Imaging bond formation between a gold atom and pentacene on an insulating surface

A covalent bond between an individual pentacene molecule and a gold atom was formed by means of single-molecule chemistry inside a scanning tunneling microscope junction. The bond formation is reversible, and different structural isomers can be produced. The single-molecule synthesis was done on ultrathin insulating films that electronically isolated the reactants and products from their environment. Direct imaging of the orbital hybridization upon bond formation provides insight into the energetic shifts and occupation of the molecular resonances.     

Nitrogen uptake from 15N-enriched fertilizer by four tree crops in an Amazonian agroforest

Mixed tree cropping systems have been proposed for sustainable nutrient management in the humid tropics. Yet, the nutrient interactions between intercropped trees have not been addressed sufficiently. In the present study we compare the temporal and spatial patterns of the uptake of applied ¹⁵N by four different tree crops in a mixed tree cropping system on a Xanthic Ferralsol in central Amazônia, Brazil, during one year. Most of the N uptake occurred during the first two weeks. Very little N was recovered by peach palm (Bactris gasipaes), more by cupuassu (Theobroma grandiflorum) and annatto (Bixa orellana) and most by Brazil nut (Bertholletia excelsa). Due to tree pruning the total accumulation of applied ¹⁵N in the above-ground biomass of annatto decreased throughout the year. It remained constant in cupuassu and peach palm and increased in Brazil nut. Brazil nut showed an extensive root activity and took up more fertilizer N applied to neighboring trees than from the one applied under its own canopy in contrast to the other three tree crops. Therefore, trees with wide-spread root systems may not need to receive N fertilizer directly but can take up N applied to other trees in the mixed cropping system. This means that such trees may effectively decrease N leaching when intercropped with trees that have dormant periods or places with low N uptake, but also exert considerable resource competition.This revised version was published online in November 2005 with corrections to the Cover Date.     

Retention of Ni,Zn and Cd by Si-associated goethite

Synthetic goethite used to study the effects of reaction time and temperature on the pH-dependent sorption of Ni, Zn and Cd was associated with amorphous silica. Ni interacted with dissolved Si and formed a Ni/Si precipitate on the goethite surface. Individual metals added at a concentration of 0.5 μmol g^?1 and sorbed during a reaction period of 504 hours (21 days) at 35°C were extracted by 0.7 M HNO₃ for 14 days. At the end of this period 11,28 and 40 percent of Ni, Cd and Zn, respectively, were not extracted whereas 20 percent of the total Fe content of the goethite and 39 percent of the associated Si were dissolved. During the sorption process metals became immobilized in the goethite particles. This effect can be related to a diffusion of metal ions into micropores. A total mobilization of sorbed metals can only be achieved by a complete dissolution of the goethite. The strong fixation of Ni, Zn and Cd by goethite suggests that additions of this Fe oxide could be used to ameliorate highly contaminated sludges or soils.     

Genetic conservation of South African wattled cranes

The wattled crane (Grus carunculatus), a species highly dependent on wetlands, is the largest and rarest of the six African crane species. The once vast range of the wattled crane now consists of only three disjunct populations. The South African population has shown dramatic declines and supplementation of this population using eggs from south-central Africa has been proposed. The objectives of this study were to compare levels of genetic variation in South African and south-central African populations to determine if such supplementation is needed, and if so, whether the south-central African populations represent a genetically similar source for supplementation. We surveyed genetic variation in samples from South Africa, Zimbabwe, and Botswana using 12 microsatellite DNA loci and a 400-bp fragment of the mitochondrial D-loop. Samples from Zimbabwe and Botswana were deemed genetically similar and pooled to increase sample size. Subsequent analyses indicate that the pooled south-central and South African populations show differentiation in microsatellite DNA genotypes, as well as mitochondrial DNA. As the results from both genetic markers indicate genetic isolation, these populations should be managed as separate entities. As no indication was seen from either microsatellite or mtDNA data that significant loss of genetic diversity has occurred within South African wattled cranes, supplementation from outside populations may not be necessary at this time.     

Spatial heterogeneity of soil properties and its mapping with apparent electrical conductivity

The site‐specific cultivation as part of the precision‐agriculture concept is more and more introduced into practical farming. However, soil information is often not available in a spatial resolution intrinsically needed for precision farming or other site‐specific soil use and management purposes. One approach to obtain spatially high‐resolution soil data is the non‐invasive measurement of the apparent electrical conductivity (EC_a). In this study, we recorded the EC_a on three fields with an EM38 (Geonics, Canada). The EC_a data were compared with (1) ground truth data obtained by conventional drilling, (2) traditional soil maps (large scale, ≤1:5,000), (3) the growth and yield of corn. The temporal variability of the EC_a due to varying soil moisture and temperature was taken into account by repeated measurements of the same fields and subsequent averaging of the EC_a values. Significant correlations (r² = 0.76) were found between the mean weighted clay content (0–1.5 m) and the EC_a. Furthermore, in soils with differently textured layers, EC_a was used to estimate the thickness of the uppermost loess layer. A comparison of EC_a and large‐scale soil maps reveals some pros and cons of EC_a measurements. The main advantages of EC_a recordings are the high spatial resolution in combination with low efforts. Yet, the EC_a signal is no direct measure for a soil type or unit. Depending on the variability of substrates and layering, the EC_a pattern can be a precise indicator for the spatial distribution of different soils. A strong conformity of the spatial variability of plant growth (derived from orthophotos and yield maps) and EC_a patterns within a field indicates that the EC_a signal per se—without conversion to traditional soil parameters—integrates the effects of various soil variables that govern soil fertility. Altogether, EC_a surveys can be a powerful tool to facilitate and improve conventional soil mapping.     

Germplasm characterization of Sesbania accessions based on isozyme analyses

Twenty-two accessions belonging to seven species of Sesbania Scop. (S. emerus, S. exasperata, S. grandiflora, S. rostrata, S. sesban, S. tetrapteraand S. virgata), were evaluated in order to characterize the isozyme electrophoretic patterns and to estimate their genetic variability and outcrossing rates. Eight isozyme systems were used: acid phosphatase (ACP), isocitrate dehydrogenase (IDH), phosphoglucomutase (PGM), malate dehydrogenase (MDH), phosphoglucoisomerase (PGI), glutamate oxaloacetate transaminase (GOT), peroxidase (PRX) and catalase (CAT), with two gel/electrode buffers and distilled water as the extraction buffer. Cotyledons were chosen for enzyme extraction. The accessions were compared considering the presence/absence of bands for each system. A dendrogram was obtained using the Jaccard similarity index and the UPGMA clustering method, classifying the accessions into eight groups, evidencing both inter- and intraspecific variability. Annual species were classified in one major group separate from the perennials (S. grandiflora, S. sesban, S. virgata), which were allocated into two other main groups. S. virgata accessions (subgenus Daubentonia) were clearly separated from the other species. Isozyme banding patterns for the accessions are presented for each system. A high incidence of intraspecific monomorphism was observed. Interpretation in terms of loci and alleles was made for S. sesbanand S. virgataaccessions, which presented polymorphism for the systems PGI, GOT (S. sesban) and MDH (S. virgata). The average expected heterozygosity estimates varied from 0.0 to 0.231 for S. sesban and 0.0 to 0.20 for S. virgata. A mean number of 2.0 alleles per locus was observed for both species. Population multilocus outcrossing rate (t _m) of 0.62 was estimated for S. sesban, indicating a partial allogamy mode of reproduction. Individual families presented multilocus rates varying from 0.00 to 1.00, indicating both self-pollination and cross-pollination.     

Effect of methionine sulphoximine on13 N-ammonium fluxes in the roots of barley and squash seedlings

¹³ N-Iabelled experiments were carried out with barley and squash seedlings to analyse the effect of methionine sulphoximine (MSO), as an inhibitor of the ammonium assimilation, on NH₄⁺ fluxes in the roots at the level (150 μM NH₄⁺) of the high affinity uptake system (HATS). MSO pretreatment in the root medium significantly accelerated NH₄⁺ influx and efflux in NH₄⁺ -fed plants of both species. Concomitantly, the level of NH₄⁺ extracted from the root and shoot tissues was strongly elevated whereas the translocation of¹³ N-Iabelled organic substances (presumably amino products) into the shoots was markedly reduced. The onset of the increased NH₄⁺ influx did not occur immediately but was indicated about 1 h after the application of MSO to the nutrient solution. Also, MSO did not directly affect the uptake of¹³ NO₃^?. The reason why MSO treatment leads to an acceleration of the NH₄⁺ influx against an increasing level in the cell tissue remains obscure. In squash, an intermediate of NO₃^? metabolism is thought to suppress NH₄⁺ uptake since NH₄⁺ influx was high in N-free- and NH₄⁺ -grown seedlings but was reduced about threefold in NO₃^? -fed squash plants; the suppression was not eliminated by MSO treatment.