Star Formation and Dynamics in the Galactic Centre

The centre of our Galaxy is one of the most studied and yet enigmatic places in the Universe. At a distance of about 8 kpc from our Sun, the Galactic centre (GC) is the ideal environment to study the extreme processes that take place in the vicinity of a supermassive black hole (SMBH). Despite the hostile environment, several tens of early-type stars populate the central parsec of our Galaxy. A fraction of them lie in a thin ring with mild eccentricity and inner radius ~0.04 pc, while the S-stars, i.e. the ~30 stars closest to the SMBH (<0.04 pc), have randomly oriented and highly eccentric orbits. The formation of such early-type stars has been a puzzle for a long time: molecular clouds should be tidally disrupted by the SMBH before they can fragment into stars. We review the main scenarios proposed to explain the formation and the dynamical evolution of the early-type stars in the GC. In particular, we discuss the most popular in situ scenarios (accretion disc fragmentation and molecular cloud disruption) and migration scenarios (star cluster inspiral and Hills mechanism). We focus on the most pressing challenges that must be faced to shed light on the process of star formation in the vicinity of a SMBH.Comment: 68 pages, 35 figures; invited review chapter, to be published in expanded form in Haardt, F., Gorini, V., Moschella, U. and Treves, A., 'Astrophysical Black Holes'. Lecture Notes in Physics. Springer 201

Plant-plant competition influences temporal dynamism of soil microbial enzyme activity.

Root-derived compounds can change rates of soil organic matter decomposition (rhizosphere priming effects) through microbial production of extracellular enzymes. Such soil priming can be affected by plant identity and soil nutrient status. However, the effect of plant-plant competition on the temporal dynamics of soil organic matter turnover processes is not well understood. This study used zymography to detect the spatial and temporal pattern of cellulase and leucine aminopeptidase activity, two enzyme classes involved in soil organic matter turnover. The effect of plant-plant competition on enzyme activity was examined using barley (Hordeum vulgare) plants grown in i) isolation, ii) intra- and iii) inter-cultivar competition. The enzyme activities of leucine aminopeptidase and cellulase were measured from portions of the root system at 18, 25 and 33 days after planting, both along the root axis and in the root associated area with detectable enzyme activity. The activities of cellulase and leucine aminopeptidase were both strongly associated with plant roots, and increased over time. An increase in the area of cellulase activity around roots was delayed when plants were in competition compared to in isolation. A similar response was found for leucine aminopeptidase activity, but only when in intra-cultivar competition, and not when in inter-cultivar competition. Therefore, plant-plant competition had a differential effect on enzyme classes, which was potentially mediated through root exudate composition. This study demonstrates the influence of plant-plant competition on soil microbial activity and provides a potential mechanism by which temporal dynamism in plant resource capture can be mediated

Much ado about nothing...so far?

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Clonal integration beyond resource sharing: implications for defence signalling and disease transmission in clonal plant networks

Item does not contain fulltextResource sharing between ramets of clonal plants is a well-known phenomenon, which allows stoloniferous and rhizomatous species to internally translocate water, mineral nutrients and carbohydrates from sites of high supply to sites of high demand. The mechanisms and implications of resource integration in clonal plants have extensively been studied in the past. Vascular ramet connections are likely to provide an excellent means to share substances other than resources, such as systemic defence signals and pathogens. The aim of this paper is to propose the idea that physical ramet connections of clonal plants can be used (1) to transmit signals, which enable members of clonal plant networks to share information about their biotic and abiotic environments, and (2) to facilitate the internal distribution of systemic pathogens in clonal plant networks and populations. We will focus on possible mechanisms as well as on potential ecological and evolutionary implications of clonal integration beyond resource sharing. More specifically, we will explore the role of physiological integration in clonal plant networks for the systemic transmission of direct and indirect defence signals after localized herbivore attack. We propose that sharing defence induction signals among ramets may be the basis for an efficient early warning system, and it may allow for effective indirect defence signalling to herbivore enemies through a systemic release of volatiles from entire clonal fragments. In addition, we will examine the role of clonal integration for the internal spread of systemic pathogens and pathogen defence signals within clonal plants. Clonal plants may use developmental mechanisms such as increased flowering and clone fragmentation, but also specific biochemical defence strategies to fight pathogens. We propose that clonal plant networks can act as stores and vectors of diseases in plant populations and communities and that clonal life histories favour the evolution of pathogens with a low virulence