Pre-peak ram pressure stripping in the Virgo cluster spiral galaxy NGC 4501

VIVA HI observations of the Virgo spiral galaxy NGC 4501 are presented. The HI disk is sharply truncated to the southwest, well within the stellar disk. A region of low surface-density gas, which is more extended than the main HI disk, is discovered northeast of the galaxy center. These data are compared to existing 6cm polarized radio continuum emission, Halpha, and optical broad band images. We observe a coincidence between the western HI and polarized emission edges, on the one hand, and a faint Halpha emission ridge, on the other. The polarized emission maxima are located within the gaps between the spiral arms and the faint Halpha ridge. Based on the comparison of these observations with a sample of dynamical simulations with different values for maximum ram pressure and different inclination angles between the disk and the orbital plane,we conclude that ram pressure stripping can account for the main observed characteristics. NGC 4501 is stripped nearly edge-on, is heading southwest, and is ~200-300 Myr before peak ram pressure, i.e. its closest approach to M87. The southwestern ridge of enhanced gas surface density and enhanced polarized radio-continuum emission is due to ram pressure compression. It is argued that the faint western Halpha emission ridge is induced by nearly edge-on ram pressure stripping. NGC 4501 represents an especially clear example of early stage ram pressure stripping of a large cluster-spiral galaxy.Comment: 22 pages, 25 figures, accepted for publication in A&

Simultaneous epidemic development of scald and net blotch on single leaf layers of a spring barley crop

Background and objectives Two pathogens growing on the same leaf compete for the same resources, i.e. space and plant nutrients. This may lead to density dependent disease development. The pathogens may also influence each other directly such that the influence of one on the other is more complex than a simple function of the area of the other pathogen. Different interaction types are, for example, competition, mutualism and exploitation. The importance of such interactions for epidemics of simultaneously occurring pathogens has received little attention. The objective of this study is to investigate the simultaneous epidemic development of Rhynchosporium secalis (causing scald) and Drechslera teres (causing net blotch) on spring barley under field conditions. Materials and methods The field trial was performed with artificial inoculation of R. secalis and D. teres on three spring barley varieties differing in their susceptibility towards the pathogens. The pathogens were inoculated in three combinations: only one was inoculated, they were inoculated together, the second pathogen was inoculated 26 days after the first. A non-inoculated treatment was included. The trial had three replications. Nine plants were harvested from each plot five times during the season. Leaves were dried and disease severity and senescence observed. Only leaves with < 50 % senescence were included in the analysis. Whole-plant disease severity over time was calculated as average of disease severity on leaves weighted by leaf area. Disease development per leaf layer was evaluated by fitting an exponential model to severity data over time for each leaf layer per variety, treatment and replicate. Association between scald and net blotch severity on individual leaves was analysed using Kendall’s tau. Results and discussion Net blotch developed on all leaf layers and reached whole-plant disease severities up to 15%. Scald did not develop on upper leaf layers and whole-plant severity was less than 2%. Disease severity curves at whole-plant level showed no effect of inoculating the other pathogen. The analysis of the growth rate of each disease per leaf layer showed a significant effect of variety and leaf layer within variety but no effect of treatment. However, we observed significant negative associations between the diseases on individual leaves for several combinations of leaf layer and variety. These results show that the individual leaf approach can provide new information and underline the importance of considering interactions between pathogens in the field. Acknowledgement This work was funded by the DARCOF II project BAR-O

The influence of the cluster environment on the large-scale radio continuum emission of 8 Virgo cluster spirals

The influence of the environment on the polarized and total power radio continuum emission of cluster spiral galaxies is investigated. We present deep scaled array VLA 20 and 6 cm observations including polarization of 8 Virgo spiral galaxies. These data are combined with existing optical, HI, and Halpha data. Ram pressure compression leads to sharp edges of the total power distribution at one side of the galactic disk. These edges coincide with HI edges. In edge-on galaxies the extraplanar radio emission can extend further than the HI emission. In the same galaxies asymmetric gradients in the degree of polarization give additional information on the ram pressure wind direction. The local total power emission is not sensitive to the effects of ram pressure. The radio continuum spectrum might flatten in the compressed region only for very strong ram pressure. This implies that neither the local star formation rate nor the turbulent small-scale magnetic field are significantly affected by ram pressure. Ram pressure compression occurs mainly on large scales (>=1 kpc) and is primarily detectable in polarized radio continuum emission.Comment: 16 pages, 10 figures, accepted for publication in A&

Arrest stress of uniformly sheared wet granular matter

We conduct extensive independent numerical experiments considering frictionless disks without internal degrees of freedom (rotation etc.) in two dimensions. We report here that for a large range of the packing fractions below random-close packing, all components of the stress tensor of wet granular materials remain finite in the limit of zero shear rate. This is direct evidence for a fluid-to-solid arrest transition. The offset value of the shear stress characterizes plastic deformation of the arrested state {which corresponds to {\em dynamic yield stress} of the system}. {Based on an analytical line of argument, we propose that the mean number of capillary bridges per particle, $\nu$, follows a non-trivial dependence on the packing fraction, $\phi$, and the capillary energy, \vareps. Most noticeably, we show that $\nu$ is a generic and universal quantity which does not depend on the driving protocol.} Using this universal quantity, we calculate the arrest stress, $\sigma_a$, analytically based on a balance of the energy injection rate due to the external force driving the flow and the dissipation rate accounting for the rupture of capillary bridges. The resulting prediction of $\sigma_a$ is a non-linear function of the packing fraction $\phi$, and the capillary energy \vareps. This formula provides an excellent, parameter-free prediction of the numerical data. Corrections to the theory for small and large packing fractions are connected to the emergence of shear bands and of contributions to the stress from repulsive particle interactions, respectively.Comment: 7 pages, g figure

Net blotch severity is best assessed at early grain filling with respect to its effect on grain weight of spring barley

Appropriate disease assessment methods and parameters reflecting whole-season disease severity levels in field plots remain important issues in studies related to plant disease epidemiology, disease resistance of crop cultivars, and disease-induced yield losses. Such methods and parameters should be yield-related to ensure relevance. Net blotch severity was determined over time in inoculated and non-inoculated field plots of three spring barley varieties by whole plot assessments and by assessments of individual leaves of single main tillers. Disease severity measures such as the area-under-disease-progress-curve, mean and maximum severity as well as severity levels at specific growth stages (GS) were derived from the data. Their relation to thousand grain weight (TGW) and their inter-correlations were examined by means of general linear model (GLM) and factor analyses (FA), respectively. All parameters of net blotch severity were significantly negatively correlated with TGW. Disease parameters derived from whole-plot assessments gave a slightly better explanation of TGW than parameters derived by assessing single main tillers. Net blotch severity at GS 70 (beginning of grain filling) of whole plot assessments yielded the highest adjusted R-squared (0.43) while the adjusted R-squared values resulting from using the same parameter of assessments of the upper three, four or all leaves of single tillers were between 0.34 and 0.35. Also, the residuals of TGW of GLM’s using disease covariates from whole-plot assessments and variety effects as independent variables exhibited less pattern related to other sources of variation than residuals of the corresponding models that used single-tiller-based disease covariates. FA revealed that all disease parameters were highly inter-correlated and co-varied along the 1 principal component axis. The results indicate that disease assessments at GS 70 are appropriate to reflect whole-season severity levels of net blotch. In this respect, the time consuming single-tiller method is in this respect not superior to the simpler whole-plot method. However, assessing individual leaf layers of single tillers allows to observe the epidemic development and thus to examine the dynamics of epidemics in much greater detail than assessing whole-plots. This showed, for example, how much each leaf layer contributed at any given time to the total disease and revealed that a substantial fraction of the total disease is being removed during the course of an epidemic by senescence of diseased lower leaves. This level of detail in examining the dynamics of epidemics cannot be achieved by the whole-plot method

Biochemical Characterization of SAC9, a Putative Phosphoinositide Phosphatase in Arabidopsis thaliana, and Its Role in Cellular Abnormalities

The phospholipid phosphatidylinositol and its phosphorylated derivatives, collectively referred to as phosphoinositides, form the basis for a multifaceted signaling pathway regulating many different cellular processes in eukaryotic cells. Phosphatidylinositol 4,5-bisphosphate, PI(4,5)P2, assumes a central position in this complex pathway. It can serve as a precursor for the generation of second messengers but can also act as a ligand to partner proteins. In order to mediate their physiological effects properly, the location and quantity of PI(4,5)P2 and other phosphoinositides have to be tightly controlled by enzymes. In general, phospholipid kinases lead to the activation of the pathway, whereas phospholipid phosphatases attenuate or terminate the signaling cascade. The SAC domain-containing protein 9 from Arabidopsis thaliana has been identified as a putative phosphoinositide phosphatase, but very little has been published on this particular protein. In my dissertation research, I broadened our knowledge of this protein and the effects seen in Arabidopsis plants carrying the mutant allele. I used molecular, genetic, and biochemical approaches to analyze the function of the putative phosphoinositide phosphatase, SAC9. To understand its physiological role, I investigated the cellular effects of a mutation in the SAC9 gene at the light microscopy, confocal microscopy, and transmission electron microscopy levels. My studies show that AtSAC9 is a soluble protein with an apparent molecular mass of 180 kDa and that it most likely is a phosphoinositide phosphatase. Furthermore, I show that the mutation of SAC9 induced unique cell wall defects that most likely have contributed to the stuntedness of the root. However, the cortical microtubule cytoskeleton was not disturbed in elongating root cells. These data are augmented by applying a novel approach for the mathematical analysis of cortical microtubule orientation