Coupling Soybean Cyst Nematode Damage to CROPGRO-Soybean

The soybean cyst nematode (SCN) Heterodera glycines Ichinohe is responsible for substantial economic losses in soybean (Glycine max L. Merr.) production throughout the U.S. Results from past efforts to quantify the severity of crop damage resulting from SCN are often subject to variable experimental conditions resulting from differences in weather, soil type, and cultivar. Because of the difficulty in accounting for these variables, a process–oriented crop growth simulation model was chosen as a platform for studying the dynamics of SCN damage and for transferring knowledge between crop production scenarios. The objective of this study was to develop and evaluate hypotheses for coupling SCN damage to the process–oriented crop growth model CROPGRO–Soybean. A monomolecular function was used to relate daily SCN damage to initial population density of SCN eggs. The equation was incorporated into the crop model in order to test two hypotheses of how SCN damage occurs. The first hypothesis was that SCN reduce daily photosynthesis (Pg), while the second hypothesis was that SCN reduce daily potential root water uptake (RWU). Canopy biomass data collected in 1997 and 1998 from a site in Iowa were used to estimate damage function parameters for two distinct coupling points, one applied to reduce daily photosynthesis (Pg) and the other applied to reduce daily potential root water uptake (RWU). Function parameters were estimated by minimizing the log transformation of root mean square error (RMSE) between predicted and measured canopy biomass collected every 2 weeks during the season in Iowa. Biomass data collected in 1997 and 1998 from an independent site in Missouri were used to validate the SCN damage models. The minimum root mean squared errors (RMSE) of canopy and grain biomass were 0.245 and 0.198 log10(kg ha–1), respectively, for the RWU coupling point, and 0.238 and 0.193 log10(kg ha–1), respectively, for the Pg coupling point at the independent site in Missouri. The damage functions transferred very well to the independent site. Validation showed that the Pg coupling point represented the variability of both canopy and final yield data slightly better than the RWU coupling point

Supersymmetry and the Cosmic Ray Positron Excess

We explore several supersymmetric alternatives to explain predictions for the cosmic ray positron excess. Light sneutrino or neutralino LSP's, and a fine-tuned model designed to provide a delta-function input, can give adequate statistical descriptions of the reported HEAT data if non-thermal production of the relic cold dark matter density dominates and/or if ``boost factors''(that could originate in uncertainties from propagation or local density fluctuations) to increase the size of the signal are included. All the descriptions can be tested at the Tevatron or LHC, and some in other WIMP detecting experiments.Comment: 15 pages, 3 figure

Injection of thermal and suprathermal seed particles into coronal shocks of varying obliquity

Context. Diffusive shock acceleration in the solar corona can accelerate solar energetic particles to very high energies. Acceleration efficiency is increased by entrapment through self-generated waves, which is highly dependent on the amount of accelerated particles. This, in turn, is determined by the efficiency of particle injection into the acceleration process. Aims. We present an analysis of the injection efficiency at coronal shocks of varying obliquity.We assessed injection through reflection and downstream scattering, including the effect of a cross-shock potential. Both quasi-thermal and suprathermal seed populations were analysed. We present results on the effect of cross-field diffusion downstream of the shock on the injection efficiency. Methods. Using analytical methods, we present applicable injection speed thresholds that were compared with both semi-analytical flux integration and Monte Carlo simulations, which do not resort to binary thresholds. Shock-normal angle θBn and shock-normal velocity Vs were varied to assess the injection efficiency with respect to these parameters. Results. We present evidence of a significant bias of thermal seed particle injection at small shock-normal angles. We show that downstream isotropisation methods affect the θBn-dependence of this result. We show a non-negligible effect caused by the crossshock potential, and that the effect of downstream cross-field diffusion is highly dependent on boundary definitions. Conclusions. Our results show that for Monte Carlo simulations of coronal shock acceleration a full distribution function assessment with downstream isotropisation through scatterings is necessary to realistically model particle injection. Based on our results, seed particle injection at quasi-parallel coronal shocks can result in significant acceleration efficiency, especially when combined with varying field-line geometry

Estimate of the impact of background particles on the X-Ray Microcalorimeter Spectrometer on IXO

We present the results of a study on the impact of particles of galactic (GCR) and solar origin for the X-ray Microcalorimeter Spectrometer (XMS) aboard an astronomical satellite flying in an orbit at the second Lagrangian point (L2). The detailed configuration presented in this paper is the one adopted for the International X-Ray Observatory (IXO) study, however the derived estimates can be considered a conservative limit for ATHENA, that is the IXO redefined mission proposed to ESA. This work is aimed at the estimate of the residual background level expected on the focal plane detector during the mission lifetime, a crucial information in the development of any instrumental configuration that optimizes the XMS scientific performances. We used the Geant4 toolkit, a Monte Carlo based simulator, to investigate the rejection efficiency of the anticoincidence system and assess the residual background on the detector.Comment: 18 pages, 9 figure

Supersymmetry and the positron excess in cosmic rays

Recently the HEAT balloon experiment has confirmed an excess of high-energy positrons in cosmic rays. They could come from annihilation of dark matter in the galactic halo. We discuss expectations for the positron signal in cosmic rays from the lightest superpartner. The simplest interpretations are incompatible with the size and shape of the excess if the relic LSPs evolved from thermal equilbrium. Non-thermal histories can describe a sufficient positron rate. Reproducing the energy spectrum is more challenging, but perhaps possible. The resulting light superpartner spectrum is compatible with collider physics, the muon anomalous magnetic moment, Z-pole electroweak data, and other dark matter searches.Comment: 4 pages, 2 figures, references added, minor wording change

Soybean Cyst Nematode Reduces Soybean Yield Without Causing Obvious Aboveground Symptoms

Field experiments were conducted at locations in northern and southern Illinois, central Iowa, and central Missouri from 1997 to 1999 to investigate the effects of Heterodera glycines on soybean growth, development, and yield. A wide range of infestation levels was present at all locations. Two locally adapted cultivars, one resistant to H. glycines, were grown at each location. Cultivars were planted in alternating four-row strips with 76 cm between rows. For each cultivar, 20 1-m-long single-row plots were sampled every 2 weeks starting 4 weeks after planting. Infection by H. glycines reduced plant height and leaf and stem weight on the resistant cultivars in the first 12 weeks after planting, and delayed pod and seed development 12 to 14 weeks after planting. Biomass accumulation was not reduced on the susceptible cultivars until 10 weeks after planting; reduction in pod and seed development occurred throughout the reproductive stages. Susceptible cultivars produced significantly lower yields than resistant cultivars, but the yield reductions were not accompanied by visually detectable symptoms

Soybean Aphid Infestation Induces Changes in Fatty Acid Metabolism in Soybean

The soybean aphid (Aphis glycines Matsumura) is one of the most important insect pests of soybeans in the North-central region of the US. It has been hypothesized that aphids avoid effective defenses by inhibition of jasmonate-regulated plant responses. Given the role fatty acids play in jasmonate-induced plant defenses, we analyzed the fatty acid profile of soybean leaves and seeds from aphid-infested plants. Aphid infestation reduced levels of polyunsaturated fatty acids in leaves with a concomitant increase in palmitic acid. In seeds, a reduction in polyunsaturated fatty acids was associated with an increase in stearic acid and oleic acid. Soybean plants challenged with the brown stem rot fungus or with soybean cyst nematodes did not present changes in fatty acid levels in leaves or seeds, indicating that the changes induced by aphids are not a general response to pests. One of the polyunsaturated fatty acids, linolenic acid, is the precursor of jasmonate; thus, these changes in fatty acid metabolism may be examples of “metabolic hijacking” by the aphid to avoid the induction of effective defenses. Based on the changes in fatty acid levels observed in seeds and leaves, we hypothesize that aphids potentially induce interference in the fatty acid desaturation pathway, likely reducing FAD2 and FAD6 activity that leads to a reduction in polyunsaturated fatty acids. Our data support the idea that aphids block jasmonate-dependent defenses by reduction of the hormone precursor

A Standard Greenhouse Method for Assessing Soybean Cyst Nematode Resistance in Soybean: SCE08 (Standardized Cyst Evaluation 2008)

The soybean cyst nematode (SCN), Heterodera glycines Ichinohe, is distributed throughout the soybean [Glycine max (L.) Merr.] production areas of the United States and Canada (Fig. 1) (26). SCN remains the most economically important pathogen of soybean in North America; the most recent estimate of soybean yield reduction in North America due to SCN totaled 34,659,000 metric tons during 2006 (34)