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Showing papers by "National University of La Plata published in 1999"


Journal ArticleDOI
TL;DR: In this article, the authors evaluated both oxidative stress and the antioxidant response system in leaves from wheat (Triticum aestivum cv. Buck Poncho) subjected sequentially to drought and watering.
Abstract: The purpose of the present work was to evaluate both oxidative stress and the antioxidant response system in leaves from wheat (Triticum aestivum cv. Buck Poncho) subjected sequentially to drought and watering. Drought was imposed by withholding water until soil water potential reached - 2.0 MPa and maintained under those conditions for 24 h. DCFDA oxidation by wheat leaves was not significantly affected by drought, but watering led to an approximately 2-fold increase in DCFDA oxidation rate. However, no significant effect either on lipid radical content or on hydroperoxide content was measured after drought and drought followed by watering. Microsomes isolated from leaves exposed to drought, and from leaves exposed to drought followed by watering, generated a significantly higher amount of hydroxyl radical as compared to microsomes isolated from control leaves, suggesting a higher production of hydroxyl radical in the cellular water-soluble phase, after drought and watering as compared to control values. The content of α-tocopherol in wheat leaves was increased 2.4-fold after drought and β-carotene content was increased by 2.6-fold after drought. Hydration lowered lipid-soluble antioxidant content to control values. Total thiol content was increased by 70% after drought, and watering did not significantly alter the enhanced values. Drought decreased by 28.5% the content of reduced ascorbic acid. Taken as a whole, active species formed at wheat membranes after exposure to moderate water stress, are efficiently removed upon rehydration by reaction with an increased content of α-tocopherol and β-carotene. Moreover, a co-ordinated response involving glutathione reductase activity, thiols and ascorbic acid is triggered to limit free radical dependent effects.

231 citations


Journal ArticleDOI
TL;DR: The data indicate that the second force phase and the increase in [Ca(2+)](i) transient after stretch result from activation of the Na(+)/H(+) exchanger (NHE) increasing [Na(+)] (i) and leading to a secondary increase in [/Ca( 2+](i), which reflects an autocrine-paracrine mechanism.
Abstract: —Myocardial stretch produces an increase in developed force (DF) that occurs in two phases: the first (rapidly occurring) is generally attributed to an increase in myofilament calcium responsiveness and the second (gradually developing) to an increase in [Ca2+]i. Rat ventricular trabeculae were stretched from ≈88% to ≈98% of Lmax, and the second force phase was analyzed. Intracellular pH, [Na+]i, and Ca2+ transients were measured by epifluorescence with BCECF-AM, SBFI-AM, and fura-2, respectively. After stretch, DF increased by 1.94±0.2 g/mm2 (P<0.01, n=4), with the second phase accounting for 28±2% of the total increase (P<0.001, n=4). During this phase, SBFI340/380 ratio increased from 0.73±0.01 to 0.76±0.01 (P<0.05, n=5) with an estimated [Na+]i rise of ≈6 mmol/L. [Ca2+]i transient, expressed as fura-2340/380 ratio, increased by 9.2±3.6% (P<0.05, n=5). The increase in [Na+]i was blocked by 5-(N-ethyl-N-isopropyl)-amiloride (EIPA). The second phase in force and the increases in [Na+]i and [Ca2+]...

209 citations


Journal ArticleDOI
TL;DR: In this paper, a search for optical microvariability in a selected sample of 23 southern AGNs, which includes both radio-quiet and radio-loud objects, was conducted.
Abstract: We present results of a search for optical mi- crovariability in a selected sample of 23 southern AGNs, which includes both radio-quiet and radio-loud objects. Microvariations were clearly detected in 60% of the radio- loud sources, with amplitudes from 2.2% up to 8% within a single night. Night-to-night variability with amplitudes of 20% was also observed in the BL Lac object 0537 441. On the contrary, no rapid variability was detected at all in 8 radio-quiet quasars. We have used microvariability data previously reported for northern objects along with our new results for southern AGNs in order to estimate duty cycles of each class of objects from the largest possible sample. Most of the microvariability in radio-loud objects could be originating in interactions between relativistic shocks and features in the inner jets, although contributions from superluminal microlensing and accretion disk instabilities can be present in some sources. It is possible that the lat- ter phenomenon is responsible for the microvariability ob- served in northern radio-quiet quasars. We suggest that the dierence in the microvariability behaviour of radio and X-ray selected BL Lacs could be due to the eect of stronger magnetic elds in the latter group of objects, elds that can prevent the formation of features like den- sity inhomogeneities and bends in the base of the jets by Kelvin-Helmholtz macroscopic instabilities.

187 citations


Journal ArticleDOI
TL;DR: In this paper, low-order nonconforming Galerkin methods are analyzed for second-order elliptic equations subjected to Robin, Dirichlet, or Neumann boundary conditions in two and three dimensions.
Abstract: Low-order nonconforming Galerkin methods will be analyzed for second-order elliptic equations subjected to Robin, Dirichlet, or Neumann boundary conditions. Both simplicial and rectangular elements will be considered in two and three dimensions. The simplicial elements will be based on P 1 , as for conforming elements; however, it is necessary to introduce new elements in the rectangular case. Optimal order error estimates are demonstrated in all cases with respect to a broken norm in H 1 (Ω) and in the Neumann and Robin cases in L 2 (Ω).

182 citations


Journal ArticleDOI
TL;DR: In this paper, the authors proposed the use of the co-occurrence matrix of the time history of the intensity of a speckle pattern and suggested a measure of the activity based on one of its second order moments.
Abstract: We describe some aspects of the measurement of dynamic speckle activity. We propose the use of the co-occurrence matrix of the time history of the intensity of a speckle pattern. Then we suggest a measure of the activity based on the use of one of its second order moments. Some numerical simulations were performed for testing purposes. The method was applied to the study of the drying of paint and was compared with gravimetric measurements. Experimental results obtained are shown and a good correlation is obtained.

174 citations


Journal ArticleDOI
TL;DR: In this paper, a two-parameter non-Hermitian quantum mechanical Hamiltonian operator that is invariant under the combined effects of parity and time reversal transformations is considered.
Abstract: We consider a two-parameter non-Hermitian quantum mechanical Hamiltonian operator that is invariant under the combined effects of parity and time reversal transformations. Numerical investigation shows that for some values of the potential parameters the Hamiltonian operator supports real eigenvalues and localized eigenfunctions. In contrast with other parity times time reversal symmetric models which require special integration paths in the complex plane, our model is integrable along a line parallel to the real axis.

173 citations


Journal ArticleDOI
TL;DR: In this article, the effect of both size specimen and boundary conditions on the splitting tensile strength, determined from the Brazilian test, were studied experimentally. And the results of the tests were compared with the theoretical predictions obtained from a closed form analytical expression based on the cohesive crack model.
Abstract: The effect of both size specimen and boundary conditions on the splitting tensile strength, determined from the Brazilian test, were studied experimentally. A total of 110 splitting tests of granite and mortar specimens were performed, using cylindrical and prismatic specimens of sizes between 17 mm and 300 mm. To analyze the effect of the boundary conditions, the specimens were tested with different widths of load bearing strip in the range of size recommended by the standards. The influence of the rupture mode (stable or unstable crack propagation) on the splitting tensile strength was also explored. The results of the tests were compared with the theoretical predictions obtained from a closed form analytical expression based on the cohesive crack model. The validity of the classical limit strength theory for larger size specimens was also analyzed. The results indicate that the splitting tensile strength depends strongly on specimen size and on the boundary conditions of the test. As the size of the specimen increases and the relative width of the bearing strip decreases, the splitting strength tends asymptotically to the minimum value coincident with the tensile strength. The dependence of the Brazilian test on the specimen size and boundary conditions closely follows the predictions of the cohesive crack model.

143 citations


Journal ArticleDOI
TL;DR: In this article, the Rossi X-ray Timing Explorer was used to detect the frequency of the kilohertz quasi-periodic oscillations (kHz QPOs) of the LMXB and atoll source 4U 1608-52.
Abstract: We present new results based on observations carried out with the Rossi X-ray Timing Explorer during the decay of an outburst of the low-mass X-ray binary (LMXB) and atoll source 4U 1608-52. Our results appear to resolve, at least in 4U 1608-52, one of the long-standing issues about the phenomenology of the kilohertz quasi-periodic oscillations (kHz QPOs), namely the lack of a unique relation between the frequency of the kHz QPOs and the X-ray flux. We show that despite its complex dependence on the X-ray flux, the frequency of the kHz QPOs is monotonically related to the position of the source in the color-color diagram. Our findings strengthen the idea that, as in the case of Z sources, in the atoll sources the X-ray flux is not a good indicator of and that the observed changes in the frequency of the kHz QPOs in LMXBs are driven by changes in . These results raise some concern about the recently reported detection of the orbital frequency at the innermost stable orbit in 4U 1820-30.

132 citations


Journal ArticleDOI
TL;DR: The results suggest that the empirical use of Eugenia uniflora L. (Myrtaceae) as antihypertensive in Northeastern Argentina is mostly due to a hypotensive effect mediated by a direct vasodilating activity, and to a weak diuretic effect that could be related to an increase in renal blood flow.

131 citations


Journal ArticleDOI
TL;DR: In this article, the authors present a general hypothesis that humans in the Pampa and Patagonia regions evolved from generalists during the Late Pleistocene/Early Holocene to specialists during the Middle Holocene, according to diversity and relative taxonomic richness counts.

130 citations


Book ChapterDOI
15 Nov 1999
TL;DR: This paper presents the main ideas underlying the Object-Oriented Hypermedia Design Method (OOHDM) and shows that Web applications are built as views of conceptual models and introduces navigational contexts as the structuring mechanism for the navigational space.
Abstract: In this paper we argue that Web applications are a particular kind of hypermedia application and show how to model their navigational structure. We argue that if we need to design applications combining hypermedia navigation with complex transactional behaviors (as in E-commerce systems), we need a systematic development approach. We present the main ideas underlying the Object-Oriented Hypermedia Design Method (OOHDM) and show that Web applications are built as views of conceptual models. We present the abstraction primitives used to design conceptual and navigational structure of Web applications and describe the view definition language. We introduce navigational contexts as the structuring mechanism for the navigational space. Further work on designing Web applications with OOHDM is also presented.

Journal ArticleDOI
TL;DR: In this article, the frequency separation between the two quasi-periodic oscillations, Δν, is always significantly smaller than the frequency of the nearly coherent oscillations seen in this source during X-ray bursts, even at the lowest inferred mass accretion rate.
Abstract: We have analyzed 17 observations of the low-mass X-ray binary and atoll source 4U 1728-34, carried out by the Rossi X-Ray Timing Explorer in 1996 and 1997. We obtain precise measurements of the frequencies of the two simultaneous kilohertz quasi-periodic oscillations (kHz QPOs) in this source. We show that the frequency separation between the two QPO, Δν, is always significantly smaller than the frequency of the nearly coherent oscillations seen in this source during X-ray bursts, even at the lowest inferred mass accretion rate, when Δν seems to reach its maximum value. We also find that Δν decreases significantly, from 349.3±1.7 Hz to 278.7±11.6 Hz, as the frequency of the lower frequency kHz QPO increases from 615 to 895 Hz. This is the first time that variations of the kHz QPO peak separation are measured in a source that shows nearly coherent oscillations during bursts.

Journal ArticleDOI
TL;DR: In this article, the effects of the zinc bath chemical composition (changed by the addition of lead or antimony) on the texture and corrosion behaviour of hot-dip galvanized steel sheets were evaluated.
Abstract: The addition of lead or antimony to the zinc bath promotes both excessive growth of the zinc crystals and a dendritic solidification in the form of spangles. This phenomenon affects not only the zinc coating texture but also its surface appearance and corrosion resistance. Therefore, the effects of the zinc bath chemical composition (changed by the addition of lead or antimony) on the texture and corrosion behaviour of hot-dip galvanized steel sheets were evaluated. Standardized salt spray tests as well as exposure to a 0.5 M NaClO4 solution test were employed. The surface texture was analyzed using X-ray diffraction (XRD). In order to study the corrosion performance, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) techniques were employed. From the experimental results it was concluded that the characteristic texture of zinc coatings was not modified by an increase in the antimony content, but an increment in the lead content produced a crystallographic preferential orientation with the pyramidal family planes parallel to the coating surface. This last effect deteriorates not only the surface appearance but also the corrosion resistance of the galvanized steel sheets.

Journal ArticleDOI
TL;DR: The genotoxicity of the pyrethroid lambda-cyhalothrin was studied using the micronucleus test in erythrocytes of Cheirodon interruptus interruptus using a validation of the MN test in fishes for the assessment of genotoxic pollutants.
Abstract: In order to develop experimental models able to detect genotoxic effects of pollutants in aquatic organisms, the genotoxicity of the pyrethroid lambda-cyhalothrin was studied using the micronucleus test in erythrocytes of Cheirodon interruptus interruptus. The frequency of micronuclei was examined in blood smears obtained from fishes exposed in vivo to three different concentrations (0.05; 0.01; 0.001 ug/l) of the compound and sacrificed at nine sampling times (24, 48, 72, 96 h and 8, 12, 15, 19 and 23 days). As a positive control fishes were exposed to 5 mg/l of cyclophosphamide. Results obtained demonstrated the genotoxic effects of the pyrethroid in the experimental model employed. The variation in the micronuclei frequencies in the different sampling times could be related to the blood cell kinetics and the erythrocyte replacement. The results could be considered as a validation of the MN test in fishes for the assessment of genotoxic pollutants.

Journal ArticleDOI
TL;DR: In this paper, the authors presented detailed evolutionary calculations for carbon-oxygen-and helium-core white dwarfs with masses ranging from M = 0.1 to 1.2 M.
Abstract: We present detailed evolutionary calculations for carbon--oxygen- and helium-core white dwarf models with masses ranging from M= 0.1 to 1.2 M⊙ and for metallicities Z = 0.001 and 0. The sequences cover a wide range of hydrogen envelopes as well. We have taken finite-temperature effects fully into account by means of a detailed white dwarf evolutionary code, in which updated radiative opacities and equations of state for hydrogen and helium plasmas are considered. The energy transport by convection is treated within the formalism of the full-spectrum turbulence theory, as given by the self-consistent model of Canuto, Goldman & Mazzitelli. Convective mixing, crystallization, hydrogen burning and neutrino energy losses are taken into account as well. The set of models presented here is very detailed and should be valuable, particularly for the interpretation of observational data on low-mass white dwarfs recently discovered in numerous binary configurations, and also for the general problem of determining the theoretical luminosity function for white dwarfs. In this context, we compare our cooling sequences with the observed white dwarf luminosity function recently improved by Leggett, Ruiz & Bergeron and we obtain an age for the Galactic disc of ≈ 8 Gyr. Finally, we apply the results of this paper to derive stellar masses of a sample of low-mass white dwarfs.

Journal ArticleDOI
TL;DR: The role of Tsalli's non-extensive information measure within an a la Jaynes Information-Theory-based formulation of Statistical Mechanics is discussed in this paper. But the role of information measure is not discussed in this paper.
Abstract: The role of Tsalli's non-extensive Information Measure within an a la Jaynes Information-Theory-based formulation of Statistical Mechanics is discussed in rather detailed fashion.

Journal ArticleDOI
TL;DR: In this article, the size effect in the Brazilian test is analyzed theoretically using a nonlinear fracture model based on cohesive crack concepts and the results obtained are compared with the classical strength limit approach.
Abstract: Splitting strength determined in the Brazilian test is assumed to be a property independent of size and uniquely related to the intrinsic material strength. However, as was experimentally demonstrated by various authors, the splitting strength depends on the specimen size. In this paper, the size effect in the Brazilian test is analyzed theoretically using a nonlinear fracture model based on cohesive crack concepts and the results obtained are compared with the classical strength limit approach. Two important variables are studied: the load-bearing strip and the geometry of the specimen. From the numerical results a closed form expression is proposed, dependent on the width of the bearing strip and on geometry. The results confirm that splitting strength decreases with the specimen size, tending towards an asymptotic solution for large size specimens. Within the size range analyzed (0.1 m to 2.5 m diameter for typical concrete) the splitting strength can vary by up to 25% in cylindrical specimens and by up to 35% in prismatic square section specimens, although this size effect is strongly dependent on the load-bearing strip. For widths of bearing strip smaller than 4% of the specimen diameter, the effect of the specimen size is negligible and the splitting strength approaches the tensile strength for any practical specimen size.

Journal ArticleDOI
TL;DR: A thermodynamics that seems to be able to treat equilibrium and nonequilibrium situations in a manner entirely similar to the conventional one is developed.
Abstract: It is shown that the Legendre-transform structure of thermodynamics can be replicated without any change if one replaces the entropy S by Fisher's information measure I. Also, the important thermodynamic property of concavity is shown to be obeyed by I. By this use of the Fisher information measure we develop a thermodynamics that seems to be able to treat equilibrium and nonequilibrium situations in a manner entirely similar to the conventional one.

Journal Article
TL;DR: In this article, the authors present a set of obras traducidas and publicadas in castellano, including Documentos de cultura, documentos de barbarie, La carcel del lenguaje, La estetica geopolitica, El posmodernismo or the logica cultural del capitalismo avanzado.
Abstract: Frederic Jameson es profesor de Literatura Comparada en la Universidad de Duke y autor de numerosas obras traducidas y publicadas en castellano , entre las que destacamos por su fuerte incidencia en los ambitos de reflexion academica: Documentos de cultura, documentos de barbarie, La carcel del lenguaje, La estetica geopolitica y El posmodernismo o la logica culturaldel capitalismo avanzado. Por su parte, Slavoj Zizek es Doctor en Filosofia, investigador del Instituto de Estudios Sociales de Liubiana, Eslovenia y profesor visitante de la New School for Social Research de Nueva York; entre sus obras mas conocidas se encuentran El sublimeobjeto de la Ideologia, Goza tu sintoma y Mirando al sesgo. Eduardo Gruner es profesor titular de Teoria Politica, en la Facultad de Ciencias Sociales de la Universidad de Buenos Aires, y de Literatura y Cine, en la Facultad de Filosofia y Letras, tambien de la Universidad de Buenos Aires; ha escrito los libros El genero culpable y Lasformas de la espada.

Journal ArticleDOI
TL;DR: In this article, the effects of the galactic magnetic field on the ultra high energy cosmic ray propagation were studied and the results were pictorially illustrated as the stretching and folding of a ''sheet'' describing the sky seen on Earth.
Abstract: We study the effects of the galactic magnetic field on the ultra high energy cosmic ray propagation. We show that the deflections of the cosmic ray trajectories can have many important implications such as (de)magnification of the cosmic ray fluxes by lensing effects (which can modify the spectrum of individual sources), the formation of multiple images of a source or the existence of regions of the sky to which the Earth is almost blind. The appearance of image pairs is related to the existence of critical curves in the magnification maps, which divide regions in the sky where the images have opposite parities. The results are pictorially illustrated as the stretching and folding of a `sheet' describing the sky seen on Earth. Making use of the most energetic AGASA events we emphasize the need to know the cosmic ray composition and the structure of the magnetic field when attempting to do detailed cosmic ray astronomy.

Journal Article
TL;DR: In this paper, a biozonation and a chronostratigraphic chart for the continental Triassic of Argentina are proposed based on the multivariate analysis of 218 taxa distributed in 15 lithostratigraphraphic units and 134 taxa from 28 paleocommunities.
Abstract: A paleobotanical biozonation and a chronostratigraphic chart for the continental Triassic of Argentina are proposed. The biostratigraphic study was based on the multivariate analysis of 218 taxa distributed in 15 lithostratigraphic units and 134 taxa from 28 paleocommunities. The biostratigraphic analysis was also supported on the Permian or Jurassic lineage of several species, their acme, the temporal turnover and the evolution of the paleofloras. This information was evaluated together with the stratigraphic location of the fossiliferous horizons, the geochronologic information and the relationship between paleocommunities and sedimentary facies and environments. Five biozones were defined: 1. Dictvoptiullum castellanosii, [ohnsionia stcizneriana and Saportaea dichotoma Biozone (CSD), Represented by a mixed forest and identified in the Barreal and lschichuca Forrnations. 2. Yabeiella marefesiaca, Scytophylllll11 bonettiae and Protoptvflloctadoxvion cormderitacnsis Biozone (MBC). Mixed forests recognized in the Potrerillos, Cortaderita (lower section) and El Alcazar Formations. 3. Ynbeiella brackebuschiana, Scytophylllll11 neuburgianum and Rhexoxvlon piatnitzkui Biozone (BNP). Mixed forests, identified in the Vera, Potrerillos and Ischigualasto Formations, and in the El Tranquilo Group. 4. Dicroidium odontopteroides and D. lancijolium Biozone (OL). Represented by a herbaceous and shrub-like paleocornmunity, defined in the Cacheuta Formation and in the upper section of the Cortaderita Formation. 5. Dictyop/iylllll11 tenuiserraium, Linguijoliuni arctum and Protocircoporoxylon nuirionoensis Biozone (DLM). Deciduous and mesoxerophytic forests, identified in the Los Colorados and Rio Blanco redbeds, the Paso Flores Formation and the Tronquimalal Group. The following local stages are defined on the bases of biozone chronology, analysis of the main stratigraphic unconformities and evolution of the basin infill: Barrealian (Early to Middle Triassic), Cortaritian (late Middle to early Late Triassic) and Flarian (upper Late Triassic).

Journal ArticleDOI
TL;DR: Evidence is presented that the drought stress syndrome in the flag leaves and ears of wheat plants, provoked by the production of ethylene is reversed by the application of a free radical scavenger, sodium benzoate or the ethylene synthesis inhibitor, aminoethoxyvinylglycine.
Abstract: In this work we present evidence that the drought stress syndrome in the flag leaves and ears of wheat plants, provoked by the production of ethylene (shortening the grain filling period and lowering the grain weight) is reversed by the application of a free radical scavenger, sodium benzoate or the ethylene synthesis inhibitor, aminoethoxyvinylglycine. Rehydration by watering also attenuated the detrimental effect of the water deficit. Consequently, the grain filling period was longer, the grain weight increased, and the total protein content was higher than that in plants watered regularly.

Journal ArticleDOI
TL;DR: Sera from 76 horses from Argentina were examined for antibodies to Sarcocystis neurona, Toxoplasma gondii and Neospora caninum, and this is the first report of S. neurona infection in horses in Argentina.

Journal ArticleDOI
TL;DR: In this paper, the authors provided an account of the possible acceleration of iron nuclei up to energies of up to 300 eV in the nearby, metally rich starburst galaxy NGC 253.
Abstract: We provide an account of the possible acceleration of iron nuclei up to energies $\ensuremath{\sim}300 \mathrm{EeV}$ in the nearby, metally rich starburst galaxy NGC 253. It is suggested that particles can escape from the nuclear region with energies of $\ensuremath{\sim}{10}^{15} \mathrm{eV}$ and can then be reaccelerated at the terminal shock of the galactic superwind generated by the starburst, avoiding the photodisintegration expected if the nuclei were accelerated in the central region of high photon density. We have also made estimates of the expected arrival spectrum, which displays a strong dependence on the energy cutoff at the source.

Journal ArticleDOI
TL;DR: In this paper, the influence of both immersion time and a small addition of titanium to the galvanizing bath on coating characteristics was investigated, and it was observed that a small amount of titanium could serve as a catalyst for the iron-aluminium reaction, allowing a greater development of the inhibition layer and delaying growth of the Fe 2 Al 5 intermetallic.
Abstract: Zinc–iron phases may develop at the steel substrate/zinc coating interface during the hot-dip galvanizing process. These phases are hard and brittle, and make the material unsuitable for the forming process. Growth of the zinc–iron phases could be controlled adding 0.18 to 0.30% of aluminium to the galvanizing bath, which reacts with iron to produce a thin layer of intermetallic Fe 2 Al 5 . The latter hinders alloying between the steel sheet and molten zinc, and is therefore referred to as the inhibition layer since it inhibits or retards the formation of Fe–Zn phases. Nevertheless, this layer is unstable and local growth of Fe–Zn phases (‘out-burst’) is found at longer immersion times. This ‘out-burst’ phenomenon depends on many factors, such as the chemical compositions of both the bath and the steel, and the immersion time. The aim of the work was to investigate the influence of both immersion time and a small addition of titanium to the galvanizing bath on coating characteristics. Thus, plain carbon steel sheets were galvanized with alloys A and B, which had different chemical compositions. The immersion time was varied between 1 and 120 s. Cross-sections of samples were observed by scanning electron microscopy. The analysis showed that, even for very short immersion times, samples galvanized with alloy A develop ‘out-bursts’ whereas those with alloy B do not, even for longer immersion times. The Fe 2 Al 5 intermetallic structure was also investigated. It was observed that samples galvanized with alloy B showed, for the same immersion time, bigger grains than the ones galvanized with alloy A. However, for both alloys, the intermetallic development was greater for longer immersion time. Such observations suggest that a small amount of titanium could serve as a catalyst for the iron–aluminium reaction, allowing a greater development of the inhibition layer and delaying growth of the Fe–Zn intermetallic.

Journal ArticleDOI
TL;DR: Results demonstrate the complexity of the rhizobial populations present in the acidic soils represented by a main group of N2-fixing rhizobia and a second group of ineffective and less-predominant isolates related to the AT strain Or191.
Abstract: Over 4 million ha of land throughout Argentina and Uruguay are used for the production of alfalfa (Medicago sativa L.) (38). Therefore, it is important to manage the N2-fixing symbiosis to maximize the production of this crop. An important constraint to this aim results from the moderately low soil pH that affects the establishment of an effective symbiosis with indigenous and inoculated rhizobia. Large areas of arable lands in the central region of Argentina have progressively acidified over the last 10 to 20 years (21, 34), where the continuous cultivation over time without crop rotation has been identified as one of the main factors that favored the acidification of soils (21, 34). It has been shown that the poor symbiosis at low pH results from a variety of influences upon the host plant (24, 27), the population of rhizobia (31), and the symbiotic interaction itself (11, 36, 41). Early studies by Munns (36, 37) compared the progress of symbiosis under neutral and acid conditions, concluding that early steps during preinfection are the more acid-sensitive events (36). This observation is in agreement with results reported by Caetano Anolles et al. in 1989, who showed a negative influence of low pH on the bacterial attachment to roots. Most of the fundamental research has sought to characterize the physiology of the interaction (11, 23, 25, 27, 28) and the effects of acidity in laboratory and in field experiments (1a, 39) and only more recently to address the identification of the bacterial determinants of acid tolerance (18, 43, 44). Particularly, Sinorhizobium meliloti strains are among the more acid-sensitive rhizobia (6, 19, 20). Most S. meliloti isolates tolerate acidity in the range between pH 5.5 and 6.0 (25). Although there is no basis to support that a higher acid tolerance of the bacteria will correspond to a better symbiotic performance under acidic conditions, it was found that acid-tolerant (AT) S. meliloti strains isolated from nodulated Medicago spp. collected in Sardinia enhanced the establishment of medic pastures in mildly acidic soils from Western Australia (23, 25). In any case, although symbiotic proficiency and acid tolerance of rhizobia are both desirable bacterial traits, they are not necessarily linked (17, 22, 25, 30, 31). While basic aspects of symbiosis have been extensively characterized, further work is still needed in order to increase our knowledge on the rhizobial ecology under suboptimal environmental conditions such as acidity. The characterization of the populations of alfalfa-nodulating rhizobia from acid soils had shown the presence of alfalfa-specific nodulating S. meliloti and another lineage represented by strain Or191 isolated from Oregon which also nodulates the common bean (13, 15). Strain Or191 was also shown to be more tolerant to acidity on agar plates (pH 5.2). Results of the genetic characterization indicate that strain Or191 is related to a previously unrecognized taxonomic group that includes strains of Rhizobium phaseoli type I (15), since renamed Rhizobium etli (32, 45). The AT strain Or191 was ineffective in alfalfa (13) but had measurable levels of symbiotic nitrogen fixation (15). So far, a detailed examination of the composition of native populations of alfalfa-nodulating rhizobia in soils from Argentina and Uruguay has not yet been carried out. In this work, we present results on the isolation and characterization of alfalfa-nodulating rhizobia from local acid soils and demonstrate the presence of two rhizobial populations with marked differences in their acid tolerance and symbiotic properties.

Journal ArticleDOI
TL;DR: It is concluded that the index of fossorial ability (IFA) discriminates among the species according to their fossorial habits within orders, but it is not equally useful in distinguishing fossorial species between orders.
Abstract: Ulnar dimensions were measured in 14 species of armadillos (Xenarthra: Dasypodidae). An index of fossorial ability (IFA) was constructed, relating the length of the olecranon process to the remaining length of the ulna. For comparative purposes, the same measurements were taken in 14 other species of mostly South American mammals belonging to 3 orders and 11 families. The fossorial habits of these mammals were classified into 3 categories: (1) species mostly cursorial and non-digging; (2) species that often dig, but to which digging plays no essential part in their alimentary strategy and are not burrowers; and (3) species that are burrowers. IFA means of the studied mammal orders were compared using one-way analysis of variance on log-transformed data. Bivariate size allometry between ulnar dimensions and body mass was assessed by fitting (least squares and geometric mean) linear regressions of log-transformed data. It is concluded that the IFA discriminates among the species according to their fossorial habits within orders, but it is not equally useful in distinguishing fossorial species between orders. In armadillos, the relationships between ulnar dimensions and body mass are isometrical. Finally, the IFA is independent of body size.

Journal ArticleDOI
TL;DR: Results suggest that transplacental transmission of N. caninum in dairy cows in Argentina is frequent and the distribution of positive and negative sera was similar when measured by ELISA, except that, depending on cut-off titre, the ELISA indicated a greater number of seropositive cows that were negative by the IFAT and N.caninum agglutination test.

Journal ArticleDOI
TL;DR: In this article, a renewed discussion on IT has been proposed, with the aim of divulging the unsolvable difficulties for obtaining relevant information from this that could be shared between laboratories, and the influence of column inlet pressure pi and the ratio L/dc on the reproducibility of IT is utilized as an example to illustrate and discuss the basic concepts.

Journal ArticleDOI
TL;DR: Fisher's information measure is shown to be a rather powerful tool for the analysis of complex, non-stationary signals, as exemplified by the Henon map and EEGs, and could be of some significance concerning the automatic detection of epileptic spikes.