Showing papers by "Simón Bolívar University published in 1999"

Leaf construction cost, nutrient concentration, and net CO 2 assimilation of native and invasive species in Hawaii

Abstract: The effects of biological invasions are most evident in isolated oceanic islands such as the Hawaiian Archipelago, where invasive plant species are rapidly changing the composition and function of plant communities. In this study, we compared the specific leaf area (SLA), leaf tissue construction cost (CC), leaf nutrient concentration, and net CO2 assimilation (A) of 83 populations of 34 native and 30 invasive species spanning elevation and substrate age gradients on Mauna Loa volcano in the island of Hawaii. In this complex environmental matrix, where annual precipitation is higher than 1500 mm, we predicted that invasive species, as a group, will have leaf traits, such as higher SLA and A and lower leaf CC, which may result in more efficient capture of limiting resources (use more resources at a lower carbon cost) than native species. Overall, invasive species had higher SLA and A, and lower CC than native species, consistent with our prediction. SLA and foliar N and P were 22.5%, 30.5%, and 37.5% higher, respectively, in invasive species compared to native ones. Light-saturated photosynthesis was higher for invasive species (9.59 μmol m−2 s−1) than for native species (7.31 μmol m−2 s−1), and the difference was larger when A was expressed on a mass basis. Leaf construction costs, on the other hand, were lower for the invasive species (1.33 equivalents of glucose g−1) than for native species (1.37). This difference was larger when CC was expressed on an area basis. The trends in the above traits were maintained when groups of ecologically equivalent native and invasive species (i.e., sharing similar life history traits and growing in the same habitat) were compared. Foliar N and P were significantly higher in invasive species across all growth forms. Higher N may partially explain the higher A of invasive species. Despite relatively high N, the photosynthetic nitrogen use efficiency of invasive species was 15% higher than that of native species. These results suggest that invasive species may not only use resources more efficiently than native species, but may potentially demonstrate higher growth rates, consistent with their rapid spread in isolated oceanic islands.

Planning as Heuristic Search: New Results

Abstract: In the recent AIPS98 Planning Competition, the hsp planner, based on a forward state search and a domain-independent heuristic, showed that heuristic search planners can be competitive with state of the art Graphplan and Satisfiability planners. hsp solved more problems than the other planners but it often took more time or produced longer plans. The main bottleneck in hsp is the computation of the heuristic for every new state. This computation may take up to 85% of the processing time. In this paper, we present a solution to this problem that uses a simple change in the direction of the search. The new planner, that we call hspr, is based on the same ideas and heuristic as hsp , but searches backward from the goal rather than forward from the initial state. This allows hspr to compute the heuristic estimates only once. As a result, hspr can produce better plans, often in less time. For example, hspr solves each of the 30 logistics problems from Kautz and Selman in less than 3 seconds. This is two orders of magnitude faster than blackbox. At the same time, in almost all cases, the plans are substantially smaller. hspr is also more robust than hsp as it visits a larger number of states, makes deterministic decisions, and relies on a single adjustable parameter than can be fixed for most domains. hspr, however, is not better than hsp accross all domains and in particular, in the blocks world, hspr fails on some large instances that hsp can solve. We discuss also the relation between hspr and Graphplan, and argue that Graphplan can also be understood as a heuristic search planner with a precise heuristic function and search algorithm.

Evaluation of Recombinant Antigens for Serodiagnosis of Chagas’ Disease in South and Central America

Abstract: The commercially available diagnostic tests for Chagas’ disease employ whole extracts or semipurified fractions of Trypanosoma cruzi epimastigotes. Considerable variation in the reproducibility and reliability of these tests has been reported by different research laboratories, mainly due to cross-reactivity with other pathogens and standardization of the reagents. The use of recombinant antigens for the serodiagnosis of Chagas’ disease is recommended to increase the sensitivity and specificity of serological tests. Expressed in Escherichia coli, as fusion products with glutathione S-transferase, six T. cruzi recombinant antigens (H49, JL7, A13, B13, JL8, and 1F8) were evaluated in an enzyme-linked immunosorbent assay for Chagas’ disease. The study was carried out with a panel of 541 serum samples of chagasic and nonchagasic patients from nine countries of Latin America (Argentina, Bolivia, Brazil, Chile, Colombia, El Salvador, Guatemala, Honduras, and Venezuela). The optimal concentration of each recombinant antigen for coating of plates was determined with the help of 125I-labelled recombinant proteins. While the specificity of the epimastigote antigen was 84% because of false positives from leishmaniasis cases, for the recombinant antigens it varied from 96.2 to 99.6%. Recombinant antigens reacted with 79 to 100% of serum samples from chronic chagasic patients. In this way, it is proposed that a mixture of a few T. cruzi recombinant antigens should be employed in a diagnostic kit to minimize individual variation and promote high sensitivity in the diagnosis of Chagas’ disease.

Reactive grasp and tabu search based heuristics for the single source capacitated plant location problem

Abstract: This paper considers the Single Source Capacitated Plant Location Problem (SSCPLP). SSCPLP is a discrete location problem. It allows capacities on the plants to be opened and constrains each client to be served by a single open plant. The following algorithms are proposed: A Reactive GRASP heuristic; a Tabu Search heuristic; and two different hybrid approaches that combine elements of the GRASP and the Tabu Search methodologies. The elements of the proposed heuristics are presented. The Reactive GRASP algorithm is a self-tuning heuristic in which the calibration process is replaced by an automated criterion for selecting the parameter value. The Tabu Search heuristic provides the framework for the first of the hybrid approaches. It consists of two phases. The GRASP methodology is used for the first one, which can be viewed as a strong diversification mechanism. The second one consists of an intensification phase. The second hybrid algorithm follows the framework of the Reactive GRASP heuristic. It...

Evaluation and Comparison of EEG Traces: Latent Structure in Nonstationary Time Series

Abstract: We explore and illustrate the use of time series decomposition methods for evaluating and comparing latent structure in nonstationary electroencephalographic (EEG) traces obtained from depressed patients during brain seizures induced as part of electroconvulsive therapy (ECT). Analysis of the patterns of change over time in the frequency structure of such EEG data provides insight into the neurophysiological mechanisms of action of this effective but poorly understood antidepressant treatment, and allows clinicians to modify ECT treatments to optimize therapeutic benefits while minimizing associated side effects. Our work has introduced new methods of time-frequency analysis of EEG series that identify the complete pattern of time evolution of frequency structure over the course of a seizure, and usefully assist in these scientific and clinical studies. New methods of decomposition of flexible dynamic models provide time domain decompositions of individual EEG series into collections of latent co...

Optimal coordination of directional overcurrent relays considering definite time backup relaying

Abstract: A methodology is presented for the consideration of definite-time backup relays in the optimal coordination of directional overcurrent relays using linear programming. It is shown that the influence of considering second-zone distance relays and breaker failure relays impose important requirements for the determination of the time dial settings of directional overcurrent relays. The paper introduces a revised formulation of the optimization problem. Results are presented for the application of the methodology on a power system with 2 generators, 9 buses, 2 transformers and 7 transmission lines.

Distribution of relaxation times from dielectric spectroscopy using Monte Carlo simulated annealing: Application to α-PVDF

Abstract: The existence of a distribution of relaxation times has been widely used to describe the relaxation function versus frequency in glass-forming liquids. Several empirical distributions have been proposed and the usual method is to fit the experimental data to a model that assumes one of these functions. Another alternative is to extract from the experimental data the discrete profile of the distribution function that best fits the experimental curve without any a priori assumption. To test this approach a Monte Carlo algorithm using the simulated annealing is used to best fit simulated dielectric loss data, ${\ensuremath{\varepsilon}}^{\ensuremath{''}}(\ensuremath{\omega}),$ generated with Cole-Cole, Cole-Davidson, Havriliak-Negami, and Kohlrausch-Williams-Watts (KWW) functions. The relaxation times distribution, $G(\mathrm{ln}(\ensuremath{\tau})),$ is obtained as an histogram that follows very closely the analytical expression for the distributions that are known in these cases. Also, the temporal decay functions, $\ensuremath{\varphi}(t),$ are evaluated and compared to a stretched exponential. The method is then applied to experimental data for $\ensuremath{\alpha}$-polyvinylidene fluoride over a temperature range $233 \mathrm{K}l~Tl~278 \mathrm{K}$ and frequencies varying from 3 MHz to 0.001 Hz. These data show the existence of two relaxation processes: the fast segmental ${\ensuremath{\alpha}}_{a}$ process associated with the glass transition and a ${\ensuremath{\alpha}}_{c}$ mode, which is slower and due to changes in conformation that can occur in the crystalline regions. The experimental curves are fitted by the simulated annealing direct signal analysis procedure, and the relaxation times distributions are calculated and found to vary with temperature. The decay function is also evaluated and it shows clearly its bimodal character and a good agreement with a KWW function with a temperature dependent $\ensuremath{\beta}$ for each mode. The relaxation plots are drawn for each mode and the Vogel-Tammann-Fulcher and Arrhenius parameters are found. The fragility parameter for polyvinylidene flouride (PVDF) is found to be 87, which characterizes this polymer as a relatively structurally strong material.

Venezuelan rainfall data analysed by using a Bayesian space-time model

Abstract: We consider a set of data from 80 stations in the Venezuelan state of Guarico consisting of accumulated monthly rainfall in a time span of 16 years. The problem of modelling rainfall accumulated over fixed periods of time and recorded at meteorological stations at different sites is studied by using a model based on the assumption that the data follow a truncated and transformed multivariate normal distribution. The spatial correlation is modelled by using an exponentially decreasing correlation function and an interpolating surface for the means. Missing data and dry periods are handled within a Markov chain Monte Carlo framework using latent variables. We estimate the amount of rainfall as well as the probability of a dry period by using the predictive density of the data. We considered a model based on a full second-degree polynomial over the spatial co-ordinates as well as the first two Fourier harmonics to describe the variability during the year. Predictive inferences on the data show very realistic results, capturing the typical rainfall variability in time and space for that region. Important extensions of the model are also discussed.

Abbreviated Measures of Food Sufficiency Validly Estimate the Food Security Level of Poor Households: Measuring Household Food Security

Abstract: This study was designed to develop an abbreviated method that captures both the qualitative and quantitative dimensions of household food security (HFS). Women in poor and very poor households (n = 238) in a peri-urban barrio in Caracas, Venezuela, provided data on food availability and their perception of food resource constraints and hunger experiences within the home. Socioeconomic data and food-related behavior that may predict HFS levels were gathered. On average, the top 12 food contributors of energy provided 81% and predicted more than 90% of the variation in households' total energy availability using stepwise regression analysis. On the other hand, a 4-point 12-item scale was shown to have face, content and construct validity with reiterative testing, factor analysis and a Chronbach's alpha coefficient of 0.92. Assessing predictors of energy availability together with a self-perceived HFS scale may provide a valid and reliable method for identifying and monitoring food security levels among poor urban households.

Fractionated crystallisation of polyethylene and ethylene/α-olefin copolymers dispersed in immiscible polystyrene matrices

Abstract: In this work, several PS/HDPE (high density polyethylene), PS/LLDPE (linear low density polyethylene) and PS/ULDPE (ultra low density polyethylene) blends were prepared in a composition range were atactic polystyrene (PS) was always the matrix component. All the types of polyethylenes employed, when dispersed into droplets, exhibited fractionated crystallisation exotherms in the temperature range between 67 and 70 °C. These low crystallisation temperatures are probably closer to the homogeneous nucleation temperature of linear polyethylene than any previously reported value, since they occur at higher supercoolings. The higher values of crystallisation temperatures previously reported could be explained by the crystallisation from heterogeneous nuclei of relatively low nucleation efficiency or by a weak nucleation capacity of the droplets interface. By applying a self-nucleation procedure we were able to corroborate that what causes the fractionated crystallisation is the lack of highly active heterogeneous nuclei (i. e., those normally active at low supercoolings in the bulk polymer) in every droplet. When polyethylene/α-olefin copolymers are finely dispersed in a PS matrix, a molecular segregation process can be induced during mixing facilitated by the heterogeneous distribution of short chain branches in the copolymers. The crystallisation of droplets that contained mostly these highly branched chains occurs at temperatures lower than 50 °C, thereby producing another low temperature exotherm which may be the lowest present in the blend and could be mistaken by the crystallisation of homogeneously nucleated crystals. We have shown that the self-nucleation technique can help to distinguish these low temperature exotherms from those originated by differences in nucleation behaviour; therefore, a plausible interpretation of all the possible fractionated crystallisation exotherms of ethylene/α-olefin copolymer droplets was made possible.

Monte Carlo study of a mixed spin-3/2 and spin-1/2 Ising ferrimagnetic model

Abstract: The magnetic properties of a mixed Ising ferrimagnetic model on a square lattice, in which the two interpenetrating square sublattices have spins that can take two values, $\ensuremath{\sigma}=\ifmmode\pm\else\textpm\fi{}1/2,$ alternated with spins that can take four values, $S=\ifmmode\pm\else\textpm\fi{}3/2,\ifmmode\pm\else\textpm\fi{}1/2,$ are studied. This model can be relevant for understanding the magnetic behavior of the new class of organometallic ferrimagnetic materials that exhibit spontaneous magnetic moments at room temperature. We carried out exact ground-state calculations for the model and employ a Monte Carlo algorithm to obtain the finite-temperature phase diagram for both the transition and compensation temperatures. The role of the different interactions in the Hamiltonian is explored. When only the nearest-neighbor interaction and the crystal-field term are included our results indicate no compensation point at finite temperature. When the next-nearest-neighbor interaction between the spins $\ensuremath{\sigma}=\ifmmode\pm\else\textpm\fi{}1/2$ exceeds a minimum value that depends on the other parameters in the Hamiltonian, a compensation point appears. The interaction between the $S=\ifmmode\pm\else\textpm\fi{}3/2$ spins, next-nearest neighbors in the lattice, has the effect of changing the compensation temperature.

Effects of fire and defoliation on the life history of native and invader C 4 grasses in a Neotropical savanna

Abstract: African grasses, introduced into Neotropical savannas to improve forage quality, have spread successfully and displaced native plants. To understand their competitive relationships, we compared biomass production and allocation, plant architecture and phenology, net photosynthesis (Pn), water relations, and nutrient content under fire and simulated herbivory between two C4 grasses, the native Trachypogon plumosus and the introduced Hyparrhenia rufa from a seasonal savanna in Venezuela. All variables were strongly influenced by the rainfall regime. Hyparrhenia produced bigger plants (in mass and size) with a large proportion of mass (>75%) allocated to leaves and culms. Its biomass production was more affected by fire than by defoliation. In contrast, Trachypogon was more affected by defoliation than by fire which promoted a flush of leaf growth even in the dry season. Fire caused up to 85% mortality in Hyparrhenia but none in Trachypogon where it increased inflorescence production. However, fire promoted abundant seed germination and fast seedling growth in Hyparrhenia, enabling it to colonize new areas. During the growing season Trachypogon had higher Pn and lower leaf water potential (Ψ) than Hyparrhenia but differences among treatments were not significant for either grass. Pn of Trachypogon ceased at a lower Ψ (−3.0 MPa) than in Hyparrhenia (−2.0 MPa), indicating its higher tolerance to water stress. During the dry season, Trachypogon leaves remained alive and retained low Pn. Leaf nutrient content was higher during the rainy season in both species. Differences in Pn could not explain the higher seasonal biomass production of Hyparrhenia. However, its water stress evasion strategy, larger biomass allocated to leaves, abundant germination and fast seedling growth appeared to be responsible for the success of Hyparrhenia as an invader of Neotropical savannas.

Leaf Photosynthesis of the Mangrove Avicennia Germinans as Affected by NaCl

Abstract: In leaves of the mangrove species Avicennia germinans (L.) L. grown in salinities from 0 to 40 ‰, fluorescence, gas exchange, and δ13C analyses were done. Predawn values of Fv/Fm were about 0.75 in all the treatments suggesting that leaves did not suffer chronic photoinhibition. Conversely, midday Fv/Fm values decreased to about 0.55-0.60 which indicated strong down-regulation of photosynthesis in all treatments. Maximum photosynthetic rate (P max) was 14.58 ± 0.22 µmol m-2 s-1 at 0 ‰ it decreased by 21 and 37 % in plants at salinities of 10 and 40 ‰, respectively. Stomatal conductance (g s) was profoundly responsive in comparison to P max which resulted in a high water use efficiency. This was further confirmed by δ13C values, which increased with salinity. From day 3, after salt was removed from the soil solution, P max and g s increased up to 13 and 30 %, respectively. However, the values were still considerably lower than those measured in plants grown without salt addition.

Direct extraction of semiconductor device parameters using lateral optimization method

Abstract: Direct extraction of semiconductor device parameters using lateral optimization method A. Ortiz-Conde, Yuansheng Ma, J. Thomson, E. Santos, J.J. Liou*, F.J. Garco a Sa nchez, M. Lei, J. Finol, P. Layman Departamento de ElectroÂnica, Universidad SimoÂn BoloÂvar, Apartado 89000, Caracas, 1080-A, Venezuela Department of ECE, University of Central Florida, Orlando, FL 32816-2450, USA Modeling and Simulation Group, Lucent Technologies, Orlando, FL, USA Motorola, 3102 North 56th Street, Phoenix, Arizona 85018, USA

Drought effects on photosynthesis of the mangrove, Avicennia germinans, under contrasting salinities

Abstract: Drought effects on leaf photosynthesis of A. germinans growing under two contrasting salinities were studied in a Venezuelan fringe mangrove. During both wet and dry seasons, severe chronic-photoinhibition at predawn was not observed but strong down regulation occurred at midday during both seasons. Carbon assimilation rates (A, μmol CO2 m−2 s−1) declined during the dry season from 11.9±1.8 to 7.0±1.5 and from 9.6±2.0 to 4.7±2.5 in plants from low and high salinity sites, respectively. Changes in carbon assimilation per unit of chlorophyll (A/Chl, mmol CO2 mol−1 Chl) were from 31.6±4.7 to 20.5±4.3 and from 21.9±4.7 to 15.2±8.2 in the low and high salinity plants, respectively. Therefore, neither changes in Chl nor seasonal differences in photoprotective down regulation could account fully for the decrease in leaf photosynthesis during drought. A reduction in CO2 diffusion due to lowered stomatal conductance was not large enough to explain such a dramatic effect of drought on leaf photosynthesis. Stomatal response could be mitigated by the capability of A. germinans for osmotic adjustment under high salinity and/or drought. However, this intracellular salt accumulation may reduce carbon assimilation capacity further by decreasing the metabolism of leaf cells, increasing dark respiration and/or photorespiration.

Application of the SSA calorimetric technique to characterize LLDPE grafted with diethyl maleate

Abstract: A linear low density polyethylene (LLDPE) was functionalized with diethyl maleate (DEM) using different amounts of a peroxide as initiator. The modified polymers were characterised by means of FTIR and 1 H NMR spectroscopy and the percentage of grafted DEM was determined. When the initiator concentration is increased from 0.44 to 0.88 g per 100 g of polymer, the degree of functionalization also increases from 0.47 to 0.8 mol-% of grafted DEM per ethylene unit. The results gathered here indicate that in this case, the grafting reaction preferentially occurs on the secondary carbons of LLDPE. Such a result was inferred by the use of the DSC based successive self-nucleation and annealing technique (SSA). The ability of SSA to induce molecular segregation during crystallisation and annealing proved to be very useful to characterise the functionalized polyolefins prepared here. When the amount of initiator was further increased to 1.76 wt.-%, the degree of functionalization only increased marginally to 0.84 mol-%. Combined results by SSA, capillary rheometry and FTIR indicated that secondary reactions occur when only part of the initiator is used in the grafting reactions. Such secondary reactions most probably produce long chain branching in the grafted polymer as indicated by the alterations in the melt flow behaviour of this sample and the modifications of its FTIR spectrum.

Molecular Simulation of Joule–Thomson Inversion Curves

Abstract: A method to determine Joule–Thomson inversion curves, using isobaric-isothermal Monte Carlo molecular simulations, is presented. The usual experimental practice to obtain the locus of points in which the isenthalpic derivative of temperature with respect to pressure vanishes is to process volumetric data by means of thermodynamic relations. This experimental procedure requires the very precise measurement of volumetric properties at conditions up to five times the fluid's critical temperature and twelve times its critical pressure. These harsh experimental conditions have hindered the publication of data for even simple fluids and mixtures. By using molecular simulation, these problems may be circumvented, since the computational effort is roughly independent of the actual value of the pressure or the temperature. In general, Joule–Thomson inversion curves obtained by molecular simulation may be used either as an unambiguous test for equations of state in the supercritical and high-pressure regions or for the prediction of real fluid behavior, should the potential be well known. Both applications are exemplified for a Lennard-Jones fluid for which the complete inversion curve is obtained.

Mandibular gland secretion in different castes of the leaf-cutter ant Atta laevigata

Abstract: Gas chromatography analyses and behavioral assays showed that Atta laevigata, a highly polymorphic ant species, has a mandibular gland secretion that varies with castes. All castes contain 4-methyl-3-heptanone as the main component and its concentration is proportional to head size. Small workers and soldiers, but not medium size workers, also contain 4-methyl-3-heptanol. Queens show variations in their chemical composition after mating, as virgin males contain a secretion dominated by 4-methyl-3-heptanol, and, in a lesser proportion, 4-methyl-3-heptanone. In mated males these proportions are inverted. The compounds 4-methyl-6-hepten-3-one, 4-methyl-4-hepten-3-one, 6-methyl-tetradecene, and 2,6-dimethyl-2-dodecene are found only in queens. The behavioral response elicited by the secretion is mainly alarm, which is elicited more strongly by glands of larger workers. The results suggest that chemical castes, behavioral castes, and morphological castes overlap in this species.

Procedure for determining diode parameters at very low forward voltage

Abstract: A technique is proposed to extract the reverse saturation current parameter and ideality factor of semiconductor junctions from the low forward voltage region of the device’s characteristics. The method involves performing a mathematical operation on the experimental data that allows to calculate the parameters at values of forward current smaller than the reverse saturation current. The procedure was tested and its accuracy verified on synthetic I‐V characteristics, with and without added simulated experimental error or noise. Good agreement is obtained between the parameters used in modeling and the extracted values. The procedure was also applied to experimentally measured IB‐VBE characteristics of a real power BJT. # 1999 Elsevier Science Ltd. All rights reserved.

“Superductile” Semicrystalline ABC Triblock Copolymers with the Polystyrene Block (A) as the Matrix

Abstract: The relationship between the microphase morphology and stress−strain behavior of a polystyrene-block-polybutadiene-block-poly(e-caprolactone) SBC triblock copolymer has been investigated with polystyrene as the major component. The semicrystalline PCL block does not change significantly the basic morphology, but it affects the form of the microdomains, so that S57B27C16 shows a “polygonal” core shell morphology in a PS matrix. This morphology seems to be the consequence of crystallization in the inner PCL cylinders inducing deformation of the microdomains, which may create interfacial stresses. These stresses can generate small microcrazes upon external loading which may be responsible for the high elongations at break in this material. Moreover, the long-range order of the microdomains, which can be affected through the preparation conditions, determines the stress−strain behavior. Copolymers with 57 and 51 wt % PS and a corresponding “PS matrix” exhibit elongations at break up to ∼800%.