Showing papers by "University of Buenos Aires published in 1994"

CO2 alters water use, carbon gain, and yield for the dominant species in a natural grassland.

Abstract: Global atmospheric CO2 is increasing at a rate of 1.5-2 ppm per year and is predicted to double by the end of the next century. Understanding how terrestrial ecosystems will respond in this changing environment is an important goal of current research. Here we present results from a field study of elevated CO2 in a California annual grassland. Elevated CO2 led to lower leaf-level stomatal conductance and transpiration (approximately 50%) and higher mid-day leaf water potentials (30-35%) in the most abundant species of the grassland, Avena barbata Brot. Higher CO2 concentrations also resulted in greater midday photosynthetic rates (70% on average). The effects of CO2 on stomatal conductance and leaf water potential decreased towards the end of the growing season, when Avena began to show signs of senescence. Water-use efficiency was approximately doubled in elevated CO2, as estimated by instantaneous gas-exchange measurements and seasonal carbon isotope discrimination. Increases in CO2 and photosynthesis resulted in more seeds per plant (30%) and taller and heavier plants (27% and 41%, respectively). Elevated CO2 also reduced seed N concentrations (9%).

Adaptive deadlock- and livelock-free routing with all minimal paths in torus networks

Abstract: This paper consists of two parts. In the first part, two new algorithms for deadlock- and livelock-free wormhole routing in the torus network are presented. The first algorithm, called Channels, is for the n-dimensional torus network. This technique is fully-adaptive minimal, that is, all paths with a minimal number of hops from source to destination are available for routing, and needs only five virtual channels per bidirectional link, the lowest channel requirement known in the literature for fully-adaptive minimal worm-hole routing. In addition, this result also yields the lowest buffer requirement known in the literature for packet-switched fully-adaptive minimal routing. The second algorithm, called 4-Classes, is for the bidimensional torus network. This technique is fully-adaptive minimal and requires only eight virtual channels per bidirectional link. Also, it allows for a highly parallel implementation of its associated routing node. In the second part of this paper, four worm-hole routing techniques for the two-dimensional torus are experimentally evaluated using a dynamic message injection model and different traffic patterns and message lengths. >

Photostimulation of seed germination during soil tillage

Abstract: SUMMARY We conducted intensively replicated field experiments in the Willamette Valley. Oregon, USA, in order to study the role of light in triggering seed germination during soil tillage. We found that the normal practice of cultivating agricultural land during daytime can increase germination of buried seed populations between 70 and 400 above the levels recorded following nighttime cultivations. Experimental reduction of the irradiance under the tillage implements during daytime cultivation decreases the number of dicotyledonous seedlings emerged. while strong artificial illumination (>300μmol m−2 s−1; 400–800nm) of the soil surface under the implements during nighttime tillage significantly increases seedling densities. These results suggest that the enhancement of seed germination caused by daytime tillage, compared with nighttime tillage, is due to light that penetrates into the soil during the actual disturbance. The detection by the seeds of this very short exposure to sunlight requires a high photosensitivity. which provides an adaptive‘purpose’for the evolution of the very-low-fluence response mechanism in phytochrome-controlled seed germination. Seedling emergence induced by nighttime control tillage was considerable in some experiments. suggesting that light perceived by seeds after cultivation or other microenvironmental factors affected by tillage may be important in triggering germination in disturbed soil.

Patch structure and dynamics in a Patagonian arid steppe

Abstract: Two patch types were recognized in the Occidental District of the Patagonian arid steppe: i) shrubs encircled by a ring of tussock grasses, and ii) tracts of scattered tussocks. Completeness of the ring of grasses around the three dominant shrubs was a function of shrub size. Average completeness was 62, 71 and 83%, respectively for the three dominant shrubs (Senecio filaginoides, Mulinum spinosum and Adesmia campestris). A model for the cyclic dynamics of the two patch types was proposed. It includes a building phase (grass ring construction), a mature phase (maximum ring completeness) and a degenerate phase. In this last phase, triggered by shrub death, completeness of the ring progressively decreases until remnant grasses become undistinguishable from the scattered tussocks patch type. Ring formation occurred independently of shrub species. Grass species were differentially associated to the two patch types and to rings of different shrub species. Cyclical patch dynamics influenced the pattern of resource utilization, since the shrub-ring patch, with a share of only 18% of cover, contributed 44% of the total primary productivity.

Signaling among neighboring plants and the development of size inequalities in plant populations

Abstract: Transgenic tobacco plants that express an oat phytochrome gene (phyA) under control of the cauliflower mosaic virus (CaMV) 35S promoter and display altered photophysiology were used to test the role of light as a vehicle of information in dominance relationships between neighboring plants. Compared with the isogenic wild type, phyA-overexpressing plants showed dramatically reduced morphological responsivity to changes in the red/far red ratio of the incident light and to the proximity of neighboring plants in spacing experiments. In transgenic canopies an increase in stand density caused the small plants of the population to be rapidly suppressed by their neighbors. In wild-type canopies, plants responded to increased density with large morphological changes, and there appeared to be an inverse relationship between the magnitude of this morphological response and the ranking of the individual plant in the population size hierarchy. In these wild-type populations, size inequality increased only moderately with density within the time frame of the experiments. Our results suggest that, in crowded stands, the ability of individual plants to acquire information about their light environment via phytochrome plays a central role in driving architectural changes that, at the population level, delay the development of size differences between neighbors.

The distribution, in European winter wheats, of genes that influence ecoclimatic adaptability whilst determining photoperiodic insensitivity and plant height

Abstract: The adaptability of wheat varieties to precise environmental conditions is known to be influenced to a large extent by photoperiod sensitive genes determining ear emergence time and by the gibberelic acid insensitive dwarfing genes that are regularly used to reduce plant height A range of European winter wheat varieties were examined to establish whether breeders in different countries have selected genotypes carrying allelic variants of photoperiodic and dwarfing genes that would be expected to provide the best levels of adaptability In most areas modern cultivars were seen to carry the genotypes predicted to promote good levels of adaptability Varieties cultivated in Germany however are still predominately of conventional height and late flowering lacking genes that would be expected to enhance adaptability and yield

Thyroid hormones and the central nervous system.

Abstract: Thyroid hormones have a significant influence on the development and maturation of the central nervous system. Among their actions, T3 and T4 have effects on the differentiation of various cell types in the rat brain and cerebellum as well as on the process of myelination. Recently, several investigators have shown effects of thyroid hormones on myelin protein gene expression. Thyroid hormones seem to have a regulatory role with regard to life span. Hyperthyroid animals appear to have a shorter life and, at advanced age, show a myelin deficit. This may be due to the damage produced by the oxidative stress generated by an excess of thyroid hormones.

Interactions between Trichoderma koningii, Fusarium solani and Glomus mosseae: Effects on plant growth, arbuscular mycorrhizas and the saprophyte inoculants

Abstract: The effect of inoculation with the saprophytic fungi Trichoderma koningii and Fusarium solani on maize ( Zea mays ) and lettuce ( Lactuca sativa ) with or without arbuscular mycorrhizal (AM) colonization by Glomus mosseae was studied in a greenhouse trial. Plant dry wt of non-AM inoculated maize and lettuce were unaffected by the presence of T. koningii and F. solani . In contrast, T. koningii decreased plant dry wt and AM colonization when inoculated into the rhizosphere before or at the same time as G. mosseae . In addition, the T. koningii population was considerably reduced when G. mosseae was inoculated 2 weeks before the saprophytic fungus. At this time T. koningii did not affect the proportion of the root length colonized by G. mosseae , but did adversely affect its metabolic activity assessed as succinate dehydrogenase activity. Although F. solani did not affect AM colonization of maize roots, its effect on AM colonization of lettuce roots was similar to that of T. koningii . The population of F. solani decreased significantly in the rhizosphere of both plants. The possible mechanisms of interaction between the saprophytic fungi and G. mosseae are discussed.

Effects of angiotensin II and angiotensin-(1-7) on the release of [3H]norepinephrine from rat atria.

Abstract: We examined the effects of angiotensin II (Ang II) and Ang-(1-7) on the release of [3H]norepinephrine elicited by nerve stimulation (2 Hz, 0.5 millisecond, for 2 minutes) in rat atria isolated with their cardioaccelerans nerves. The stimulation-induced release of [3H]norepinephrine was increased 50% by 3 x 10(-8) mol/L of either peptide. No further increase in [3H]norepinephrine release was observed with peptide concentrations up to 3 x 10(-7) mol/L. This effect was completely blocked by the nonselective angiotensin receptor antagonist saralasin (1 x 10(-7) mol/L). The type 1 angiotensin receptor antagonist DuP 753 (1 x 10(-6) mol/L) entirely prevented the increases in [3H]norepinephrine caused by Ang II and Ang-(1-7). On the other hand, the type 2 angiotensin receptor antagonist PD 123319 (1 x 10(-6) mol/L) prevented the increase in [3H]norepinephrine release elicited by Ang-(1-7) but not by Ang II. These results suggest that Ang-(1-7), like Ang II, could have a neuromodulatory function in rat atria via activation of specific angiotensin receptor subtypes, which could be the subtype 1 angiotensin receptor for Ang II and subtypes 1 and 2 for Ang-(1-7).

Odour and colour information in the foraging choice behaviour of the honeybee

Abstract: 1. Honeybees Apis mellifera ligustica were trained to work on a patch with artificial rewarding and non-rewarding flowers, coupled to an air extractor. The perceptual colour distance between the rewarding and the non-rewarding flowers was varied and the flower choice and the repellent scent-marking behaviour of the bees were recorded. 2. The discrimination between rewarding and non-rewarding flowers depended on their colour distance, improving with a greater colour difference. This task was guided thus visually and was not affected by activating the air extractor. 3. The scent-marking activity was only observable when the colour information of both groups of flowers was the same or very similar. This thus represents the first reported case of a modulation of an olfactory activity through the visual input provided by colour distances. When the air extractor was activated, rejections associated with the scent-marking behaviour disappeared, thus confirming the olfactory nature of this behaviour. 4. Honeybees are thus capable of using one or more sensory cues to enhance their foraging efficiency, according to the environmental situation. This great plasticity allows them to attain an enhanced efficiency while foraging. 5. We successfully applied the model of colour choice behaviour of the honeybee. Since the original theory was developed for Apis mellifera carnica, this work also constitutes the first attempt to describe the behaviour of the honeybee race, Apis mellifera ligustica, using the postulated model, and reaffirms thus its generality.

Phonological awareness, spelling and reading abilities in Spanish‐speaking children

Abstract: The relationship between phonological awareness, spelling and reading abilities was studied comparing first grade (N=39) Spanish-speaking skilled and less skilled readers' performance on three measures: phoneme segmentation, word spelling and word reading. Results showed that skilled readers performed at ceiling on the three tasks, whereas less skilled readers had a better performance on spelling than on reading, and most of them reached segmentation criterion in the phonological awareness task. Performance on this task was significantly correlated with word spelling, while no association was found between phoneme segmentation and word reading. The less skilled readers could spell many words they could not read, and children in both groups spelled most of the words in a conventional way. It is agreed that certain characteristics of Spanish phonetic structure — small number of vowels, simple syllabic structure — and the relative transparency of its orthography may account for the early development of phonemic segmentation skills and allow the mastery of sound-letter correspondence rules that is reflected in good spelling performance.

Chagas disease in north-west Argentina: risk of domestic reinfestation by Triatoma infestans after a single community-wide application of deltamethrin

Abstract: The risk of domestic reinfestation by Triatoma infestans after a single community-wide application of deltamethrin (2·5% suspension concentrate at 25 mg active ingredient/m2) was studied in Amama, north-west Argentina, where no insecticide spraying had been done by official control services. The percentage of infested houses fell from 88% before spraying in 1985 to nil during the 6 months after spraying, and thereafter increased from 5% in 1986 to 96% before a second treatment in 1992, fitting closely to a logistic model (r2 = 0·997). Significant risk factors associated with domestic reinfestation determined from stepwise logistic regression and one-factor analysis were the density of T. infestans in bedrooms just before spraying and the surface structure of indoor walls. Peak densities of bugs in 1988–1989 significantly differed between levels of both risk factors. Our study suggests the existence of stable determinants of infestation linked to the household which, in the absence of effective control measures, would also determine the speed of house recolonization and the ensuing bug densities. Plastering of mud walls before application of insecticides to all domestic and peridomestic structures is supported by the study.

Odour learning and decision-making during food collection in the leaf-cutting ant Acromyrmex lundi

Abstract: Recruited workers of the leaf-cutting antAcromyrmex lundi learn the odour of the food fragment initially carried by a successful scout, and use this cue as a decision criterion during food collection. Citral, a natural deterrent odour, can also be associated as a food signal, showing how powerful olfactory learning is in this species. The role of both food-odour learning and a previously reported leaf-marking pheromone during diet selection by leaf-cutting ants is discussed.

Single intracerebral injection of apotransferrin in young rats induces increased myelination

Abstract: Three-day-old rats were injected intracranially with 210 or 350 ng of apotransferrin (aTf) to study the possible neurotrophic effects of this iron transport protein. Treated animals and appropriate co

Artemisia annua L.: dedifferentiated and differentiated cultures

Abstract: Dedifferentiated and differentiated tissue cultures ofArtemisia annua L. for artemisinin production were carried out. The calluses were initiated on MS medium supplemented with sucrose (30 g l-1), myoinositol (100 mg l-1) and RT vitamins. The auxins used were naphtaleneacetic acid (NAA), indoleacetic acid (IAA), indolebutyric acid (IBA) and 2,4-dichlorophenoxyacetic acid (2,4-d). These were added to the basal medium either singly or in combination. The best results were obtained with 2.4-d (4.5 μM : μ0.02 d-1) and NAA (5.4 μM : μ 0.06 d-1). Cell suspensions were established on the same media without agar. Suspension cultures showed different morphological characteristics according to the plant growth regulator supplied. Organized cultures were initiated from callus obtained on 2,4-d (4.5 μM) and from bud cultures. Medium containing 6-benzylaminepurine (BA) (8.9 μM)+NAA (0.54 μM); Zeatin (45.62 μM)+NAA (5.37 μM) or BA (8.9 μM) stimulated both organogenesis in callus (frequency of induction =50%) and semi-organized tissue in shoot buds. BA (13.32 μM)+NAA (1.08 μM) or BA (13.32 μM) only stimulated multiple shoot cultures (frequency of induction =80%). Regarding artemisinin content, while the values obtained were 1.13 and 0.78 mg gDW-1 in primary callus, artemisinin was not detected in cell suspension and only traces of it were found in multiple shoot cultures.

Terranes of Southern Gondwanaland and Their Control in the Andean Structure (30°–33°S Latitude)

Abstract: A revision of the present knowledge of the late Proterozoic-Palaeozoic accretionary history in this segment of southwestern Gondwanaland enables us to discuss its role in the control of the geometry of Andean tectonics. The previous terrane boundaries were reactivated by extension in early Mesozoic times during the break-up of Gondwanaland. The asymmetric rift systems that formed were located in the hanging wall of the Proterozoic-Palaeozoic sutures. Cenozoic shortening was controlled by tectonic inversion involving basement in the inner areas of the Coastal and Principal Cordilleras, while in the eastern Principal Cordillera and Precordillera a thin-skinned fold-and-thrust belt developed. The boundary between the Sierras Pampeanas and the Precordillera has been tested by deep-seismic reprocessing, and shows oblique discontinuities down to more than 30 km. The boundary between the Precordillera and Frontal Cordillera coincides with the slope of the early Palaeozoic continental margin of Gondwanaland. It is concluded that most of the present morphostructural boundaries match crustal discontinuities inherited from the earlier accretionary history.