differential equations is simplified. Since the integrals in these equations complicate a numerical treatment. This reduces the balance to one single partial differential equation. An approximate analytical solution is given. If the resulting equation is applied to large gas fluxes, the instability of the coalescence process causes large bubbles and gas plugs to develop. From the instability the volume fraction of the large bubbles and gas plugs may be predicted. Additives may hinder the coalescence process. Experiments show that coalescence hindrance changes the coalescence kernel only by a factor. Calculations are done for bubble columns and vertical pipe flow.

Vibration of a periodically supported beam on an elastic half-space

Abstract: The steady-state vibration of a periodically supported beam on an elastic half-space under a uniformly moving harmonically varying load is investigated. The concept of equivalent stiffness of a half-space is used for problem analysis. It is shown that the half-space can be replaced by a set of identical springs placed under each support of the beam. The equivalent stiffness of these springs is a function of the frequency of the beam vibrations and of the phase shift of vibrations of neighboring supports. It is found that the equivalent stiffness is equal to zero for some relationship between the frequency and the phase shift. The reason for this is that the surface waves generated by all supports can come to any support in phase, providing an infinite displacement. It is demonstrated that the equivalent stiffness has a real and an imaginary part. The imaginary part arises due to radiation of waves in the half-space. The expressions are derived for the steady-state response of the beam to the moving load. The limiting case of a constant load is considered, showing that the load moving with the Rayleigh wave velocity causes resonance in the system.

Learning and Behavioral Stability — An Economic Interpretation of Genetic Algorithms

Abstract: This article tries to connect two separate strands of literature concerning genetic algorithms. On the one hand, extensive research took place in mathematics and closely related sciences in order to find out more about the properties of genetic algorithms as stochastic processes. On the other hand, recent economic literature uses genetic algorithms as a metaphor for social learning. This paper will face the question of what an economist can learn from the mathematical branch of research, especially concerning the convergence and stability properties of the genetic algorithm. It is shown that genetic algorithm learning is a compound of three different learning schemes. First, each particular scheme is analyzed. Then it is shown that it is the combination of the three schemes that gives genetic algorithm learning its special flair: A kind of stability somewhere in between asymptotic convergence and explosion.

Parameter identification of gradient enhanced damage models with the finite element method

Abstract: In this contribution an algorithm for parameter identification of gradient enhanced damage models is proposed, in which non-uniform distributions of the state variables such as stresses, strains and damage variables are taken into account. To this end a least-squares functional consisting of experimental data and simulated data is minimized, whereby the latter are obtained with the Finite-Element-Method. In order to improve the efficiency of the minimization process, a gradient-based optimization algorithm is applied, and therefore the corresponding sensitivity analysis for the coupled variational problem is described in a systematic manner. For illustrative purpose, the performance of the algorithm is demonstrated for a square panel under tension, in which an isotropic gradient damage model is used.

Prediction of existing gully erosion in vineyard parcels of the NE Spain: a logistic modelling approach

Abstract: This paper presents a study of determining factors and a method to predict the existence of gully erosion in vineyard parcels. The Alt Penedes-Anoia region (Catalonia, NE Spain), mainly dedicated to the cropping of vineyards for high quality wine production, was selected as the case study area. Single factors related to the existence of gully erosion were determined by means of statistical tests (Student's t-test and chi-square). The existence of gully erosion was predicted by means of a multivariate procedure. A stepwise selection of variables (relief, soil, land use/management characteristics) was performed, which allowed the identification of factors that present a significant relationship with the existence of gully erosion. The selected factors, slope degree and planar slope form, were considered as independent variables in a logistic regression of binary response. The model computes the probability that gullies exist in given vineyard parcels, and it can be implemented in a raster-based geographical information system (GIS). The assessment of the model in 52 parcels, different from the training data set, yielded an overall accuracy of 84.6%. The predictive model can be applied for areas with similar conditions, but the modelling approach can be applied in other different areas.