5,610 research outputs found
An ecophysiological approach to crop losses exemplified in the system wheat, leaf rust, and glume blotch
No full textThe motive of the author to embark upon the research reported here was the need to develop new concepts to approach the problem of crop losses, and eventually the problem of their prevention by means of disease control and plant breeding. The introduction of ecophysiology as an approach, explained above, was the first result. An ecophysiological treatment of processes like growth and development of plants in health and disease necessitates adjustments of current thoughts on experimental techniques and on organization in research (Van der Wal and Cowan, 1974).In order to measure plant or aegricorpus responses concurrently with environmental factors in climate chamber and field experiments during periods Of months, a great variety of instruments had to be bought, modified, or devised. The climate chamber had to be adjusted to allow for high light intensities with a view to growing wheat under conditions approximating those of early summer in the Netherlands; the yield of the spring wheat 'Kolibri' was c. 0.5 kg.m -2, which is nearly equivalent to the average field production of the country. Regulations for the prevention and control of contamination by aphids and mildew without any use of chemicals were issued after detailed experimentation (Van der Wal, unpubl.). Several instruments have been developed, usually with the help of others; this has led to two publications (Schurer and Van der Wal, 1972; Tegelaar and Van der Wal, 1974). These efforts resulted in the experimental designs described in the appendix. Field experiments, conducted in the same period as the climate chamber experiments reported here, will be published later. It is felt that the techniques to record growth of pathogens are still inadequate.The results reported in the appendix show that the 'state' of the plant at the time of infection is a major determinant of the plant's future behaviour, which plant breeders and pathologists can express in terms of resistance and tolerance. The conceptual framework presented above may contribute to a future revision of crop husbandry and crop protection practices
Setting the stage for death : new themes in the euthanasia debate
Full text linkMaas, P.J. van der [Promotor]Wal, G. van der [Promotor]Heide, A. van der [Copromotor]Philipsen, B.D. [Copromotor
Electrochemical characterization of the bacterial cell surface
No full textBacterial cells are ubiquitous in natural environments and also play important roles in domestic and industrial processes. They are found either suspended in the aqueous phase or attached to solid particles. The adhesion behaviour of bacteria is influenced by the physico-chemical properties of their cell surfaces, such as hydrophobicity and cell wall charge. The charge in the bacterial wall originates from carboxyl, phosphate and amino groups. The degree of dissociation of these anionic and cationic groups is determined by the pH and the activity of the surrounding electrolyte solution. Almost all bacterial cells are negatively charged at neutral pH, because the number of carboxyl and phosphate groups is generally higher than that of the amino groups. The presence of the charged cell wall groups leads to the spontaneous formation of an electrical double layer. The purpose of the present investigation is to elucidate the structure of the electrical double layer of bacterial cell surface. Such a study serves at least two goals. It allows the quantification of electrostatic interactions in the adhesion process and it contributes to gain better insight into the availability of (in)organic compounds for bacterial cells.The characteristics of the electrical double layer of bacterial cell surfaces have been revealed by applying a combination of experimental techniques, which include: chemical cell wall analysis, potentiometric proton titration and electrokinetic studies such as micro-electrophoresis, static conductivity and dielectric dispersion measurements.For the present study five Gram-positive bacterial strains, including four coryneforms and a Bacillus brevis, have been selected. Cell walls of these bacterial strains have been isolated and were subsequently subjected to chemical analyses and proton titration studies. Both methods provide information on the number of carboxyl, phosphate and amino groups.The chemical analysis of isolated cell walls involves the quantitative determination of both peptidoglycan and protein content. These analyses indicate that the chemical composition of the walls of the coryneforms are very similar, but considerably different from that of Bacillus brevis. Peptidoglycan is an important cell wall constituent of the coryneform bacteria and determines about 23 to 31 % of the cell wall dry weight. The protein fractions are somewhat lower, between 7 to 14%. The cell wall structure of the Bacillus brevis strain is more complex and multi-layered. It contains a thin peptidoglycan layer, which only determines 5 % of the cell wall dry weight. On the other hand, the protein content of these walls is higher than 56%. These proteins most likely can be attributed to a so-called S(urface)-layer, which is the outermost cell wall layer.The surface charge density of the bacterial cells is assessed by proton titrations of isolated cell walls at different electrolyte concentrations. Rather high values, i.e. between 0.5 and 1.0 C/m 2are found at neutral pH. The absence of hysteresis in the titration curves leads to the conclusion that the charging process can be considered as reversible. It also implies that the cell wall charge is continuously in equilibrium with the surrounding electrolyte solution, at any pH and salt concentration. This observation considerably facilitates the interpretation of the titration curves, because it allows a rigorous (thermodynamic) analysis. The anionic and cationic groups in the bacterial wall could be identified and their numbers determined by representing the differential titration curves as functions of pH and cell wall charge. The carboxyl and phosphate groups are almost entirely titrated in the pH range accessible by proton titration, allowing precise estimation of their numbers. These numbers compare very well with those based on a chemical analysis of the isolated cell walls. Estimates for the number of amino groups were less accurate, because these groups are only partly titrated in the pH range were precise titration measurements are feasible. Nevertheless, it could be concluded that the number of amino groups in the bacterial wall are lower than those of the carboxyl groups.Information about the ionic composition of the countercharge has been obtained from Esin-Markov analysis of the titration curves and from estimates of the cell wall potential based on a Donnan-type model. The Esin-Markov analysis is purely thermodynamic and based on first principles, whereas the Donnan model requires several assumptions about the structure of the bacterial wall. Both approaches lead to the same conclusion that at salt concentrations below 0.01 M the cell wall charge is predominantly compensated by counterions, with the excluded co-ions hardly contributing to the countercharge. This observation has considerably facilitated the interpretation of the electrokinetic properties of bacterial cell suspensions.Electrophoresis, static conductivity and dielectric response are related (electrokinetic) techniques and therefore share common physical bases. This also implies that the physical and mathematical problems that have to be solved in order to interpret the experimental data are very similar. Analytical solutions only exist for colloidal particles for which the electrical double layer is very thin compared to the particle dimensions. Most bacterial cells are relatively large colloidal particles and therefore the largeKa theory may be of help in the evaluation of their electrokinetic properties. However, the original theories do not include surface conductance in the hydrodynamically stagnant layer. Therefore, they had to be extended to account for the finite conductivity of ions in the bacterial wall.Static conductivity and dielectric dispersion both show that the counterions in the bacterial wall give rise to a considerable surface conductance. From a comparison of the mobile charge with the total cell wall charge it is inferred that the mobilities of the counterions in the bacterial wall are of the same order but somewhat lower than those in the electrolyte solution.Due to surface conductance the electrophoretic mobility may be strongly retarded compared to the classical Helmholtz-Smoluchowski theory, especially at low electrolyte concentrations. In 1 mM and 10 mM electrolyte solution, the Helmholtz-Smoluchowski equation underestimates the ζ-potential by approximately a factor of 2 and 1.3, respectively.Resolving the fundamentals of the electrochemical characteristics of bacterial cell surfaces is a key step towards a quantitative understanding of the electrostatic interactions of bacterial cells with their surroundings. The success of such an investigation depends on the state of the art of the disciplines involved. Both microbiology and colloid chemistry have the microscopically small particle as object of study. Until recently there has hardly been any exchange of scientific knowledge between these two disciplines, despite their common interest. Colloid chemists prefered to study relatively simple particles to test their basic theories and bacterial cells were considered far too complex to serve as model colloids. However, the progress that has been made during the last decades in both colloid chemistry and microbiology provide the right tools for a successful cooporation. The present study is born from such a symbiosis and shows that many physicochemical characteristics of bacterial cell surfaces are accessible with (classical) colloid chemical techniques. In fact, for testing more advanced colloid chemical theories bacteria may even be better model particles than the generally used ionorganic colloids, because of their ability to produce a homogeneous population of identical cells.For the time being only Gram-positive strains have been considered, because of their relatively less complex cell wall structures. Nevertheless, the techniques used may mutatis mutandis also be applied to Gram-negative cells. In fact, such a study would be highly interesting, because it would contribute to a more complete description of the composition of the electrical double layer of bacterial cell surfaces
Policies and guidelines on end-of-life care decision-making in Dutch health care institutions
Full text linkPhilipsen, B.D. [Promotor]Wal, G. van der [Promotor]Pasman, H.R.W. [Copromotor]Heide, A. van der [Copromotor
The struggle behind I'm all right : Response shifts and self-presentation in small-cell lung cancer patients
Full text linkWal, G. van der [Promotor]Hak, A. [Copromotor]The, B.A.M. [Copromotor
Unbearable suffering: A study into suffering in end-of-life cancer patients and requests for euthanasia
Full text linkPhilipsen, B.D. [Promotor]Wal, G. van der [Promotor]Kerkhof, A.J.F.M. [Copromotor
Diagnostic reasoning and diagnostic error in medicine
Full text linkTimmermans, D.R.M. [Promotor]Wal, G. van der [Promotor]Thijs, A. [Copromotor]Wagner, C. [Copromotor
Career preference of medical students and career choice of recent graduates : Factors influencing the preference for a choice of a medical speciality in general and in public health in particular
Full text linkWal, G. van der [Promotor]Cate, T.H.J. ten [Promotor
A 3D Glacial Isostatic Adjustment model for Northwestern Europe
No full textThe Earth is subjected to 100,000 year cycles of glaciation and deglaciation. The deformations induced by glacial and oceanic loading and the continuous attempt at recovery of the isostatic equilibrium within the solid Earth, are referred to as Glacial Isostatic Adjustment (GIA). This process is ongoing still and yields a large contribution to present day surface deformation and sea level change in formerly glaciated areas. In order to accurately model GIA, the lateral viscosity variations within the interior of the Earth are accounted for (Kaufmann et al., 2000; Steffen et al., 2006; Wu and van der Wal, 2003). Additionally, an increased level of accuracy is obtained by adopting a combination of linear and non-linear viscoelasticity as demonstrated by Barnhoorn et al. (2011); Forno and Gasperini (2007); van der Wal et al. (2013);Wu and Wang (2008). During the Last Glacial Maximum, the British-Irish and Fennoscandian Ice Sheets covered large parts of Northwestern Europe. The interior of the Earth in this area is known to consist of material of very heterogeneous tectonic origin (Artemieva et al., 2006). Additionally, research in this area is promoted by the availability of the independent regional ice model Bradley2018 (Bradley, personal communication), an RSL observation database for the Rhine-Meuse Delta (Hijma and Cohen, 2019), and a collection of GPS derived uplift rates throughout Europe (Teferle, personal communication). At the Astrodynamics and Space Missions research group of Delft University of Technology, a 3D GIA FEM model has been developed to model GIA in Antarctica (Blank et al., 2017). This model follows the work by Wu (2004) and van der Wal et al. (2013), and is complemented with an iterative algorithm to solve the sea level equation in accordance with Kendall et al. (2005). This research aims to provide a single GIA model best suited for the prediction of GIA induced vertical surface deformation in Northwestern Europe, by adapting the existing model. In doing so, a better understanding of the interior of the Earth in Northwestern Europe can be achieved.The response of the Earth is dictated by the composite rheology creep flow laws for olivine (Hirth and Kohlstedt, 2003). By varying the grain size as well as the water content of the mantle material, and by implementing a global temperature model of the Earth’s interior, four 3D composite rheology Earth models are obtained. The fifth Earth model considered is the radially symmetric VM5a viscosity profile developed by Peltier et al. (2015) in conjunction with the global ice model ICE-6G_C. The performance of all five Earth model configurations in combination with both the ICE-6G_C model and the Bradley2018 model is analysed in terms of relative sea level and uplift rates.It is found that the Bradley2018 model is the preferred ice model for GIA modelling in Northwestern Europe. The ICE-6G_C model outperforms the Bradley2018 model at far-field RSL sites, which is attributed to its superior representation of global eustatic sea level rise. The 3D composite rheologies lead to improved fits to RSL observations for the majority of the investigated measurement sites compared to the 1D scenario. The dry 4 mm grain size rheology yields the best overall performance out of all rheological configurations considered. A preference towards wet rheology exists in regions of Sveco-Norwegian tectonic origin. The strongest rheology is preferred in the mid-west of Scotland. No definitive connection is found between the local tectonic origin and preferred rheology fromRSL simulations. It is believed that this analysis may benefit from the inclusion of laterally varying grain sizes and water content inferred from geophysical observations, as well as the extension of the variable space for the water content.For both ice models, an improved fit to observed uplift rates can be obtained through the application of a 3D composite rheology. The GPS derived uplift rates can be reproduced best using the dry 10 mm grain size rheology in combination with the Bradley2018 ice history. This model is deemed to be best suited for simulation of GIA induced uplift rates in Northwestern Europe. The second-best performance in terms of uplift rate is found using the 1D Earth model. In Scandinavia the 4 mm dry rheology yields uplift rates equal to roughly half the observed uplift rates, while the uplift rates for the 10 mm and 4 mm wet rheologies are near-zero. In the far-field, where other surface deformation mechanisms may infer a larger deformation rate than GIA (Fokker et al., 2018), no model could reproduce the observed uplift rates. The presence of a high viscosity anomaly beneath Eastern Fennoscandia is captured by the 3D rheologies and results in a shift of the centre of positive and negative uplift rates. As the spatial distribution of minima and maxima in both uplift rates and RSL change rates is sensitive to the inclusion of a 3D rheology, this should be accounted for in future regional sea level change and surface deformation projections.Aerospace Engineerin
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