Pulsatile extracorporeal circulation during on-pump cardiac surgery enhances aortic wall shear stress

Controversy on superiority of pulsatile versus non-pulsatile extracorporeal circulation in cardiac surgery still continues. Stroke as one of the major adverse events during cardiopulmonary bypass is, in the majority of cases, caused by mobilization of aortic arteriosclerotic plaques that is inducible by pathologically elevated wall shear stress values. The present study employs computational fluid dynamics to evaluate the aortic blood flow and wall shear stress profiles under the influence of antegrade or retrograde perfusion with pulsatile versus non-pulsatile extracorporeal circulation. While, compared to physiological flow, a non-pulsatile perfusion resulted in generally decreased blood velocities and only moderately increased shear forces (48 Pa versus 20 Pa antegradely and 127 Pa versus 30 Pa retrogradely), a pulsatile perfusion extensively enhanced the occurrence of turbulences, maximum blood flow speed and maximum wall shear stress (1020 Pa versus 20 Pa antegradely and 1178 Pa versus 30 Pa retrogradely). Under these circumstances arteriosclerotic embolism has to be considered. Further simulations and experimental work are necessary to elucidate the impact of our findings on the scientific discourse of pulsatile versus non-pulsatile extracorporeal circulation.     

Life histories of desert geophytes —the demographic consequences of reproductive biomass partitioning patterns

Summary During five consecutive growing seasons (winters) ca. 110 plants of the desert geophyte Bellevalia desertorum were marked individually in a 25 m² plot on a south-facing slope in the central Negev Desert of Israel. The number of rosette leaves of each plant was recorded, as well as whether it flowered and produced seeds. Multiple regression analysis of the data on the B. desertorum individuals showed that the reproductive state of a plant was determined by 1) its previous size and 2) the current conditions (rainfall), but not by previous conditions, nor by previous reproductive activity. Plant surveys supported these findings. These demographic results were consistent with the current understanding of the reproductive resource allocation pattern of B. desertorum. Flowering was most affected by rainfall until January of the same season, the number of leaves by rainfall until March and seed set was by the total annual amount. Surveys in populations ofB. eigii, a species of more mesic habitats in the Negev Desert suggested that in this species there is a negative effect of previous reproduction in combination with the previous and current conditions, which is also expected from its biomass partitioning pattern.The significance of demographic studies of individual plants in natural populations for the interpretation of experimentally determined resource partitioning patterns was discussed.     

Gibberellic-acid-induced cell elongation in pea epicotyls: Effect on polyploidy and DNA content

In gibberellic-acid(GA₃)-treated epicotyls of dwarf peas (Pisum sativum L.) grown in the light, DNA (per cell and per epicotyl) is followed. Histofluorometric DNA determinations show that GA₃-promoted cell elongation is not accompanied by increased endomitosis, but chemical estimations show an increased DNA content per epicotyl. This difference must therefore be the result of increased mitotic activity in the GA₃-treated tissue. Epicotyls of seedlings grown with or without cotyledons under continuous light with GA₃ are tetraploid, as are those of ecotylized embryos grown in darkness. These epicotyls reach no more than half the length of octaploid epicotyls of seedlings grown in darkness. This result provides evidence for a relationship between polyploidy and final possible cell length.     

Factors affecting the estimation of the relative amount of chromophore and chromophore area by the two-wavelength method of patau and ornstein

Summary Factors influencing the calculation of the relative amount of chromophore and the chromophore area by the two-wavelength method are examined. The study was carried out with the help of models and further tested on Feulgen stained preparations. Except for certain restrictions the difference between the chromophore area as calculated from the two transmissions measurements and the chromophore area obtained by planimetry can be used as a guide for determining the proper measuring conditions, including the choise of the two wavelengths.     

Annual plant community responses to density of small-scale soil disturbances in the Negev Desert of Israel

We investigated whether plant diversity and productivity in small-scale soil disturbances, which is known to be higher than in undisturbed soil, decreases as the density of the disturbances increases. We studied this in an experiment with soil diggings (15 cm diameter and 15 cm depth) dug at a range of densities, on a north- and a south-facing slope of a watershed in the central Negev Desert of Israel. The diggings were similar to the commonly occurring pits made by porcupines (Hystrix indica) as they forage for below-ground plant parts. We used four levels of digging density, within the naturally occurring range in the region, represented by a rectangular plot with rows of diggings dug at four distances between diggings. The plots were laid out in a blocked design with three replications on both slopes, with each block containing all four levels of digging density. In the spring of 1992, 1994 and 1995 we measured plant density, species richness and plant productivity in the diggings, and in adjacent equal-sized undisturbed control areas (“soil matrix”) and on the mounds made by the removed excess soil. Plant density, species richness and productivity of annual plants were higher in the diggings than in the undisturbed matrix, while these responses were very low on the mounds. Plant density, species richness and productivity in the diggings, but not in the matrix or mounds, decreased as digging density increased. This effect varied slightly with location within a watershed and with annual rainfall. The density of seeds captured in the diggings from outside the digging during the 1995 dispersal season decreased with increasing digging density, but only on one of the slopes. At the highest digging density, plant density and species number in the diggings did not decrease down the slope, as expected if interference between diggings in runoff water capture were the cause of the digging density effect. There was a weak decrease in biomass production in 1994–1995 down the slope. We used a simple mathematical model to estimate whether the distribution of rainfall intensities that occurred during the winter of 1994–1995 could result in differences between digging densities in the amount of water captured by the diggings, and whether this could explain the observed effect of digging density. The model showed that there were four events during which less water was captured by the diggings at high digging densities, except in the topmost row of diggings. Soil moisture measurements, however, showed very little difference between diggings at different digging densities. We explain our findings as the result of the interaction between the properties of the disturbance patch with its surroundings, as the diggings capture resources in the form of runoff water, and seeds moved primarily by wind. The additional resources and seeds captured in diggings increase plant density, species richness and productivity relative to the undisturbed matrix. However, the contrast in plant responses between the disturbed patches and undisturbed soil diminishes at higher digging densities. We explain this as interference among diggings at close proximity. As we did not detect a decrease in plant responses down the slopes, we conclude that interference is due to interception of the wind-driven, non-directional flow of seeds. Interception of the down-slope flow of runoff water by upslope diggings is insufficient to affect plant density, determined at the beginning of the season. Later in the season, runoff interception may become important for biomass production. Received: 16 August 1996 / Accepted: 26 October 1997     

The effect of plant litter on ecosystem properties in a Mediterranean semi‐arid shrubland

Abstract. We studied the distribution of litter in a shrubland of the Negev with a semi‐arid Mediterranean climate of less than 200 mm of rainfall per year. Our focus was on the effects of litter on properties of landscape patches relevant to ecosystem processes (water runoff and soil erosion), annual plant community responses (seedling density, biomass production and species richness), and animal activity (soil disturbance by termites). Three 60‐m transects, extending across a pair of opposing north‐ and south‐facing slopes and their drainage channel, showed that litter accumulates not only under shrubs, but to a lesser extent also on the crusted inter‐shrub open areas. We used 35 experimental units (‘cells’, 0.5m × 1 m), each containing a crust and a shrub patch. Because runoff flows from crusted patches and is intercepted by shrub patches, the latter were in the lower third of the cells. Leaf litter was added in single and double amounts providing ca. 0.5 and 1.0 cm litter depth, to either, both, or none of the patches. Litter addition significantly decreased the amount of runoff, regardless of the location and amount of litter applied. Litter on the crust increased species number and seedling density of species with low abundance. Adding a double litter layer increased annual plant biomass production, while a single amount had no effect. Litter addition to the shrub patch affected neither biomass nor species richness. Litter addition to both patches at both quantities caused a large increase in termite activity. Termites caused disturbance by disrupting the crust, which may contribute to the reduction in runoff amounts. In the open, flat crust patches, annual plant communities are limited in their productivity and species richness, as there are few structures stopping the outflow of water, soil and seeds. Litter adds such structures, but affects the plant communities only when added to litter‐free crust. Litter accumulation and its patchy distribution have large impacts on landscape patch properties affecting resource distribution, plant productivity and diversity, and animal activity. Therefore, understanding litter distribution in relation to the patchy structure of the landscape of semi‐arid shrubland should be viewed as an important component of shrubland management.     

Two-wavelength cytophotometry: the choice of the wavelengths from a practical point of view.

Methods in use for the choice of the two-wavelengths and the effect of a faulty choice of the two wavelengths in the two-wavelength cytophotometric method of Patau and Ornstein are studied by means of a computer simulation and in practice upon pararosaniline-Schiff stained nuclei. A simplification of the measuring strategy is proposed without affecting the accuracy.     

Patterns of biotic community organization and reorganization: A conceptual framework and a case study

Reorganization is an important concept for confronting complex adaptive systems (CAS) theory with ecological reality. However, little work has been done to translate the reorganization idea into a practical conceptual framework. This paper focuses on community-level reorganization, the process of re-forming patterns in relation to the distribution of species density (abundance) and their frequency of occurrence in space (incidence). We assert that changes in species positions in the incidence–abundance (IA) phase plane depict community reorganization in response to environmental changes. This is because species positions in the IA plane represent the most prominent organizational features of a biotic community.We identified four sequential levels of species reorganization in the IA phase plane: reshuffling of species, species appearance, species disappearance and whole-assemblage shift. We propose that the sequential levels represent an increase in reorganization intensity depending on the extent of environmental change.We formulate the Environmental Change and Re-Organization (ECRO) hypothesis specifying that ecosystems respond predictably in terms of community reorganization in the IA phase plane to external drivers. The predictable reorganization follows the four sequential levels of organization in response to intensity of environmental change. We suggest that the response of a species assemblage to external drivers in sequential levels of reorganization is independent of ecosystem properties, type of environmental variables, taxa, and spatio-temporal scales.We tested the predictability of reorganization according to the ECRO hypothesis, using annual plant assemblages of a semiarid shrubland. These assemblages exhibit CAS properties as suggested by Levin: high species diversity, high numbers of individuals, local interactions among individuals in relation to water consumption, and annually selected species subsets for replication.We investigated annual plant assemblage organization and reorganization for 12 years in response to disturbance and resource input, using the IA phase plane. The field study supports our assertion that reorganization of species assemblages in response to environmental change can be represented by species repositioning in the IA phase plane, and that community reorganization follows the four sequential levels of reorganization in response to the intensity of environmental change.Our conceptual framework and experimental studies demonstrate that hypotheses related to core CAS concepts of organization and reorganization can be tested by linking them with community ecology concepts of species patterns in the IA phase plane. We also discuss the relationship between reorganization in the IA phase plane and resilience, regime shift and ecosystem functioning as affected by species response and effects traits.     

Factors affecting the estimation of the relative amount of chromophore and chromophore area by the two-wavelength method of Patau and Ornstein.

Factors influencing the calculation of the relative amount of chromophore and the chromophore area by the two-wavelength method are examined. The study was carried out with the help of models and further tested on Feulgen stained preparations. Except for certain restrictions the difference between the chromophore area as calculated from the two transmissions measurements and the chromophore area obtained by planimetry can be used as a guide for determining the proper measuring conditions, including the choise of the two wavelengths.