No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

The rhizosphere and rhizoplane environments of higher plant roots are specialised microhabitats for soil organisms. These organisms include nematodes which are capable of responding to attractants from roots over several centimetres (Rode, 1962, 1969). The aim of this project was to investigate the various aspects of chemotaxis in host finding by nematodes, and the relative importance of electrotaxis in this process. This involved experimentation on root diffusate as a whole, on various ions etc which may be attractive components of root diffusate, on interference by lectins on the host finding process and finally into nematode response to applied electrical fields with comparison to field strengths measured at root surfaces. The working hypothesis used throughout this project was the occurrence of long distance attraction via a non-specific factor which is replaced by a more specific factor as the distance between host and parasite is reduced. Examination of the diffusate as a whole clearly demonstrated that nematode attraction is a directed response. It also gave some support to the theory of successive attractive factors, with each successive factor being of higher molecular weight and lower diffusibility. The response to H+, OH-, Na+, five amino acids and cAMP was tested. The two pH extremes were equally attractive, the Na+ and amino acids were neither attractive nor repulsive and the cAMP was repulsive at the higher concentration used. These results suggested that the ions might have a secondary involvement in attraction by stimulating the initial movement of the nematodes but would be non-specific. Any attraction to amino acids noted by other workers e.g. Bird (1959) might be due to the acidic nature of the amino acids tested. Experiments were carried out using concanavalin A to interfere with host finding as suggested by Marban-Mendoza et al. (1987) through its effects on surface carbohydrates. The experiments produced some evidence to support this theory, and further experiments were made to try to elucidate the mechanism. The results from this further work indicate the effect in pH dependent via changes in aggregate size, but also that there may be a dilution involvement as well. Application of electrical fields to the nematodes produced directional movement, but at voltages approximately 1,000 times that measured at the root surfaces with the vibrating probe. It is therfore concluded that chemotaxis is the primary means by which nematodes locate their host plants, but the possibility of electrotaxis being used to locate specific feeding sites is discussed.

We report the discovery of a stellar overdensity 8 degrees north of the centre of the Small Magellanic Cloud (SMC; Small Magellanic Cloud Northern Over-Density; SMCNOD), using data from the first 2 yr of the Dark Energy Survey (DES) and the first year of the MAGellanic SatelLITEs Survey (MagLiteS). The SMCNOD is indistinguishable in age, metallicity and distance from the nearby SMC stars, being primarily composed of intermediate-age stars (6 Gyr, Z=0.001), with a small fraction of young stars (1 Gyr, Z=0.01). The SMCNOD has an elongated shape with an ellipticity of 0.6 and a size of similar to 6 degrees x 2 degrees. It has an absolute magnitude of M-V congruent to -7.7, r(h) = 2.1 kpc, and mu v(r < r(h)) = 31.2 mag arcsec(-2). We estimate a stellar mass of similar to 10(5) M-circle dot, following a Kroupa mass function. The SMCNOD was probably removed from the SMC disc by tidal stripping, since it is located near the head of the Magellanic Stream, and the literature indicates likely recent Large Magellanic Cloud-SMC encounters. This scenario is supported by the lack of significant H-1 gas. Other potential scenarios for the SMCNOD origin are a transient overdensity within the SMC tidal radius or a primordial SMC satellite in advanced stage of disruption.

A series of selectively fluorinated tolans of the general formulae C(_6)F(_5)-C=C-C(_6)H(_4)X and C(_6)H(_5)-C=C-C(_6)F(_4)X (where X = I, Br, CI) have been synthesized via homogeneous palladium-catalysed Sonogashira cross-coupling and organolithium chemistry. Several of their crystal structures have been solved from X-ray diffraction data, and their molecular packing is described in terms of arene-perfluoroarene and halogen-halogen interactions. Diffraction-quality crystals of a number of binary arene-perfluoroarene complexes of hexafluorobenzene and octafluoronaphthalene with several mismatched polyaromatic hydrocarbons have been obtained and their crystal structures solved from X-ray diffraction data. All of the structures have been shown to consist of infinite stacks of alternating components. The individual structures are compared and contrasted in detail, and those of the HFB complexes are found to closely resemble those predicted from ab initio DFT calculations, which implies that the interactions are over 90 % electrostatic in nature, in contrast with previous calculations on related complexes. A number of selectively fluorinated 4,4'-bis(phenylethynyl)tolan (BPET) derivatives containing fluorinated and non-fluorinated phenyl rings, have been synthesized from palladium-catalysed Sonogashira cross-coupling of various tolan-based precursors. They are observed to strongly absorb in the UV range 336 - 342nm, which are directly comparable to the absorptions for similarly fluorinated 1,4-bis(phenylethynyl)benzene derivatives which suggests that an effective conjugation length (ECL) of 3-4 repeat units is applicable for these phenylene ethynylene systems. They are observed to fluoresce very strongly in the range 372-410 nm. The diethynylbenzene derivatives 1,4-diethynyltetrafluorobenzene and 1,4- diethynyl-2,5-difluorobenzene have been synthesized from the hydrodesilation of their trimethylsilylated precursors. Their crystal structures have been solved from X-ray diffraction data, and are described in terms of C=C-H F and C=C-H π(C=C) interactions.

Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal.