Mejora del olmo para su resistencia contra los escolítidos

Dutch Elm Disease (DED) forms a complex system of three elements closely interacting: a host, a pathogen and an insect vector. Elm breeding programmes for resistance to DED have been exclusively based on selecting or obtaining pathogen-resistant trees, thus stressing only the host–pathogen side of the interaction and missing the vector component. Resistance against the bark beetle vectors would involve mechanisms other than those implicated in fungus resistance. As a consequence the search for resistance to the vectors would offer more than just a different way of obtaining resistant elms as it would allow incorporation of a new type of resistance into pathogen-resistant trees. Suppressing twig crotch feeding by elm bark beetles would greatly reduce infection on healthy elms. Evidence of preferences by elm bark beetles among different elm species has been documented in the field and in controlled experiments, demonstrating that U. minor is preferred over U. laevis and U. glabra, whereas intraspecific preferences among individual elms or clones remain largely unexplored. The selection of an individual plant by an insect is a two-step process that involves finding and accepting the host. Plant chemicals can play a key role in both of these steps, acting either as long-range stimuli, such as attractants or repellents, or as close-range contact cues, such as feeding stimulants and deterrents, operating after landing on the plant surface. Research on the chemical aspects of elm selection by bark beetles is here reviewed, and prospects for the future are discussed.La enfermedad de la grafiosis de los olmos constituye un sistema formado por tres elementos estrechamente relacionados: un hospedante, un patógeno y un insecto vector. Los programas de mejora de olmos frente a la grafiosis se han basado en seleccionar u obtener árboles resistentes al patógeno, poniendo el énfasis sólo en la parte patógenohospedante de la interacción y olvidándose del componente vector. La resistencia frente a los escolítidos vectores supondría mecanismos distintos de aquéllos implicados en la resistencia frente al hongo. Como consecuencia, la búsqueda de resistencia frente a los vectores ofrecería no sólo una via diferente de obtener olmos resistentes, sino además la posibilidad de incorporar un nuevo tipo de resistencia a los árboles resistentes al patógeno. La supresión de la alimentación en las ramillas por los escolítidos del olmo reduciría notablemente la infección en los olmos sanos. La evidencia de preferencias en los escolítidos del olmo entre especies diferentes de olmos ha sido documentada en el campo y en experimentos controlados, demostrándose que U. minor es más preferido que U. laevis o que U. glabra, si bien, la existencia de preferencias intraespecíficas entre árboles o clones permanece inexplorada. La selección de una determinada planta por un insecto es un proceso doble que supone encontrar y aceptar al hospedante. Los compuestos químicos vegetales pueden representar un papel clave en ambas fases, actuando bien como estímulos a larga distancia, tales como atrayentes o repelentes, o bien como señales a corta distancia, tales como fagoestimulantes o fagodisuasores, que operan después del aterrizaje sobre la superficie vegetal. Se revisa aquí la investigación sobre los aspectos químicos en la selección de los olmos por los escolítidos y se discuten las perspectivas futuras

Fusion of strings vs. percolation and the transition to the quark-gluon plasma

In most of the models of hadronic collisions the number of exchanged colour strings grows with energy and atomic numbers of the projectile and target. At high string densities interaction between them should melt them into the quark-gluon plasma state. It is shown that under certain assumptions about the the string interaction, a phase transition to the quark gluon plasma indeed takes place in the system of many colour strings. It may be of the first or second order (percolation), depending on the particular mechanism of the interaction. The critical string density is about unity in both cases. The critical density may have been already reached in central Pb-Pb collisions at 158 A GeV.Comment: 16 pages, 3 Postscript figure

Universal geometrical scaling of the elliptic flow

The presence of scaling variables in experimental observables provide very valuable indications of the dynamics underlying a given physical process. In the last years, the search for geometric scaling, that is the presence of a scaling variable which encodes all geometrical information of the collision as well as other external quantities as the total energy, has been very active. This is motivated, in part, for being one of the genuine predictions of the Color Glass Condensate formalism for saturation of partonic densities. Here we extend these previous findings to the case of experimental data on elliptic flow. We find an excellent scaling for all centralities and energies, from RHIC to LHC, with a simple generalization of the scaling previously found for other observables and systems. Interestingly the case of the photons, difficult to reconcile in most formalisms, nicely fit the scaling curve. We discuss the possible interpretations of this finding in terms of initial or final state effects.Comment: 6 pages, 4 figures, accepted for publication in Phys Rev

Limiting fragmentation in heavy-ion collisions and percolation of strings

The observed limiting fragmentation of charged particle distributions in heavy ion collisions is difficult to explain as it does not apply to the proton spectrum itself. On the other hand, string percolation provides a mechanism to regenerate fast particles, eventually compensating the rapidity shift (energy loss) of the nucleons. However a delicate energy-momentum compensation is required, and in our framework we see no reason for limiting fragmentation to be exact. A prediction, based on percolation arguments, is given for the charged particle density in the full rapidity interval at LHC energy $(\sqrt s =5500 GeV)$.Comment: 9 pages, 2 figures (2 eps files), late

Particle production azimuthal asymmetries in a clustering of color sources model

The collective interactions of many partons in the first stage of the collisions is the usual accepted explanation of the sizable elliptical flow. The clustering of color sources provides a framework of partonic interactions. In this scheme, we show a reasonable agreement with RHIC data for pT<1.5 GeV/c in both the dependence of v2 transverse momentum and in the shape of the nuclear modified factor on the azimuthal angle for different centralities. We show the predictions at LHC energies for Pb-Pb. In the case of proton-proton collisions a sizable v2 is obtained at this energy.Comment: To appear in Journal of Physics

Percolation Effects in Very High Energy Cosmic Rays

Most QCD models of high energy collisions predict that the inelasticity $K$ is an increasing function of the energy. We argue that, due to percolation of strings, this behaviour will change and, at $\sqrt{s} \simeq 10^4$ GeV, the inelasticity will start to decrease with the energy. This has straightforward consequences in high energy cosmic ray physics: 1) the relative depth of the shower maximum $\bar{X}$ grows faster with energy above the knee; 2) the energy measurements of ground array experiments at GZK energies could be overestimated.Comment: Correction of equation (19) and figures 3 and 4. 4 pages, 4 figure