Bi-cropping fodder maize in an existing (grass)clover sward

Organic cultivation of fodder maize is still considered to be difficult. Weed pressure, soil structure degradation during harvest and low nutrient efficiency are some of the common problems. Directly sowing maize in a (grass)clover sward using special-ized drilling machines can solve these problems. In a bi-cropping experiment under organic conditions we found similar yields as the reference treatment (ploughing) when the maize was sown in a clover sward, the roots of the remaining sward were cut one week after sowing and an additional fertiliser was applied

Microsomal superoxide anion production and NADPH-oxidation in a series of 22 aziridinylbenzoquinones

Several 2,5-bis(1-aziridinyl)-1,4-benzoquinones (BABQs) can be activated to alkylating species by reduction of the quinone moiety. On the other hand, cytotoxicity of these compounds can be induced by redox cycling. A series of BABQs and their methylated analogues (BMABQs) with different substituents at the 3- and 6-position was synthesized in order to investigate the influence of the substituents on the reduction of the quinone moiety and on the generation of superoxide anion radicals with rat liver microsomes. Superoxide anion production (SAP) ranged from 3.7±0.1 to 742±74 nmoles/min/mg protein with quinone concentrations of 10 nmoles/ml. NADPH-oxidation was measured under the same conditions and it correlated well (r = 0.88, P < 0.001) with SAP. It ranged from 1.4±0.2 to 494±60 nmoles/min/mg protein. SAP for 22 B(M)ABQs showed a good correlation with the summated electronic substituent constant θpara,total (r = 0.86, P < 0.001). It can be concluded that superoxide anion production by 22 B(M)ABQs in rat liver microsomes can be predicted from structural features of the compounds

Effects of elevated plant selenium levels on reproduction and root-nematode interactions

2011 Spring.Includes bibliographical references.Selenium is an important element in soils throughout the western United States, and its presence has important consequences for the ecology of these seleniferous sites. Some plants in seleniferous areas have evolved mechanisms to hyperaccumulate Se to 0.1 - 1% of their dry weight. Other plants accumulate moderate Se levels between 0.01 - 0.1% of their dry weight. In the studies described in this thesis, facets of the evolution of Se accumulation and the associated ecology of Se hyperaccumulators are considered. First, I examined the effect of increasing Se on reproductive parameters of Se accumulators and hyperaccumulators. The reproductive parameters were measured through cross-pollinations of greenhouse-grown accumulator plants receiving different Se concentrations. In the accumulator Brassica juncea , increasing Se concentrations in plant tissues caused decreases in biomass, pollen germination, seed weight, seed production, and seed germination. In some cases, however, interactions of similar Se concentrations in both parents actually proved beneficial to reproduction. The hyperaccumulator Stanleya pinnata showed no effect of increased Se concentration on pollen germination. These data provide interesting insight into the potential reproductive cost of Se accumulation, and the apparent evolution of physiological tolerance mechanisms in hyperaccumulators to avoid these reproductive problems. To further investigate the effect of Se on reproduction, S. pinnata plants were taken to a field site with hives of the European honey bee (Apis mellifera) to examine the effect of Se in floral tissues on potential pollinators. The bees and other pollinators showed no preference for or against Se in the flower and foraged on both high- and low-Se plants equally. Because the bees showed no preference, the honey of bees in seleniferous sites was analyzed for the presence of Se, and there were small amounts (up to 2 mg kg-1 FW) of Se found in this honey. These findings are important for bee keepers in seleniferous areas, as they show no evidence of toxic Se levels in their honey and they may even market their honey as Se-enriched and beneficial for human health. The finding that bees do not discriminate between high and low-Se plants does warrant further studies on the potential health effects of the ingested Se on the pollinators and the movement of Se into the food chain. Next, to further investigate the ecology of Se hyperaccumulators, I examined the interactions of hyperaccumulator roots with root-associated nematodes. Selenium hyperaccumulators S. pinnata and Astragalus bisulcatus growing in the field have root Se concentrations between 100 and 1,500 mg Se kg-1 DW, a toxic concentration to most above-ground herbivores. Therefore, it was expected that with increasing root Se concentrations, there would be reduced levels of nematodes associated with plants. There was no significant negative correlation with increasing Se concentration, and even roots containing >1,000 mg Se kg-1 Se harbored nematode herbivores. However, when nematodes extracted from field-harvested plants were used to inoculate greenhouse-grown S. pinnata , plants treated with Se did harbor significantly fewer nematodes several months later. These findings are of significance, both because they suggest the presence of Se-tolerant and potentially Se-specialist nematodes in seleniferous sites, and for the possible use of Se as a pesticide for nematodes in non-seleniferous sites. Furthermore, the roots of hyperaccumulators were examined for the spatial distribution and speciation of Se using X-ray Absorption Spectroscopy (XAS). The majority of the Se was found in the cortex and epidermis of the root, with lower levels in the wood. Organic Se of the C-Se-C type (Se bonded to two carbon atoms, similar to methyl-selenocysteine) was the predominant form of Se in the hyperaccumulator roots, together with a small fraction of inorganic selenite. The findings presented in this thesis may also have relevance for hyperaccumulators of other elements, such as arsenic, cadmium, nickel or zinc, as these metals may also protect roots from nematodes and other root herbivores, and may have similar effects on reproduction. Further investigations may reveal other herbivores that are deterred by root hyperaccumulation, as well as more evidence of specialist herbivores that have evolved tolerance in response to the hyperaccumulator's elemental defense. Beyond insight into the ecological and co-evolutionary relationships between roots and herbivores, the results presented here also have applications in agriculture. Since Se is both a nutrient and a toxin, depending on its concentration, Se could be used as an alternative to organic pesticides in controlling root nematode and herbivore levels in organic and subsistence farming. With careful monitoring, the resulting plants may be considered Se-fortified food with enhanced nutritional value. Finally, the findings presented here provide a framework for follow-up studies investigating the evolution of plant Se hyperaccumulation and the associated effects of (hyper)accumulated plant Se on ecological interactions in seleniferous habitats

Spin-dependent transport in metal/semiconductor tunnel junctions

This paper describes a model as well as experiments on spin-polarized tunnelling with the aid of optical spin orientation. This involves tunnel junctions between a magnetic material and gallium arsenide (GaAs), where the latter is optically excited with circularly polarized light in order to generate spin-polarized carriers. A transport model is presented that takes account of carrier capture in the semiconductor surface states, and describes the semiconductor surface in terms of a spin-dependent energy distribution function. The so-called surface spin-splitting can be calculated from the balance of the polarized electron and hole flow in the semiconductor subsurface region, the polarized tunnelling current across the tunnel barrier between the magnetic material and the semiconductor surface, and the spin relaxation at the semiconductor surface. Measurements are presented of the circular-polarization-dependent photocurrent (the so-called helicity asymmetry) in thin-film tunnel junctions of Co/Al2O3/GaAs. In the absence of a tunnel barrier, the helicity asymmetry is caused by magneto-optical effects (magnetic circular dichroism). In the case where a tunnel barrier is present, the data cannot be explained by magneto-optical effects alone; the deviations provide evidence that spin-polarized tunnelling due to optical spin orientation occurs. In Co/τ-MnAl/AlAs/GaAs junctions no deviations from the magneto-optical effects are observed, most probably due to the weak spin polarization of τ-MnAl along the tunnelling direction; the latter is corroborated by bandstructure calculations. Finally, the application of photoexcited GaAs for spin-polarized tunnelling in a scanning tunnelling microscope is discussed.

Mechanism of the hydrodenitrogenation of neopentylamine and adamantylamine on sulfided NiMo/Al2O3

Neopentanethiol, 1-adamantanethiol, and 2-adamantanethiol were primary products and neopentane and adamantane were secondary products in the hydrodenitrogenation of neopentylamine, 1-adamantylamine, and 2-adamantylamine, respectively, over sulfided NiMo/Al2O3. Dialkylamines and dialkylimines were formed as primary products in the reactions of 2-adamantylamine and neopentylamine as well. None of the three amines can react by ammonia elimination and a classic SN2 substitution of the NH2 group by H2S is not possible for the adamantylamines either. The formation of di(2-adamantyl)imine and di(neopentyl)imine indicates that dehydrogenation and hydrogenation reactions occur and that imine or iminium-cation intermediates play an important role. NH2-SH substitution takes place by dehydrogenation of the amine to an imine or iminium cation, which adds H2S and eliminates NH3. The secondary character of adamantane and neopentane demonstrates that hydrogenolysis of the aliphatic C-N bond does not take place over sulfided NiMo/Al2O3 below 340°C. Even though 1-adamantylamine can neither react by classic SN2, E1, and E2 reactions, nor via an imine or iminium cation, it formed 1-adamanethiol at 300°C. This reaction might take place by an SN1 reaction or by adsorption of the amine at a surface vacancy, followed by a shift of the adamantyl group to a neighboring sulfur ato

Correlated Timing and Spectral Variations of the Soft X-ray Transient Aquila X-1: Evidence for an Atoll classification

Based on Rossi X-ray Timing Explorer data, we discuss the classification of the soft X-ray transient Aquila X-1 in the Z/atoll scheme, and the relation of its kilohertz quasi-periodic oscillations (kHz QPO) properties to the X-ray colors. The color-color diagram shows one elongated ("banana") structure and several "islands" of data points. The power spectra of the island are best represented by a broken power-law, whereas those of the banana by a power-law below ~ 1 Hz plus an exponentially cut-off component at intermediate frequencies (30-60 Hz). The parameters of these two components change in correlation with the position of the source in the color-color diagram. Based on the pattern that the source shows in the color-color diagram and its aperiodic variability we conclude that Aquila X-1 is an atoll source. We have also investigated the possible correlation between the frequency of the kHz QPO and the position of the source in the color-color diagram. The complexity seen in the frequency versus count rate diagram is reduced to a single track when the frequency is plotted against hard or soft color.Comment: 9 pages, 5 figures. Accepted for publication in Ap

Facilitated engraftment of human hematopoietic cells in severe combined immunodeficient mice following a single injection of Cl²MDP liposomes

Transplantation of normal and malignant human hematopoietic cells into severe combined immunodeficient (SCID) mice allows for evaluation of long-term growth abilities of these cells and provides a preclinical model for therapeutic interventions. However, large numbers of cells are required for successful engraftment in preirradiated mice due to residual graft resistance, that may be mediated by cells from the mononuclear phagocytic system. Intravenous (i.v.) injection of liposomes containing dichloromethylene diphosphonate (Cl2MDP) may eliminate mouse macrophages in spleen and liver. In this study outgrowth of acute myeloid leukemia (AML) cells and umbilical cord blood (UCB) cells in SCID mice conditioned with a single i.v. injection of Cl2MDP liposomes in addition to sublethal total body irradiation (TBI) was compared to outgrowth of these cells in SCID mice that had received TBI alone. A two- to 10-fold increase in outgrowth of AML cells was observed in four cases of AML. Administration of 107 UCB cells reproducibly engrafted SCID mice that had been conditioned with Cl2MDP liposomes and TBI, whereas human cells were not detected in mice conditioned with TBI alone. As few as 2 x 104 purified CD34+ UCB cells engrafted in all mice treated with Cl2MDP liposomes. In SCID mice treated with macrophage depletion unexpected graft failures were not observed. Histological examination of the spleen showed that TBI and Cl2MDP liposomes i.v. resulted in a transient elimination of all macrophage subsets in the spleen, whereas TBI had a minor effect. Cl2MDP liposomes were easy to use and their application was not associated with appreciable side-effects. Cl2MDP liposome pretreatment in combination with TBI allows for reproducible outgrowth of high numbers of human hematopoietic cells in SCID mice