Showing papers by "Instituto Politécnico Nacional published in 2002"

Phosphate Availability Alters Architecture and Causes Changes in Hormone Sensitivity in the Arabidopsis Root System

Abstract: The postembryonic developmental program of the plant root system is plastic and allows changes in root architecture to adapt to environmental conditions such as water and nutrient availability Among essential nutrients, phosphorus (P) often limits plant productivity because of its low mobility in soil Therefore, the architecture of the root system may determine the capacity of the plant to acquire this nutrient We studied the effect of P availability on the development of the root system in Arabidopsis We found that at P-limiting conditions (<50 microM), the Arabidopsis root system undergoes major architectural changes in terms of lateral root number, lateral root density, and primary root length Treatment with auxins and auxin antagonists indicate that these changes are related to an increase in auxin sensitivity in the roots of P-deprived Arabidopsis seedlings It was also found that the axr1-3, axr2-1, and axr4-1 Arabidopsis mutants have normal responses to low P availability conditions, whereas the iaa28-1 mutant shows resistance to the stimulatory effects of low P on root hair and lateral root formation Analysis of ethylene signaling mutants and treatments with 1-aminocyclopropane-1-carboxylic acid showed that ethylene does not promote lateral root formation under P deprivation These results suggest that in Arabidopsis, auxin sensitivity may play a fundamental role in the modifications of root architecture by P availability

TCP westwood: end-to-end congestion control for wired/wireless networks

Abstract: TCP Westwood (TCPW) is a sender-side modification of the TCP congestion window algorithm that improves upon the performance of TCP Reno in wired as well as wireless networks. The improvement is most significant in wireless networks with lossy links. In fact, TCPW performance is not very sensitive to random errors, while TCP Reno is equally sensitive to random loss and congestion loss and cannot discriminate between them. Hence, the tendency of TCP Reno to overreact to errors. An important distinguishing feature of TCP Westwood with respect to previous wireless TCP "extensions" is that it does not require inspection and/or interception of TCP packets at intermediate (proxy) nodes. Rather, TCPW fully complies with the end-to-end TCP design principle. The key innovative idea is to continuously measure at the TCP sender side the bandwidth used by the connection via monitoring the rate of returning ACKs. The estimate is then used to compute congestion window and slow start threshold after a congestion episode, that is, after three duplicate acknowledgments or after a timeout. The rationale of this strategy is simple: in contrast with TCP Reno which "blindly" halves the congestion window after three duplicate ACKs, TCP Westwood attempts to select a slow start threshold and a congestion window which are consistent with the effective bandwidth used at the time congestion is experienced. We call this mechanism faster recovery. The proposed mechanism is particularly effective over wireless links where sporadic losses due to radio channel problems are often misinterpreted as a symptom of congestion by current TCP schemes and thus lead to an unnecessary window reduction. Experimental studies reveal improvements in throughput performance, as well as in fairness. In addition, friendliness with TCP Reno was observed in a set of experiments showing that TCP Reno connections are not starved by TCPW connections. Most importantly, TCPW is extremely effective in mixed wired and wireless networks where throughput improvements of up to 550% are observed. Finally, TCPW performs almost as well as localized link layer approaches such as the popular Snoop scheme, without incurring the overhead of a specialized link layer protocol.

Analysis and design of a 1.8-GHz CMOS LC quadrature VCO

Abstract: This paper presents a quadrature voltage-controlled oscillator (QVCO) based on the coupling of two LC-tank VCOs A simplified theoretical analysis for the oscillation frequency and phase noise displayed by the QVCO in the 1/f/sup 3/ region is developed, and good agreement is found between theory and simulation results A prototype for the QVCO was implemented in a 035-/spl mu/m CMOS process with three standard metal layers The QVCO could be tuned between 164 and 197 GHz, and showed a phase noise of -140 dBc/Hz or less across the tuning range at a 3-MHz offset frequency from the carrier, for a current consumption of 25 mA from a 2-V power supply The equivalent phase error between I and Q signals was at most 025/spl deg/

MAFRA - A MApping FRAmework for Distributed Ontologies

Abstract: Ontologies as means for conceptualizing and structuring domain knowledge within a community of interest are seen as a key to realize the Semantic Web vision. However, the decentralized nature of the Web makes achieving this consensus across communities difficult, thus, hampering efficient knowledge sharing between them. In order to balance the autonomy of each community with the need for interoperability, mapping mechanisms between distributed ontologies in the Semantic Web are required. In this paper we present MAFRA, an interactive, incremental and dynamic framework for mapping distributed ontologies.

Preparation, characterization and photocatalytic activity of ZnO, Fe2O3 and ZnFe2O4

Abstract: ZnO, Fe 2 O 3 and ZnFe 2 O 4 were synthesized by precipitation or coprecipitation at constant pH After aging, washing and drying, the solids were calcined at 800 C The samples were characterized by XRD, TGA, N 2 physisorption, UV-Vis and TPR The photocatalytic degradation of phenol was studied in a batch reactor and TiO 2 (Degussa, P-25) was used as a reference catalyst Pure ZnO and Fe 2 O 3 were obtained at 800 C, whereas ZnFe 2 O 4 and α-Fe 2 O 3 (segregated) were detected in the zinc ferrite sample TPR experiments showed the reduction peaks in the range of 350-800 C only with Fe 2 O 3 and ZnFe 2 O 4 , attributed to Fe 3+ to Fe 2+ reduction and complete dehydroxylation The photodegradation of phenol (samples calcined at 800 C) showed small but significant variations that decreased in the order: TiO 2 > ZnFe 2 O 4 > ZnO > Fe 2 O 3 Subproducts such as hydroquinone, catechol and benzoquinone were mainly detected In particular, attention was focused on our prepared materials because after 24 h they did not dissolve as happened with TiO 2

Description logics of minimal knowledge and negation as failure

Abstract: We present description logics of minimal knowledge and negation as failure (MKNF-DLs), which augment description logics with modal operators interpreted according to Lifschitz's nonmonotonic logic MKNF. We show the usefulness of MKNF-DLs for a formal characterization of a wide variety of nonmonotonic features that are both commonly available inframe-based systems, and needed in the development of practical knowledge-based applications: defaults, integrity constraints, role, and concept closure. In addition, we provide a correct and terminating calculus for query answering in a very expressive MKNF-DL.

Low stray inductance bus bar design and construction for good EMC performance in power electronic circuits

Abstract: Ten years after the publication of the EC Directive 89/336 on electromagnetic compatibility, the impact of this directive on design and lay-out of modern electrical and electronic equipment can be observed. Many research and development studies have proposed and evaluated detailed improvements in the area of component design, component selection, circuit lay-out, shielding and active and passive filtering. New and innovative solutions to minimize noise, especially common mode conducted electromagnetic interference (EMI), in power electronic circuits continue to be developed. In this paper, the authors investigate to what extent EMI caused by power electronic devices in hard switching inverter topologies can be minimized using ultra-low inductive planar busbars. The concept followed in this study is to tackle EMI directly at the source where most EMI is generated; in other words, to reduce the parasitic magnetic energy stored in the inverter DC link to reduce high voltage spikes during switching. A planar busbar was built, tested and analyzed. Measurements show the validity of the theoretical, but simple, design procedure for planar busbars in power converters.

Open source software projects as virtual organisations: competency rallying for software development

Abstract: Contributes to the identification and testing of factors important for the success of open source software (OSS) projects. We present an analysis of OSS communities as virtual organisations, and apply Katzy and Crowston's (2000) competency rallying (CR) theory to the case of OSS development projects. CR theory suggests that project participants must develop necessary competencies, identify and understand market opportunities, marshal competencies to meet the opportunity and manage a short-term co-operative process. Using data collected from 7477 OSS projects hosted by the SourceForge system, we formulate and test a set of specific hypotheses derived from CR theory. The empirical data analysis supports the majority of these hypotheses, suggesting that CR theory provides a useful lens for studying OSS projects.

Characterization of rhizobia that nodulate legume species of the genus Lespedeza and description of Bradyrhizobium yuanmingense sp. nov.

Abstract: Legume species belonging to the genus Lespedeza are annual or perennial herb or shrub plants that grow in the northern hemisphere. They are known for the formation of root nodules, but little information is available about their microsymbionts. In this study, 58 root-nodule isolates from Lespedeza spp., obtained from China and the USA, were characterized using numerical taxonomic analysis of phenotypic features, SDS-PAGE analysis of whole-cell proteins, DNA-DNA hybridization, 16S rRNA gene sequence analysis and cross-nodulation with selected legume species. From the results generated using these approaches, it was concluded that Lespedeza spp. were promiscuous hosts for rhizobia. Four main clusters of bacteria, which included 35 of the strains isolated from Lespedeza spp., were defined upon numerical taxonomic analysis; these groups corresponded to those determined from analyses of protein electrophoretic and DNA-DNA hybridization data. The four clusters were found to define strains belonging to one of four species, Sinorhizobium saheli, Bradyrhizobium japonicum, Bradyrhizobium elkanii or a novel species of the genus Bradyrhizobium. The strains of B. japonicum and B. elkanii were all from the USA soil samples, and their representative strains could not nodulate soybean. The seven strains found to represent the novel Bradyrhizobium sp. were from China. These were differentiated from recognized species of the genus Bradyrhizobium by all of the taxonomic methods used here; hence, it is proposed that the novel strains isolated from Lespedeza spp. represent a novel species of the genus Bradyrhizobium, Bradyrhizobium yuanmingense. The type strain of the novel species, CCBAU 10071(T) (= CFNEB 101(T)), formed ineffective nodules on Medicago sativa and Melilotus albus but did not nodulate soybean. The other 23 bacterial strains isolated from Lespedeza spp. were found to form single branches or small groups (two to three strains) that were related to Bradyrhizobium, Mesorhizobium, Rhizobium and Sinorhizobium spp. on the basis of numerical taxonomic analysis, indicating the possibility that other rhizobial species are also associated with Lespedeza spp.

Stability improvements of an LCL-filter based three-phase active rectifier

Abstract: Three-phase active rectifiers guarantee sinusoidal input currents and controllable dc voltage at the price of a high switching frequency ripple that can disturb and reduce efficiency of other EMI sensitive equipment connected to the grid. This problem could be solved choosing a high value for the ac inductors making them expensive and bulky. Moreover the dynamic of the system becomes poor with so high value of inductance. On the contrary to adopt an LCL-filter configuration allows to use reduced values of the inductances (preserving dynamic) and to reduce the switching frequency pollution emitted in the grid. However the stability of the system should be rigorously studied. A poor analysis made on qualitative considerations could lead to excessive damping (unnecessary increase of the losses) or insufficient damping (the system seems to be stable but it is not). In this paper the damping, both passive (based on the use of resistors) and active (based on the modification of the control algorithm), is studied using the z-plane root locus approach and looking to dynamic performances and losses. In fact it is necessary to verify the dynamic effects by the introduction of damping resistors or by the modification of the control algorithm to perform active damping. The analysis is validated both with simulation and experiments.

Terahertz semiconductor-heterostructure lasers

Abstract: Injection lasers based on interminiband transitions in GaAs/AlGaAs heterostructures are operated in the THz range. Single-mode emission is achieved at 4.4THz, with output powers of 2.5mW and thresholds of few hundred A/cm/sup 2/ up to 50K.

Measurement of Cell Volume Changes by Fluorescence Self-Quenching

Abstract: At high concentrations, certain fluorophores undergo self-quenching, i.e., fluorescence intensity decreases with increasing fluorophore concentration. Accordingly, the self-quenching properties can be used for measuring water volume changes in lipid vesicles. In cells, quantitative determination of water transport using fluorescence self-quenching has been complicated by the requirement of relatively high (mM) and often toxic loading concentrations. Here we report a simple method that uses low (μM) loading concentrations of calcein-acetoxymethyl ester (calcein-AM) to obtain intracellular concentrations of the fluorophore calcein suitable for measurement of changes in cell water volume by self-quenching. The relationship between calcein fluorescence intensity, when excited at 490 nm (its excitation maximum), and calcein concentration was investigated in vitro and in various cultured cell types. The relationship was bell-shaped, with the negative slope in the concentration range where the fluorophore undergoes fluorescence self-quenching. In cultured retinal pigment epithelial cells, calcein fluorescence and extracellular osmolarity were linearly related. A 25-mOsm hypertonic challenge corresponded to a decrease in calcein fluorescence with high signal-to-noise ratio (>15). Similar results were obtained with the fluorophore BCECF when excited at its isosbestic wavelength (436 nm). The present results demonstrate the usefulness of fluorescence self-quenching to measure rapid changes in cell water volume.

Soft elastic response of stretched sheets of nematic elastomers: a numerical study

Abstract: Stretching experiments on sheets of nematic elastomers have revealed soft deformation modes and formation of microstructure in parts of the sample. Both phenomena are manifestations of the existence of a symmetry-breaking phase transformation from a random, isotropic phase to an aligned, nematic phase. The microscopic energy proposed by Bladon et al. (Phys. Rev. E 47 (1993), R 3838) to model this transition delivers a continuum of symmetry-related zero-energy states, which can be combined in different ways to achieve a variety of zero-energy macroscopic deformations. We replace the microscopic energy with a macroscopic effective energy, the so-called quasiconvexification. This procedure yields a coarse-grained description of the physics of the system, with (energetically optimal) fine-scale oscillations of the state variables correctly accounted for in the energetics, but averaged out in the kinematics. Knowledge of the quasiconvexified energy enables us to compute efficiently with finite elements, and to simulate numerically stretching experiments on sheets of nematic elastomers. Our numerical experiments show that up to a critical, geometry-dependent stretch, no reaction force arises. At larger stretches, a force is transmitted through parts of the sheet and, although fine phase mixtures disappear from most of the sample, microstructures survive in some pockets. We reconstruct from the computed deformation gradients a possible composition of the microstructure, thereby resolving the local orientation of the nematic director.

Activation tagging using the En-I maize transposon system in Arabidopsis

Abstract: A method for the generation of stable activation tag inserts was developed in Arabidopsis using the maize (Zea mays) En-I transposon system. The method employs greenhouse selectable marker genes that are useful to efficiently generate large populations of insertions. A population of about 8,300 independent stable activation tag inserts has been produced. Greenhouse-based screens for mutants in a group of plants containing about 2,900 insertions revealed about 31 dominant mutants, suggesting a dominant mutant frequency of about 1%. From the first batch of about 400 stable insertions screened in the greenhouse, four gain-in-function, dominant activation-tagged, morphological mutants were identified. A novel gain-in-function mutant called thread is described, in which the target gene belongs to the same family as the YUCCA flavin-mono-oxygenase that was identified by T-DNA activation tagging. The high frequency of identified gain-in-function mutants in the population suggests that the En-I system described here is an efficient strategy to saturate plant genomes with activation tag inserts. Because only a small number of primary transformants are required to generate an activation tag population, the En-I system appears to be an attractive alternative to study plant species where the present transformation methods have low efficiencies.

Ultra-microhardness testing procedure with Vickers indenter

Abstract: Depth-sensing indentation equipment is widely used for evaluation of the hardness and Young's modulus of materials. The depth resolution of this technique allows the use of ultra-low loads. However, aspects related to the determination of the contact area under indentation should be cautiously considered when using this equipment. These are related to the geometrical imperfections of the tip, the diamond pyramidal punch and the formation of pileup or the presence of sink-in, which alter the shape and size of the indent. These and other aspects, such as the thermal drift of the equipment and the scattering at the zero indentation depth position related to surface finishing, are discussed in this work. A study concerning the hardness and the Young's modulus results determined by Vickers indentation on different materials was performed. Samples of fused silica, BK7 glass, aluminium, copper and mild steel (for which the values of Young's modulus were previously known) were tested using indentation loads in the range 10–1000 mN. Moreover, two methods are proposed for performing the indentation geometrical calibration of the contact area; these are compared with a former method proposed by Oliver and Pharr (OP). The present methods are based on: (i) analysis of the punch profile using atomic force microscopy (AFM); and (ii) a linear penetration-depth function correction (LM), based on knowledge of the values of the Young's modulus of several materials. By applying these methods to the indentation load/indentation depth results, it was possible to draw some conclusions about the benefit of the AFM and LM methods now under proposal.

Movement-related modulation of neural activity in human basal ganglia and its L-DOPA dependency: recordings from deep brain stimulation electrodes in patients with Parkinson's disease.

Abstract: Through electrodes implanted for deep brain stimulation in three patients (5 sides) with Parkinson's disease, we recorded the electrical activity from the human basal ganglia before, during and after voluntary contralateral finger movements, before and after L-DOPA. We analysed the movement-related spectral changes in the electroencephalographic signal from the subthalamic nucleus (STN) and from the internal globus pallidus (GPi). Before, during and after voluntary movements, signals arising from the human basal ganglia contained two main frequencies: a high β (around 26 Hz), and a low β (around 18 Hz). The high β (around 26 Hz) power decreased in the STN and GPi, whereas the low β (around 18 Hz) power decrease was consistently found only in the GPi. Both frequencies changed their power with a specific temporal modulation related to the different movement phases. L-DOPA specifically and selectively influenced the spectral power changes in these two signal bands.

Spatiotemporal analysis of Portevin–Le Châtelier deformation bands: Theory, simulation, and experiment

Abstract: In many solid solutions plastic deformation becomes unstable at sufficiently high temperature due to dynamic strain aging, i.e., repeated breakaway of dislocations from their solute clouds and recapture by mobile solutes, producing stress serrations in constant strain-rate tests or strain bursts in constant stress-rate tests. The instabilities of this well-known Portevin--Le Ch\^atelier (PLC) effect are closely connected with localization of strain in ``PLC deformation bands'' with a width of the order of the specimen thickness and sometimes propagating like a soliton along the specimen. In the present work, the nucleation and propagation of PLC deformation bands is studied by means of a multizone laser scanning extensometer, providing information on local strain along the main part of the specimen, in addition to the conventional measurement of stress serrations. This enables one to differentiate clearly between the bands of types A, B, and C, and to explore their ranges of existence at various temperatures, stresses and strain rates as well as transitions between them along the stress-strain curve. The laser extensometer provides independent data on propagation rate, concentrated strain and width of the bands. These experimental data are compared with a theoretical space-time analysis of propagating PLC bands, which explicitly combines a physical description of the kinetics of dynamic strain aging and plastic deformation. This model provides not only analytical predictions for the above band parameters and their dependences on deformation rate and specimen thickness for Type-A PLC bands, but---by considering types B and C as perturbation modes---is also able to explain the observed transitions between the various types of deformation bands. Moreover, the effect of strain hardening on the appearance of PLC strain localization is elucidated. The analytical predictions are validated by numerical simulations of the model and by comparing them to the experimental findings reported here.

An Empirical Comparison of Particle Swarm and Predator Prey Optimisation

Abstract: In this paper we present and discuss the results of experimentally comparing the performance of several variants of the standard swarm particle optimiser and a new approach to swarm based optimisation. The new algorithm, which we call predator prey optimiser, combines the ideas of particle swarm optimisation with a predator prey inspired strategy, which is used to maintain diversity in the swarm and preventing premature convergence to local suboptima. This algorithm and the most common variants of the particle swarm optimisers are tested in a set of multimodal functions commonly used as benchmark optimisation problems in evolutionary computation.

The role of prostaglandin E2 in the immunopathogenesis of experimental pulmonary tuberculosis

Abstract: Prostaglandins (PG) are potent mediators of intercellular communication, and PGE2 at high concentration is immunosuppressive for T-cell-mediated immunity. We studied the kinetics of PGE2 production and the expression of the enzymes related to its synthesis during the course of experimental pulmonary tuberculosis. Secondly, we analysed the pathological and immunological changes produced by the pharmacological suppression of PG production. In BALB/c mice infected via the trachea with Mycobacterium tuberculosis H37Rv there is an initial phase of partial resistance, dominated by type 1 cytokines plus tumour necrosis factor-alpha (TNF-alpha) and expression of the inducible form of nitric oxide synthase (iNOS), followed by a phase of progressive disease. During the early phase of the infection some activated macrophages located in the alveolar-capillary interstitium and in granulomas showed strong PGE2 immunostaining. However, PGE2 concentrations were relatively low and stable. Animals in this early phase of infection were treated with niflumic acid, a potent and specific blocker of cyclo-oxygenase 2, the rate-limiting enzyme of PG production. In comparison with control animals, the suppression of PG synthesis produced higher inflammation and expression of TNF-alpha, interleukin-1alpha and interferon-gamma (IFN-gamma), but almost complete disappearance of iNOS expression, which coexisted with a significant increment of bacterial load. The late progressive phase in this experimental model is characterized by progressive pneumonia, small granulomas and diminished expression of IFN-gamma, TNF-alpha and iNOS in coexistence with high expression of IL-4. Strong PGE2 immunostaining was seen in foamy macrophages localized in the pneumonic areas, and the PGE2 concentration was four-fold higher in this late phase of infection than during the early phase. When PG production was suppressed in animals suffering advanced phase infection, a significant reduction of pneumonia and bacillus load with striking increment of granuloma size was seen, and the expression of IFN-gamma, TNF-alpha and iNOS was also improved. These findings demonstrate a significant participation of PGE2 in the pathogenesis of pulmonary tuberculosis, showing that during the early phase of the infection there are low PGE2 concentrations which contribute to iNOS expression permitting the temporal control of bacillus growth, while the high PGE2 concentrations during the late phase of the disease contribute to down-regulate cell-mediated immunity, permitting disease progression.