Showing papers by "University of Buenos Aires published in 1996"

A global analysis of root distributions for terrestrial biomes

Abstract: Understanding and predicting ecosystem functioning (e.g., carbon and water fluxes) and the role of soils in carbon storage requires an accurate assessment of plant rooting distributions. Here, in a comprehensive literature synthesis, we analyze rooting patterns for terrestrial biomes and compare distributions for various plant functional groups. We compiled a database of 250 root studies, subdividing suitable results into 11 biomes, and fitted the depth coefficient β to the data for each biome (Gale and Grigal 1987). β is a simple numerical index of rooting distribution based on the asymptotic equation Y=1-βd, where d = depth and Y = the proportion of roots from the surface to depth d. High values of β correspond to a greater proportion of roots with depth. Tundra, boreal forest, and temperate grasslands showed the shallowest rooting profiles (β=0.913, 0.943, and 0.943, respectively), with 80-90% of roots in the top 30 cm of soil; deserts and temperate coniferous forests showed the deepest profiles (β=0.975 and 0.976, respectively) and had only 50% of their roots in the upper 30 cm. Standing root biomass varied by over an order of magnitude across biomes, from approximately 0.2 to 5 kg m-2. Tropical evergreen forests had the highest root biomass (5 kg m-2), but other forest biomes and sclerophyllous shrublands were of similar magnitude. Root biomass for croplands, deserts, tundra and grasslands was below 1.5 kg m-2. Root/shoot (R/S) ratios were highest for tundra, grasslands, and cold deserts (ranging from 4 to 7); forest ecosystems and croplands had the lowest R/S ratios (approximately 0.1 to 0.5). Comparing data across biomes for plant functional groups, grasses had 44% of their roots in the top 10 cm of soil. (β=0.952), while shrubs had only 21% in the same depth increment (β=0.978). The rooting distribution of all temperate and tropical trees was β=0.970 with 26% of roots in the top 10 cm and 60% in the top 30 cm. Overall, the globally averaged root distribution for all ecosystems was β=0.966 (r 2=0.89) with approximately 30%, 50%, and 75% of roots in the top 10 cm, 20 cm, and 40 cm, respectively. We discuss the merits and possible shortcomings of our analysis in the context of root biomass and root functioning.

Maximum rooting depth of vegetation types at the global scale.

Abstract: The depth at which plants are able to grow roots has important implications for the whole ecosystem hydrological balance, as well as for carbon and nutrient cycling. Here we summarize what we know about the maximum rooting depth of species belonging to the major terrestrial biomes. We found 290 observations of maximum rooting depth in the literature which covered 253 woody and herbaceous species. Maximum rooting depth ranged from 0.3 m for some tundra species to 68 m for Boscia albitrunca in the central Kalahari; 194 species had roots at least 2 m deep, 50 species had roots at a depth of 5 m or more, and 22 species had roots as deep as 10 m or more. The average for the globe was 4.6±0.5 m. Maximum rooting depth by biome was 2.0±0.3 m for boreal forest. 2.1±0.2 m for cropland, 9.5±2.4 m for desert, 5.2±0.8 m for sclerophyllous shrubland and forest, 3.9±0.4 m for temperate coniferous forest, 2.9±0.2 m for temperate deciduous forest, 2.6±0.2 m for temperate grassland, 3.7±0.5 m for tropical deciduous forest, 7.3±2.8 m for tropical evergreen forest, 15.0±5.4 m for tropical grassland/savanna, and 0.5±0.1 m for tundra. Grouping all the species across biomes (except croplands) by three basic functional groups: trees, shrubs, and herbaceous plants, the maximum rooting depth was 7.0±1.2 m for trees, 5.1±0.8 m for shrubs, and 2.6±0.1 m for herbaceous plants. These data show that deep root habits are quite common in woody and herbaceous species across most of the terrestrial biomes, far deeper than the traditional view has held up to now. This finding has important implications for a better understanding of ecosystem function and its application in developing ecosystem models.

Smoking and low antioxidant levels increase oxidative damage to sperm DNA.

Abstract: Our previous studies have shown that men with low ascorbate intake have markedly increased oxo8dG in the DNA of their sperm. Because cigarette smoke is high in oxidants and depletes plasma and tissue antioxidants, oxidative DNA damage in sperm and tocopherol and ascorbate levels in seminal plasma were determined in smokers and non-smokers. The level in sperm DNA of oxo8dG, an oxidative lesion of guanine, was 50% higher in smokers compared to nonsmokers (p = 0.005). The concentration of alpha-tocopherol in seminal plasma was decreased in smokers by 32% (p = 0.03). Smoking and low antioxidant levels increase oxidative damage to sperm DNA. We discuss the possibility that paternal smoking causes mutations in sperm that lead to cancer, birth defects, and genetic diseases in offspring.

Detection of coloured stimuli by honeybees: minimum visual angles and receptor specific contrasts

Abstract: Honeybees Apis mellifera were trained to distinguish between the presence and the absence of a rewarded coloured spot, presented on a vertical, achromatic plane in a Y-maze. They were subsequently tested with different subtended visual angles of that spot, generated by different disk diameters and different distances from the decision point in the device. Bees were trained easily to detect bee-chromatic colours, but not an achromatic one. Chromatic contrast was not the only parameter allowing learning and, therefore, detection: α min, the subtended visual angle at which the bees detect a given stimulus with a probability P 0 = 0.6, was 5° for stimuli presenting both chromatic contrast and contrast for the green photoreceptors [i.e. excitation difference in the green photoreceptors, between target and background (green contrast)], and 15° for stimuli presenting chromatic but no green contrast. Our results suggest that green contrast can be utilized for target detection if target recognition has been established by means of the colour vision system. The green-contrast signal would be used as a far-distance signal for flower detection. This signal would always be detected before chromatic contrast during an approach flight and would be learned in compound with chromatic contrast, in a facilitation-like process.

Rooting depth, water availability, and vegetation cover along an aridity gradient in Patagonia

Abstract: Above- and belowground biomass distribu- tion, isotopic composition of soil and xylem water, and carbon isotope ratios were studied along an aridity gradi- ent in Patagonia (44-45~ Sites, ranging from those with Nothofagus forest with high annual rainfall (770 ram) to Nothofagus scrub (520 mm), Festuca (290 mm) and Stipa (160 mm) grasslands and into desert vegetation (125 ram), were chosen to test whether root- ing depth compensates for low rainfall. Along this gradi- ent, both mean above- and belowground biomass and leaf area index decreased, but average carbon isotope ra- tios of sun leaves remained constant (at -27%o), indicat- ing no major differences in the ratio of assimilation to stomatal conductance at the time of leaf growth. The depth of the soil horizon that contained 90% of the root biomass was similar for forests and grasslands (about 0.80-0.50 m), but was shallower in the desert (0.30 m). In all habitats, roots reached water-saturated soils or ground water at 2-3 m depth. The depth profile of oxy- gen and hydrogen isotope ratios of soil water corre- sponded inversely to volumetric soil water contents and showed distinct patterns throughout the soil profile due to evaporation, water uptake and rainfall events of the past year. The isotope ratios of soil water indicated that high soil moisture at 2-3 m soil depth had originated from rainy periods earlier in the season or even from past rainy seasons. Hydrogen and oxygen isotope ratios of xylem water revealed that all plants used water from re- cent rain events in the topsoil and not from water-saturat- ed soils at greater depth. However, this study cannot ex- plain the vegetation zonation along the transect on the basis of water supply to the existing plant cover. Al- though water was accessible to roots in deeper soil layers in all habitats, as demonstrated by high soil moisture, earlier rain events were not fully utilized by the current plant cover during summer drought. The role of seedling establishment in determining species composition and vegetation type, and the indirect effect of seedling estab- lishment on the use of water by fully developed plant cover, are discussed in relation to climate change and vegetation modelling.

Isolation and selection of biosurfactant-producing bacteria.

Abstract: Haemolysis has been used as an initial selection criterion for the primary isolation of surfactant-producing bacteria. Only 37 of 492 strains of different origins had haemolytic activity. These 37 strains, together with 49 non-haemolytic ones chosen at random, were studied for surface activity. Only five strains, all of them haemolytic, tested positive. Haemolysis and biosurfactant-production are thus probably associated.

Crystal structure of a T-cell receptor β-chain complexed with a superantigen

Abstract: SUPERANTIGENS (SAgs) are viral or bacterial proteins that act as potent T-cell stimulants and have been implicated in a number of human diseases, including toxic shock syndrome1,2, diabetes mellitus3 and multiple sclerosis4. The interaction of SAgs with the T-cell receptor (TCR) and major histocompatibility complex (MHC) proteins results in the stimulation of a disproportionately large fraction of the T-cell population2. We report here the crystal structures of the β-chain of a TCR complexed with the Staphylococcus aureus enterotoxins C2 and C3 (SEC2, SEC3). These enterotoxins, which cause both toxic shock and food poisoning, bind in an identical way to the TCR β-chain. The complementarity-determining region 2 (CDR2) of the β-chain and, to lesser extents, CDR1 and hypervariable region 4 (HV4), bind in a cleft between the two domains of the SAgs. Thus, there is considerable overlap between the SAg-binding site and the peptide/MHC-binding sites of the TCR. A model of a TCR–SAg–MHC complex constructed from the crystal structures of (1) the β-chain–SEC3 complex, (2) a complex between staphylococcal enterotoxin B (SEB) and an MHC molecule5, and (3) a TCR Vα domain6, reveals that the SAg acts as a wedge between the TCR and MHC to displace the antigenic peptide away from the TCR combining site. In this way, the SAg is able to circumvent the normal mechanism for T-cell activation by specific peptide/MHC complexes.

Phytochrome A Mediates the Promotion of Seed Germination by Very Low Fluences of Light and Canopy Shade Light in Arabidopsis

Abstract: Seeds of the wild type (WT) and of the phyA and phyB mutants of Arabidopsis thaliana were exposed to single red light (R)/far-red light (FR) pulses predicted to establish a series of calculated phytochrome photoequilibria (Pfr/P). WT and phyB seeds showed biphasic responses to Pfr/P. The first phase, i.e. the very-low-fluence response (VLFR), occurred below Pfr/P = 10-1%. The second phase, i.e. the low-fluence response, occurred above Pfr/P = 3%. The VLFR was similarly induced by either a FR pulse saturating photoconversion or a subsaturating R pulse predicted to establish the same Pfr/P. The VLFR was absent in phyA seeds, which showed a strong low-fluence response. In the field, even brief exposures to the very low fluences of canopy shade light (R/FR ratio

Solar Ultraviolet-B Radiation Affects Seedling Emergence, DNA Integrity, Plant Morphology, Growth Rate, and Attractiveness to Herbivore Insects in Datura ferox.

Abstract: To study functional relationships between the effects of solar ultraviolet-B radiation (UV-B) on different aspects of the physiology of a wild plant, we carried out exclusion experiments in the field with the summer annual Datura ferox L. Solar UV-B incident over Buenos Aires reduced daytime seedling emergence, inhibited stem elongation and leaf expansion, and tended to reduce biomass accumulation during early growth. However, UV-B had no effect on calculated net assimilation rate. Using a monoclonal antibody specific to the cyclobutane-pyrimidine dimer (CPD), we found that plants receiving full sunlight had more CPDs per unit of DNA than plants shielded from solar UV-B, but the positive correlation between UV-B and CPD burden tended to level off at high (near solar) UV-B levels. At our field site, Datura plants were consumed by leaf beetles (Coleoptera), and the proportion of plants attacked by insects declined with the amount of UV-B received during growth. Field experiments showed that plant exposure to solar UV-B reduced the likelihood of leaf beetle attack by one-half. Our results highlight the complexities associated with scaling plant responses to solar UV-B, because they show: (a) a lack of correspondence between UV-B effects on net assimilation rate and whole-plant growth rate, (b) nonlinear UV-B dose-response curves, and (c) UV-B effects of plant attractiveness to natural herbivores.

Circulating antibodies against rat parotid gland M3 muscarinic receptors in primary Sjögren's syndrome

Abstract: In this study we demonstrate that IgG present in the sera of patients with primary Sjogren's syndrome (PSS) could bind and activate muscarinic acetylcholine receptors (mAChRs) of rat parotid gland. These antibodies were able to inhibit in a non-competitive manner the binding of 3H-quinuclidinyl benzilate (QNB) to mAChRs of purified rat parotid gland membranes. Moreover, IgG from PSS could modify biological effects mediated by mAChR activation; i.e. decrease cAMP, increase phosphoinositide turnover without affecting cGMP. Atropine and 4-DAMP blocked all of these effects, and carbachol mimicked them, confirming the M3 subtype mAChRs mediated PSS IgG action. Neither binding nor biological effect were obtained with IgG from sera of normal women. The prevalence of cholinergic antibody was 100% in PSS, and was independent of Ro/SS-A and La/SS-B antibodies. It could be concluded that antibody against mAChRs may be another serum factor to be considered in the pathophysiology of the development of PSS.

Robust Estimation in the Logistic Regression Model

Abstract: A new class of robust and Fisher-consistent M-estimates for the logistic regression models is introduced. We show that these estimates are consistent and asymptotically normal. Their robustness is studied through the computation of asymptotic bias curves under point-mass contamination for the case when the covariates follow a multivariate normal distribution. We illustrate the behavior of these estimates with two data sets. Finally, we mention some possible extensions of these M-estimates for a multinomial response.

Hippocampal cGMP and cAMP are differentially involved in memory processing of inhibitory avoidance learning.

Abstract: Cyclic GMP (cGMP) and cyclic AMP (cAMP) have been proposed to participate in the early and late stages of long-term potentiation (LTP), respectively. Here we report on the effect of post-training intrahippocampal infusion of membrane-permeable analogues of these cyclic nucleotides on the consolidation of inhibitory avoidance learning in rats and on the effect of this task on hippocampal cGMP and cAMP levels. Bilateral intrahippocampal microinjection of 8 Br-cGMP (1.25 micrograms per side) enhanced retention test performance when given immediately (0 min), but not when given 180 min, after training. In marked contrast, intrahippocampal infusion of the same dose of 8 Br-cAMP facilitated memory consolidation when given 180 min, but not 0 min, after training. Rats submitted to an inhibitory avoidance task showed a significant increase in the amount of cGMP in the hippocampus at 0 and 30 min after training, and in the amount of cAMP 30 and 180 min after training. Taken together, these results indicate that cGMP-regulated processes in the hippocampus play an important role in the early stages of memory consolidation and that cAMP signalling pathways are involved in the late post-training memory processing of inhibitory avoidance learning.

Quantitative Spin-Trapping Studies of Weakly Illuminated Titanium Dioxide Sols. Implications for the Mechanism of Photocatalysis

Abstract: Initial quantum yields φ for the formation of DMPO-OH adducts in clear, aerated, unbuffered TiO2 sols irradiated at 295 nm were determined by kinetic electron paramagnetic resonance spectrometry as function of the 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trap concentration, photon irradiance Io, and added dichlorobenzene DCB, at 298 K. φ steadily increases with [DMPO] in the range 0.3 mM to 0.3 M, extrapolating to φ[DMPO]→∞ = 0.54 ± 0.10, in contrast with the constant yields obtained for the homogeneous generation of OH radicals in the photodissociation of H2O2 at 254 nm under similar conditions. In TiO2 sols at [DMPO] = 0.6 mM, φ (≈0.002) remains constant over a 20-fold variation of Io, but decreases upon addition of comparable DCB concentrations. These observations prove that (1) DMPO reacts with photogenerated holes and/or OH radicals on the surface of TiO2 particles rather than in the solution bulk, (2) it is possible to approach quantitative carrier trapping, (3) DCB is as reactive as DMPO toward...

Ecosystem responses to changes in plant functional type composition: An example from the Patagonian steppe

Abstract: . Grass cover along a grazing intensity gradient in Patagonia decreases, whereas bare soil and shrub cover increases. Our objective was to study the effect of a change in the dominant plant functional type on soil water balance, primary production, herbivore biomass, roughness, and albedo. Using a soil water balance model, we found increases in evaporation and deep drainage, and a decrease in total transpiration along the grazing intensity gradient. Above-ground primary production, estimated from transpiration, decreased along the grazing intensity gradient because shrubs did not fully compensate for the decrease in grass production. Using a statistical model, we calculated herbivore biomass from estimates of above-ground primary production. Estimated herbivore biomass was lowest in the shrub-dominated extreme of the grazing gradient. Roughness increased from the grass-dominated to the shrub-dominated community. Albedo had a maximum at an intermediate position along the gradient. Our results suggest that changes in plant functional type composition, independent of changes in biomass, affect ecosystem functioning and the exchange of energy and material with the atmosphere. Grasses and shrubs proved to be appropriate plant functional types to link structure and function of ecosystems.

Biological nitrogen fixation in Azospirillum strain-maize genotype associations as evaluated by the 15N isotope dilution technique

Abstract: Few studies of the inoculation of cereal crops with N2-fixing bacteria have included more than one or two plant genotypes. In a recent study performed in Argentina using 12 different maize genotypes, it was found in 2 consecutive field experiments that several of them responded consistently, either negatively or positively, to inoculation with a mixture of strains of Azospirillum spp. The present study in post was performed to investigate the effect of inoculation of individual strains (and a mixture) of Azospirillum spp., and their nitrate reductase negative (NR-) mutants, on the growth of four of these maize genotypes. Two of these genotypes were grown in 15N-labelled soil with the aim of quantifying any contributions of biological N2 fixation. Two genotypes (Morgan 318 and Dekalb 4D-70) produced similar increases in grain yield when they were inoculated with a mixture of Azospirillum spp. strains or fertilized with the equivalent of 100 kg N ha-1. The other genotypes (Dekalb 2F-11 and CMS 22) showed little response to inoculation or N fertilization. The Morgan 318 and Dekalb 4D-70 genotypes showed a large increase in total N accumulation, suggesting that the response was due to increased N acquisition, but not due to bacterial nitrate reductase as the NR- mutants generally caused plant responses similar to those of the parent strains. Despite problems with the stabilization of the 15N enrichment in the soil, the 15N isotope dilution results indicated that there were very significant biological nitrogen fixation (BNF) contributions to the Dekalb 4D-70 and CMS 22 maize genotypes.

Autosomal dominant pattern of distal subungual onychomycosis caused by Trichophyton rubrum.

Abstract: mis, and subcutaneous fat without evidence of epidermotropism.2, 5, 7 Our patient demonstrates that ~//8 CTCL can present both clinically and histologically as typical mycosis fungoides. This supports the view that definition of a new clinicopathologic entity is not indicated.6, 8 However, the rapid progression described in this patient, which is not often seen in mycosis fungoides, may be related to the nature of these cells; ",//8 TCR rearrangement occurs earlier during T-cell maturation in the thymus. If this rearrangement fails, call3 TCRs are expressed. 11 CD7 is also an early marker of T-cell differentiation. Therefore T cells expressing CD7 and ~/Ki TCR can be regarded as less mature and may respond differently to the various cytokines presumed to play a role in the progression of mycosis fungoides. 12 The altered response may lead to either spread of the eruption or to the development of tumors within a short period, thus resulting in more aggressive behavior.

Oxidant Defense Systems in Testes from Zinc-Deficient Rats

Abstract: Previous studies have demonstrated that zinc deficiency can be associated with high rates of oxidative damage to testes lipids, proteins, and DNA in male rats. In the present work, different aspects of the oxidant defense system (enzymes and lipid-soluble antioxidant substances) were characterized in the testes of control and zinc-deficient rats. Seventeen-day-old males were given free access to either a control (25 microg Zn/g) or a zinc-deficient (0.5 microg Zn/g) diet, or the 25 microg Zn/g diet at a level of food intake similar to that of zinc-deficient rats. Animals were sacrificed 14 days after the initiation of the diet. The activities of copper-zinc superoxide dismutase (CuZn SOD) and glutathione reductase (GRed) were significantly higher (34% and 23%, respectively) in testes from the zinc-deficient animals than in those of the ad libitum controls. In testes, the activities of manganese superoxide dismutase (Mn SOD) and glutathione peroxidase (GPx), and the concentration of alpha-tocopherol and ubiquinol-9 and -10 were similar among the groups. However, the ratio of reduced/total concentration of both ubiquinols was higher in the zinc-deficient and restrict-fed animals than in the ad libitum controls. Testes homogenates from the zinc-deficient rats showed a low susceptibility to Fe(II)-induced oxidation, which could be explained in part by a lower peroxidation index, mainly due to the decreased testicular content of the fatty acid 20:4 observed in these animals. In summary, both undernutrition and zinc-deficiency can cause an oxidative stress situation in testes, for which cells tend to compensate by increasing select components of the oxidant defense system.

Molecular epidemiology of adenovirus acute lower respiratory infections of children in the south cone of South America (1991-1994).

Abstract: A collection of 165 adenovirus strains isolated from nasopharyngeal aspirates of children hospitalized for acute lower respiratory infection in Argentina, Chile, and Uruguay between 1991 and 1994 was studied by restriction enzyme analysis (work performed in the Department of Virology, University of Umea). Of the isolates, 71% (n = 117) were identified as members of subgenus B. Of these, 101 (61.2%) corresponded to genome type 7h, four (2.4%) to genome type 3p2, four (2.4%) to genome type 11a, one (0.6%) to genome type 7b, and one (0.6%) to genome type 7c. Two isolates that were neutralized as serotype 3 and four isolates that were neutralized as serotype 7 exhibited novel BamHI cleavage profiles corresponding to three new genome types denominated 3x, 7i, and 7j. Subgenus C members represented 28.5% of all typed isolates. Five different genome types of Ad1, seven genome types of Ad2, and three genome types of Ad5 were identified of, which two, two, and one, respectively, were found to correspond to new DNA variants. Only one isolate (0.6%) corresponded to Ad4 of subgenus E. Ad7h was isolated from 17 of the 18 fatal cases recorded among the patients included in the study.

Maize genotype effects on the response to Azospirillum inoculation

Abstract: Two field experiments were carried out in Northern Argentina, during the 1989–1990 and 1990–1991 growing seasons, on Argentinian and Brazilian maize genotypes. The inoculant consisted of a mixture of four Azospirillum brasilense strain isolated from surface-sterilized maize roots in Argentina and three A. lipoferum strains isolated from surface-sterilized maize or sorghum roots in Brazil. Establishment of the inoculated strains was confirmed by the antibiotic resistance of the strains in the highest dilution vials. In all treatments, numbers of Azospirillum spp. were increased and the inoculated strains were found in the highest dilutions. While grain yields of the different genotypes varied between 1700 and 7300 kg ha-1, total N accumulation was much less variable. Significant inoculation effects on total N accumulation and on grain yields were consistently negative with one Argentinian genotype and positive with four Argentinian and two Brazilian genotypes. Significant inoculation effects on leaf nitrate reductase activity at the flowering stage, observed in the range-55% to +176%, indicated the presence of various interactions between the plant NO inf3 sup- metabolism and Azospirillum spp. Three Brazilian and one Argentinian maize genotype showed significant decreases in leaf nitrate reductase due to inoculation while four Argentinian genotypes showed significant increases in leaf nitrate reductase activity. The results of the present study, were consistent over the two field experiments and strongly indicate that more detailed plant genotype-Azospirillum spp. strain interaction studies, taking the entire N metabolism in the plant into account, are needed to allow better inoculation results of cereal crops.