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Results for “Spatial heterogeneity”

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PubMed2006

Dynamics of spatial heterogeneity of stomatal closure in Tradescantia virginiana altered by growth at high relative air humidity.

The spatial heterogeneity of stomatal closure in response to rapid desiccation of excised well-watered Tradescantia virginiana leaves grown at moderate (55%) or high (90%) relative air humidity (RH) was studied using a chlorophyll fluorescence imaging system under non-photorespiratory conditions. Following rapid desiccation, excised leaves grown at high RH had both a greater heterogeneity and a higher average value of PSII efficiency (Phi(PSII)) compared with leaves grown at moderate RH. Larger decreases in relative water content resulted in smaller decreases in water potential and Phi(PSII) of high RH-grown leaves compared with moderate RH-grown leaves. Moreover, the Phi(PSII) of excised high RH-grown leaves decreased less with decreasing water potential, implying that the stomata of high RH-grown leaves are less sensitive to decreases in leaf water potential compared with moderate RH-grown leaves. After desiccation, some non-closing stomata were distributed around the main vein in high RH-grown leaves. Direct measurements of stomatal aperture showed 77% stomatal closure in the margins after 2 h desiccation compared with 40% closure of stomata in the main-vein areas in high RH-grown leaves. Faster closure of stomata in leaf margins compared with main-vein areas of leaves grown at high RH was related to substantially lower relative water content in these areas of the leaves.

Desiccation↗

PubMed2005

Population dynamics in spatially heterogeneous systems with drift: The generalized contact process.

We investigate the time evolution and stationary states of a stochastic, spatially discrete, population model (contact process) with spatial heterogeneity and imposed drift (wind) in one and two dimensions. We consider in particular a situation in which space is divided into two regions: an oasis and a desert (low and high death rates). Carrying out computer simulations we find that the population in the (quasi) stationary state will be zero, localized, or delocalized, depending on the values of the drift and other parameters. The phase diagram is similar to that obtained by Nelson and coworkers from a deterministic, spatially continuous model of a bacterial population undergoing convection in a heterogeneous medium.

Algorithms↗

PubMed1991

Bifurcating spatially heterogeneous solutions in a chemotaxis model for biological pattern generation.

We consider a simple cell-chemotaxis model for spatial pattern formation on two-dimensional domains proposed by Oster and Murray (1989, J. exp. Zool. 251, 186-202). We determine finite-amplitude, steady-state, spatially heterogeneous solutions and study the effect of domain growth on the resulting patterns. We also investigate in-depth bifurcating solutions as the chemotactic parameter varies. This numerical study shows that this deceptively simple-chemotaxis model can produce a surprisingly rich spectrum of complex spatial patterns.

Animals↗

PubMed1996

Spatial heterogeneity of blood flow in the dog heart. I. Glucose uptake, free adenosine and oxidative/glycolytic enzyme activity.

The spatial heterogeneity of myocardial perfusion and metabolism was studied in 11 anaesthetized dogs under resting conditions. In each heart local myocardial blood flow was assessed using the tracer microsphere technique in 256 samples (mean mass: 83.1 mg) taken from the left anterior ventricular wall. In the same samples, the following biochemical parameters were determined: accumulation of [3H]-deoxyglucose (a measure of glucose uptake), free cytosolic adenosine (S-adenosylhomocysteine accumulation technique, a measure of tissue oxygenation and a possible mediator of blood flow regulation), and the specific activities of oxidative (citrate synthase, cytochrome-c-oxidase) and glycolytic (hexokinase, phosphoglycerate kinase) enzymes. Capillary density and mitochondrial and myofibril volume densities were determined by morphometry. Myocardial perfusion in each sample (average 0.77 ml min-1 g-1) varied between 0.1 and 2.5 times the mean (coefficient of variation 0.30+/-0.02). [3H]-deoxyglucose was deposited locally in proportion to perfusion. Samples showing low flow (<0.2 ml min-1 g-1) did not exhibit increased levels of cytosolic adenosine. The specific activities of the oxidative and glycolytic enzymes, however, were uniformly distributed between low and high flow areas. Furthermore, capillary density and mitochondrial and myofibril densities were similar in high and low flow regions. The results show firstly that local glucose metabolism in the heart occurs in proportion to local blood flow, suggesting that high flow regions have a higher than average metabolic rate. Secondly, regions of low flow are not compromized by critical oxygenation and most likely have a lower than average oxygen demand and finally, the homogeneous distribution of oxidative and glycolytic enzymes, as well as the homogeneous myocardial ultrastructure, suggest that areas with high and low blood flow under resting conditions may increase their metabolic rate to similar levels when required.

Adenosine↗

PubMed2000

Spatially heterogeneous dynamics in supercooled liquids.

Although it has long been recognized that dynamics in supercooled liquids might be spatially heterogeneous, only in the past few years has clear evidence emerged to support this view. As a liquid is cooled far below its melting point, dynamics in some regions of the sample can be orders of magnitude faster than dynamics in other regions only a few nanometers away. In this review, the experimental work that characterizes this heterogeneity is described. In particular, the following questions are addressed: How large are the heterogeneities? How long do they last? How much do dynamics vary between the fastest and slowest regions? Why do these heterogeneities arise? The answers to these questions influence practical applications of glass-forming materials, including polymers, metallic glasses, and pharmaceuticals.

Journal Article↗

PubMed2004

Modelling a spatially heterogeneous biofilm and the bulk fluid: selected results from benchmark problem 2 (BM2).

The numerical simulation of mass transfer and conversion in spatially heterogeneous biofilms on the meso-scale requires an accurate description of the hydrodynamics in the biofilm systems and of spatial effects. This leads to systems of three-dimensional nonlinear partial differential equations that are numerically very expensive to solve and to data requirements that are not easy to meet. In this paper several modeling approaches to reduce the physical complexity and, hence, accelerate the computation are compared. They range from a mere reduction of dimensionality by lumping the problem along a secondary flow direction to global mass balances or empirical correlations, at the core of which a one-dimensional boundary value problem must be solved. It is found that even strongly simplified models can describe the qualitative behaviour of the model with regard to variations in the geometrical and hydrodynamic model parameters quite well. In order to obtain also quantitatively reliable results the hydrodynamics must be considered in an appropriate manner.

Bacteria↗

PubMed1999

Spatial heterogeneity of bacterial populations along an environmental gradient at a shallow submarine hydrothermal vent near Milos Island (Greece).

The spatial heterogeneity of bacterial populations at a shallow-water hydrothermal vent in the Aegean Sea close to the island of Milos (Greece) was examined at two different times by using acridine orange staining for total cell counts, cultivation-based techniques, and denaturing gradient gel electrophoresis (DGGE) analysis of PCR-amplified 16S rRNA gene fragments. Concurrent with measurements of geochemical parameters, samples were taken along a transect from the center of the vent to the surrounding area. Most-probable-number (MPN) counts of metabolically defined subpopulations generally constituted a minor fraction of the total cell counts; both counting procedures revealed the highest cell numbers in a transition zone from the strongly hydrothermally influenced sediments to normal sedimentary conditions. Total cell counts ranged from 3.2 x 10(5) cells ml(-1) in the water overlying the sediments to 6.4 x 10(8) cells g (wet weight) of sediment(-1). MPN counts of chemolithoautotrophic sulfur-oxidizing bacteria varied between undetectable and 1.4 x 10(6) cells g(-1). MPN counts for sulfate-reducing bacteria and dissimilatory iron-reducing bacteria ranged from 8 to 1.4 x 10(5) cells g(-1) and from undetectable to 1.4 x 10(6) cells g(-1), respectively. DGGE revealed a trend from a diverse range of bacterial populations which were present in approximately equal abundance in the transition zone to a community dominated by few populations close to the center of the vent. Temperature was found to be an important parameter in determining this trend. However, at one sampling time this trend was not discernible, possibly due to storm-induced disturbance of the upper sediment layers.

Acridine Orange↗

PubMed2003

Control of spatially heterogeneous and time-varying cellular reaction networks: a new summation law.

A hallmark of a plethora of intracellular signaling pathways is the spatial separation of activation and deactivation processes that potentially results in precipitous gradients of activated proteins. The classical metabolic control analysis (MCA), which quantifies the influence of an individual process on a system variable as the control coefficient, cannot be applied to spatially separated protein networks. The present paper unravels the principles that govern the control over the fluxes and intermediate concentrations in spatially heterogeneous reaction networks. Our main results are two types of control summation theorems. The first type is a non-trivial generalization of the classical theorems to systems with spatially and temporally varying concentrations. In this generalization, the process of diffusion, which enters as the result of spatial concentration gradients, plays a role similar to other processes such as chemical reactions and membrane transport. The second summation theorem is completely novel. It states that the control by the membrane transport, the diffusion control coefficient multiplied by two, and a newly introduced control coefficient associated with changes in the spatial size of a system (e.g., cell), all add up to one and zero for the control over flux and concentration. Using a simple example of a kinase/phosphatase system in a spherical cell, we speculate that unless active mechanisms of intracellular transport are involved, the threshold cell size is limited by the diffusion control, when it is beginning to exceed the spatial control coefficient significantly.

Animals↗

PubMed1995

Proton spin diffusion for spatial heterogeneity and morphology investigations of polymers.

The diffusion of Zeeman spin order in the laboratory frame for a system with microscopic spatial heterogeneity, due to either molecular mobility or chemical heterogeneity, has been analysed by solving the diffusion equation in the limit of two different models. By simulation of proton spin diffusion nuclear magnetic resonance (NMR) spectral areas for poly(ethylene oxide) we have demonstrated, for both models, the sensitivity of these observables to phase domain size and to the dimensionality of the diffusion process. The possibility to obtain information about the morphology is analysed. Results from calculations based on the two models are compared with experimental spin diffusion results from 1H NMR spectra of poly(ethylene oxide). Two methods for filtering the z-magnetization from the different phase domains in a solid are discussed. The efficiency of a magnetization dipolar filter based on magic and polarization echoes is presented.

Magnetic Resonance Spectroscopy↗

PubMed2003

Multi-scale variation in spatial heterogeneity for microbial community structure in an eastern Virginia agricultural field.

To better understand the distribution of soil microbial communities at multiple spatial scales, a survey was conducted to examine the spatial organization of community structure in a wheat field in eastern Virginia (USA). Nearly 200 soil samples were collected at a variety of separation distances ranging from 2.5 cm to 11 m. Whole-community DNA was extracted from each sample, and community structure was compared using amplified fragment length polymorphism (AFLP) DNA fingerprinting. Relative similarity was calculated between each pair of samples and compared using geostatistical variogram analysis to study autocorrelation as a function of separation distance. Spatial autocorrelation was found at scales ranging from 30 cm to more than 6 m, depending on the sampling extent considered. In some locations, up to four different correlation length scales were detected. The presence of nested scales of variability suggests that the environmental factors regulating the development of the communities in this soil may operate at different scales. Kriging was used to generate maps of the spatial organization of communities across the plot, and the results demonstrated that bacterial distributions can be highly structured, even within a habitat that appears relatively homogeneous at the plot and field scale. Different subsets of the microbial community were distributed differently across the plot, and this is thought to be due to the variable response of individual populations to spatial heterogeneity associated with soil properties.

Bacteria↗

PubMed2004

Spatial heterogeneity of endocardial voltage amplitude in viable, chronically dysfunctional myocardium.

BACKGROUND: Although electromechanical mapping has been used to assess cardiac physiology, interpretation is dependent upon the spatial variability of endocardial voltage and local shortening in normal and viable dysfunctional myocardium, which is currently unknown. METHODS: NOGA mapping was performed in 13 pigs with an established model of viable dysfunctional myocardium produced by a chronic LAD stenosis, and five uninstrumented controls. Voltage maps (122 +/- 7 points each) were obtained in the closed-chest anesthetized state, and (18)F-2-deoxyglucose uptake and TTC staining confirmed viability. RESULTS: There were systematic regional variations in voltage amplitude in both chronically-instrumented and control animals. Unipolar voltage was ~15% higher in LAD-supplied versus remote myocardium (10.8 +/- 0.3 vs. 8.9 +/- 0.4 mV, p < 0.001), with a similar relative difference in controls (14.0 +/- 0.5 vs. 12.0 +/- 0.4 mV, p < 0.02). In contrast, bipolar voltage was ~35% lower in the LAD territory of both groups (2.2 +/- 0.2 vs. 3.5 +/- 0.2 mV, p < 0.01 and 3.1 +/- 0.3 vs. 5.1 +/- 0.3 mV in controls, p < 0.01). The relative dispersion (SD/mean) of voltage was large, but significantly lower for unipolar versus bipolar measurements (39 +/- 1% vs. 70 +/- 2%, p < 0.001). Variability between hearts was partially related to end-systolic volume (r = 0.58, p < 0.05). Linear local shortening measurements were insensitive to detect anterior hypokinesis. CONCLUSIONS: Our data demonstrates significant regional and spatial heterogeneity of endocardial voltage and NOGA-derived linear shortening in normal and viable dysfunctional myocardium, with large confidence intervals for individual measurements. Even though the absence of necrosis in this model precludes assessment of the sensitivity and specificity of NOGA mapping to identify infarction, our findings highlight important methodological limitations in using electromechanical mapping to determine viability.

Animals↗

PubMed1996

Fitness patterns and phenotypic plasticity in a spatially heterogeneous environment.

We analyse patterns of the means and variances of genotypic fitnesses across different niches in a randomly mating haploid population. The population inhabits a spatially heterogeneous environment where it is subject to mutation and weak multilocus additive selection, with different election coefficients in different niches. Approximate analytical expressions are derived for the stationary mean and variance of genotypic fitnesses among the niches in terms of environmental and genetic parameters. As a special case, we analyse an environment described by a variable t, distributed among the niches with mean t(star) and variance D(star) and quadratic decrease in correlation between environments as a function of the difference in values of t. If the niches have the same qualities, the mean and variance of genotypic fitnesses evolve to be quadratic functions of t that achieve their maximum and minimum, respectively, at t(star). With unequal niche qualities, these are non-polynomial functions that attain their extrema at different, usually intermediate values of t, although the coefficient of variation of the genotypic fitnesses still attains its minimum near t(star). The functions involve the total mutation rate, the combination of the loci to genotypic fitnesses, and the frequency and quality distributions of the niches. Thus, for this relatively simple model the norms of reaction may be calculated in terms of the detailed properties of the environmental heterogeneity, and the genetic system.

Biological Evolution↗

PubMed2005

[Spatial heterogeneity of soil moisture after raining at forest-grassland landscape boundary in hilly area of Loess Plateau].

Soil moisture is a main factor limiting vegetation restoration in semi-arid region. In this paper, the spatial variability of different layers soil moisture after raining at the forest-grassland boundary in hilly area of Loess Plateau were studied by traditional and geostatistical analysis methods. The results showed that the moisture content in surface (0 - 10 cm) and subsurface soil layer (10 - 20 cm) of grassland was higher than that of forestland. The two layers soil moisture content at forest-grassland boundary showed a small variation but an obvious ecological distribution. By using moving split-window techniques, it was obtained that the width of edge influence in surface and subsurface layer was 8 and 6 m, respectively. Geostatistical analyses showed that the spatial distribution of two layers soil moisture had a pure nugget effect in grassland, linear model in forestland, and spherical model in forest-grassland boundary. The spatial heterogeneity of two layers soil moisture was higher at forest-grassland boundary than at forestland and grassland, which had a stronger spatial dependence and autorelation. Kriging maps expressed the spatial structural characters. The distribution of soil moisture in two layers showed a strip shape near forest edge, and a patch shape far from the edge.

China↗

PubMed2005

Root proliferation and seed yield in response to spatial heterogeneity of below-ground competition.

Here, we tested the predictions of a 'tragedy of the commons' model of below-ground plant competition in annual plants that experience spatial heterogeneity in their competitive environment. Under interplant competition, the model predicts that a plant should over-proliferate roots relative to what would maximize the collective yield of the plants. We predict that a plant will tailor its root proliferation to local patch conditions, restraining root production when alone and over-proliferating in the presence of other plants. A series of experiments were conducted using pairs of pea (Pisum sativum) plants occupying two or three pots in which the presence or absence of interplant root competition was varied while nutrient availability per plant was held constant. In two-pot experiments, competing plants produced more root mass and less pod mass per individual than plants grown in isolation. In three-pot experiments, peas modulated this response to conditions at the scale of individual pots. Root proliferation in the shared pot was higher compared with the exclusively occupied pot. Plants appear to display sophisticated nutrient foraging with outcomes that permit insights into interplant competition.

Ecosystem↗

PubMed1992

Spatial heterogeneity of the metabolic response of RIF-1 tumors to a vasoactive agent evaluated in vivo by one-dimensional 31P chemical-shift imaging.

Localized 31P NMR spectroscopy was used to evaluate the spatial heterogeneity of the metabolic response of RIF-1 tumors to hydralazine. Volume localized 31P spectra were obtained from subcutaneous RIF-1 tumors using one-dimensional chemical-shift imaging, before and 20 min after treatment with 5 mg/kg hydralazine, administered intravenously. Following treatment all of the tumors showed an overall decrease in the ratio of nucleoside triphosphate (NTP) to inorganic phosphate (Pi) and a decrease in pH. However, spatial localization revealed that the reduction in NTP/Pi was not uniform within some tumors. This was partly due to regional differences in the levels of metabolites existing before treatment. Normal tissue adjacent to the tumor did not show a significant decrease in high-energy metabolites or pH.

Animals↗

PubMed1996

Spatial heterogeneity of myocardial blood flow presages salvage versus necrosis with coronary artery reperfusion in conscious baboons.

BACKGROUND: Myocardial blood flow distribution is known to be heterogeneous. It is also known that not all of the area at risk (AAR) infarcts with coronary artery occlusion (CAO) and coronary artery reperfusion (CAR). The goal of the present study was to determine whether the proportion of AAR that is salvaged or infarcted can be predicted by the pre-CAO level of myocardial blood flow, which varies considerably in individual samples as a result of natural heterogeneity. METHODS AND RESULTS: The effects of 90-minute CAO followed by 5- to 7-day CAR were examined in six conscious baboons instrumented with aortic and left atrial catheters and coronary artery occluders. AAR was determined by dual perfusion. Myocardial blood flow was measured by radioactive microspheres before and after CAO and CAR. The AAR was cut into small pieces (0.21 +/- 0.01 g) and separated into two categories; salvaged (n = 252) or infarcted (n = 133). Analysis of myocardial blood flow distribution revealed two distinct populations (P < .01); infarcted tissues demonstrated higher pre-CAO myocardial blood flow than salvaged tissues. Importantly, 50% of the salvaged tissue samples were characterized by pre-CAO myocardial blood flows of < 0.90 mL.min-1.g-1 compared with 29% for infarcted samples, whereas 51% of infarcted samples were characterized by pre-CAO myocardial blood flows of > 1.12 mL.min-1.g-1 compared with 22% of salvaged samples. Endocardial analyses were qualitatively similar to transmural analyses. CONCLUSIONS: This study suggests that heterogeneity of pre-CAO myocardial blood flows can predict the proportion of myocardium salvaged by CAR and can further explain the spatial heterogeneity of infarction that occurs after CAR, potentially independent of CAR injury.

Animals↗

PubMed2002

[Effect of mutual interference and spatial heterogeneity on predation of adult Chrysopa septempunctata].

The more wheat stalks the adult Chrysopa septempunctata encountered, the more interference was met during the predation of C. septempunctata, which means that the predation efficiency was low when the spatial heterogeneity was complex. The coefficient of mutual interference within C. septempunctata was 0.9390, and that between C. septempunctata and Propylaea japonica was 0.8722. The frontier was greater than the latter. The correlation of competition strength(I) and the number of natural enemy (P) could be described as I = 0.0640 + 1.1127 1gP.

Animals↗

PubMed1997

Spatial heterogeneity of the volume effect for radiation pneumonitis in mouse lung.

PURPOSE: In a previous study to determine the effect of partial volume irradiation on damage and morbidity from pneumonitis in mouse lung, a critical determinant of the volume effect was the spatial location of the irradiated subvolume within the lung. The goals of the present study were to (a) define the dose-volume effect curves for radiation pneumonitis in mouse lung, (b) define the threshold volume, and (c) further investigate the spatial heterogeneity of the radiosensitivity of mouse lung. METHODS AND MATERIALS: Eight fractional volumes ranging from 94% to 17% of the lungs of C3Hf/Kam mice were irradiated with single doses ranging from 12 to 22 Gy, depending on the volume irradiated. The fractional volumes irradiated were determined from computed tomographic scans of mouse lung. To determine the effect of location of irradiated subvolume, equivalent volumes in the base and the apex were irradiated by shielding the prescribed adjacent volume in the apex or base respectively. Dose-response curves of breathing rate at 22 weeks and lethality at 28 weeks were constructed for each subvolume irradiated in the apex or base and fitted by logit analysis, and ED50s and LD50s with 95% confidence limits obtained, respectively. Lungs from dead mice or mice sacrificed when moribund were examined for histologic signs of pneumonitis. RESULTS: Irradiation of any of the eight subvolumes in the base yielded a consistently lower isoeffect dose for both assays of radiation pneumonitis than if the same irradiated subvolume was located in the apex. Plots of isoeffect dose for breathing rate as a function of subvolume irradiated in the base or apex showed that these curves were not linear but exhibited a plateau between irradiated volumes of 70% and 80% in both the apex and base. A similar curve was obtained for lethality and volume irradiated in the base. A threshold volume, i.e., irradiation of that volume that should produce no changes in breathing rate or mortality, was dependent on the location of the irradiated subvolume. CONCLUSION: The response of mouse lung to partial volume irradiation is heterogeneous and is critically dependent on the specific location of the irradiated subvolume in the lung, i.e., a given subvolume in the base is consistently more sensitive than the same subvolume in the apex using either breathing rate or lethality as assays of radiation pneumonitis. We suggest that this heterogeneity is due to the anatomy of the tracheobronchial tree, i.e., to the distribution of non-gas exchange-conducting airways in the irradiated volume. These data have implications for the modeling of dose-volume effects in the lung and the prediction of normal tissue complication probabilities for radiation pneumonitis in humans.

Animals↗