Perspectives on allelopathy in mexican traditional agroecosystems: A case study in tlaxcala

Agroecosystems in Tlaxcala, Mexico, are surrounded by trees and water channels and have a great variety of cultivated and noncultivated plants. The main results of a study carried out on a traditional agroecosystem in Santa Inés, Tlaxcala are presented. Some ecological aspects of polycultures, plant covers (dry leaves ofAlnus firmifolia, Berula erecta, andJuncus sp.), and the allelopathic potential of crops and noncultivated plants (fresh and dry material) were analyzed. The main plants (trees, shrubs, and herbs) present in the agroecosystem were identified. The total number of weeds in plots where plant covers were added was reduced. The number of nodules ofRhizobium phaseoli and the production of bean and squash increased with plant covers. Corn, beans, and squash showed a clear allelopathic effect, as well asChenopodium murale, Tradescantia crassifolia, Melilotus indicus, andAmaranthus hybridus, among other weeds. The contribution of allelopathy in studies of traditional agroecosystems is of great importance for the management of species in space and time. Allelopathy can be the basis of biological control of pests and weeds and of the discovery of new useful substances.     

Minima equalization: A useful strategy in automatic processing of microscopic images

Specifically stained features in microscopic images rarely have a unique range of grey levels which would permit selection of the features by simple thresholding. The spaces between features and immediately surrounding them are often as dark or darker than the more lightly stained features. An algorithm for minima equalization which facilitates the extraction and segmentation of such features was designed and is explained in intuitive terms. It is applied to the analysis of cross-sections of peripheral myelinated nerve fibres. It is shown that the binary image obtained can be combined with a gradient image to give a binary image which accurately reflects the thickness of the myelin in the original image. Using silver-impregnated nerve endings and bile canaliculi stained for thiamine pyrophosphatase, binary images like those prepared manually from thick specimens using a camera lucida can be obtained using the minima equalization procedure. The image processor is used to develop a composite image by combining images at various focal planes through the thick specimen. This image is then processed to obtain the binary image.     

Endothelium and high blood pressure

Due to its strategic anatomical position, the endothelium is constantly exposed to the different risk factors for atherosclerosis. During the last decade it has become clear that hypertension profoundly affects endothelial function. Depending on the form of hypertension, endothelium-dependent relaxation is impaired in most vascular beds. In spontaneous hypertension, the production of nitric oxide, which in endothelial cells is formed from L-arginine via the constitutively expressed enzyme endothelial nitric oxide synthase, represents the main mediator of endothelium-dependent vasodilation and seems to be enhanced. On the other hand, the release of endothelium-dependent contracting factors such as prostaglandin H2 and thromboxane A2 have been demonstrated in this model of hypertension. Similar results have been obtained in the forearm circulation of patients with essential hypertension. In contrast, in models of salt-sensitive hypertension no release of vasoconstrictor prostanoids can be found indicating a decreased production of nitric oxide. Thus, in spontaneous hypertension an increased production of nitric oxide seems to occur, which is ineffective due to either the simultaneous release of endothelium-dependent vasoconstrictors and/or inactivation of nitric oxide, or due to anatomical changes such as hypertension-induced intimal thickness which inhibits its action on vascular smooth muscle cells. In summary, in hypertension, endothelium-dependent vasodilation is blunted and the endothelial L-arginine nitric oxide pathway is altered. These changes seem to represent a consequence rather than a cause of hypertension.     

Colors of dope and yamn of spun-dyed cellulose acetate

A method of color control of bulk-dyed cellulose acetate is described, based on the transmittance spectrum of the dope. Control can be effected only if the spectrum is recorded from thick samples which amplify the absorption from small amounts of impurities that later cause unacceptable color differences on the yarn. The calculation of the reflectance spectrum, and hence the color, of the yarn from the transmittance spectrum of the dope by Kubelka-Munk theory is not satisfactory due to faint turbidity of the ‘bright’ dope. The reflectance spectrum of the yarn can be calculated from a series expansion of the experimental Lambert law absorption coefficient with very satisfactory results. A fully computer-managed color-control method is employed successfully in industrial practice.     

Preparation and dielectric measurements of aluminate-chromate sodalite ceramics

Dense aluminate-chromate sodalite ceramics have been prepared by the solid-state reaction technique. Calcium aluminate-chromate did not exhibit dielectric anomalies at the phase-transitions. Strontium aluminate-chromate showed dielectric maximum at its phase-transition (18°C). The value of its spontaneous polarization at -30°C was 2 μC/cm².     

Mechanisms for Frequency Control in Neuronal Competition Models

We investigate analytically a firing rate model for a two-population network based on mutual inhibition and slow negative feedback in the form of spike frequency adaptation. Both neuronal populations receive external constant input whose strength determines the system's dynamical state-a steady state of identical activity levels or periodic oscillations or a winner-take-all state of bistability. We prove that oscillations appear in the system through supercritical Hopf bifurcations and that they are antiphase. The period of oscillations depends on the input strength in a nonmonotonic fashion, and we show that the increasing branch of the period versus input curve corresponds to a release mechanism and the decreasing branch to an escape mechanism. In the limiting case of infinitely slow feedback we characterize the conditions for release, escape, and occurrence of the winner-take-all behavior. Some extensions of the model are also discussed.     

Percutaneous endoscopic gastrostomy (PEG) in cancer patients

Ninety-nine cancer patients underwent PEG placement attempt at Rosewell Park Cancer Institute between January 1, 1985, and December 1, 1987. Ninety-eight of these were successful and were retrospectively reviewed to determine if cancer patients constitute a high-risk group for PEG placement. Procedure-related mortality was 2% and morbidity was 19%. Morbidity of 17% was noted at less than 30 days and 2% had late complications. Six complications were considered serious with peritonitis in 3 and tube loss in 3 patients; an additional 4 patients had a failure of adequate GI tract decompression which was the indication for their PEG placement. Ascites was a major factor in morbidity with 4 of 5 patients with ascites having complications including the 2 deaths. Overall major morbidity was not increased in cancer patients without ascites including a group of patients with carcinomatosis (18 patients) and 22 patients requiring preoperative dilatation and/or tumor ablative procedures. We conclude that morbidity in cancer patients is not increased if one excludes those with ascites from the procedure.     

The contribution of structural disorder to diffuse phase transitions in ferroelectrics

Simple crystal-chemical arguments were used to suggest that in the ferroelectric perovskite lead-scanium-tantalate (PbSc_0.5Ta_0.5O₃) the B-site cations in this simple ABO₃ structure should be close to the boundary between order and disorder. Both polycrystal ceramic and single crystal materials of this composition have been grown. In this study X-ray powder diffraction is used to identify the strong superlattice reflections associated with the ordering of scandium and tantalum ions in the B-site, and to demonstrate quantitatively how the degree of ordering can be modified by suitable thermal treatment. Thermal changes associated with the ferroelectric Curie temperature have been measured by differential scanning calorimetry and show very clearly the manner in which the diffuse (broadened) transition in this crystal is sharpened by increase in the B-cation ordering.     

Direct analytical procedure for determination of volatile organic acids in raw municipal wastewater

A direct analytical method for identification and determination of the individual volatile acids in raw sewage was developed. The proposed procedure is rapid, omitting tedious sample pretreatment and thus avoiding possible losses involved in steam distillation, evaporation or extraction. It consists of direct injection of raw sewage into a gas chromatograph, including Carbowax 20 M on acid washed Chromosorb W column and a flame ionization detector. Sample preparation is confined to addition of solid metaphosphoric acid to the raw sewage, and removal of precipitated proteins and suspended solids by centrifugation.The direct injection method proved to be practicable, accurate and rapid. Volatile acids content in raw municipal sewage in Haifa, Israel, was found to be in the range of 150–160 mg l⁻¹, of which 120–125 mg l⁻¹ was acetic acid, 30–33 mg l⁻¹ propionic acid, 6–8 mg l⁻¹ butyric acid, 2 mg l⁻¹ isovaleric acid, and 0.5–1 mg l⁻¹ valeric acid.