Thermoinhibition of seed germination

Thermoinhibition describes the inability of seeds to germinate at high temperatures, although germination proceeds immediately when the temperature is reduced below a certain threshold level. This phenomenon is distinct from thermodormancy, where some form of dormancy-breaking treatment is required before germination can proceed at the favourable temperature. Like seed dormancy, thermoinhibition is manifested in a number of different ways, ranging from simple high-temperature induced changes to the structures enclosing the embryo which prevent radicle emergence, to the interaction of a number of different factors, and probable expression of certain genes inhibitory to germination which may be temperature regulated. Thermoinhibition occurs in a large number of important crop species, so that an understanding of this phenomenon is both of scientific interest and practical importance

Prevalence of methicillin-resistant Staphylococcus aureus among large commercial pig herds in South Africa

The prevalence of nasal carrier status of methicillin-resistant Staphylococcus aureus (MRSA) in pigs has been described elsewhere, but is unknown in South Africa. To address concerns that exist regarding the zoonotic risk that carriers pose to workers, the herd-level prevalence of MRSA was determined among 25 large (> 500 sows) commercial pig herds in South Africa, representing 45% of the large commercial herds in the country. From each herd, the nasal contents of 18 finisher pigs were sampled at the abattoir, pooled into three and selectively cultured to determine the presence of MRSA. A herd was classified as MRSA-positive if one or more of the three pooled samples cultured positive. Three of the 25 herds tested positive for MRSA, equating to a 12% herd prevalence (95% CI: 7% – 23%) among South African commercial piggeries. The prevalence of nasal MRSA carriers among large commercial pig herds in South Africa was low compared to what has been reported elsewhere and suggests a relatively low zoonotic MRSA risk to workers in South African commercial piggeries and abattoirs

Fodder Resources and Grazing Management in a Savanna Environment: an Ecosystem Approach

The desirability for students of social and natural sciences to pool their abilities and resources in investigations concerned with man and his environment has often been noted, but genuine efforts in this direction are conspicuously less frequent. Admittedly, examples could be quoted of studies which synthetise the findings of individual researchers, many of which have been commissioned by those involved in the planning for and implementation of development. But few cases are known to the present authors of researchers who have tried to im-prove on the relevance, consistency, and coherenoe of their basic investigations into different, though related, aspects by seeking right from the start an interdisciplinary approaoh to a jointly delimited theme. Even among students of strongly interdependent disciplines such as geography and ecology, as represented by the present authors, these 'joint ventures' are believed to be rare. Those who set out on this challenging path therefore not only have to face occasional difficulties due to different frames of reference and jargon, but also and more important, the problem of designing a functionally integrated approach without being able to resort to previous endeavours for guidance

Aspects of the dynamics of colloidal suspensions: Further results of the mode-coupling theory of structural relaxation

Results of the idealized mode-coupling theory for the structural relaxation in suspensions of hard-sphere colloidal particles are presented and discussed with regard to recent light scattering experiments. The structural relaxation becomes non-diffusive for long times, contrary to the expectation based on the de Gennes narrowing concept. A semi-quantitative connection of the wave vector dependences of the relaxation times and amplitudes of the final $\alpha$-relaxation explains the approximate scaling observed by Segr{\`e} and Pusey [Phys. Rev. Lett. {\bf 77}, 771 (1996)]. Asymptotic expansions lead to a qualitative understanding of density dependences in generalized Stokes-Einstein relations. This relation is also generalized to non-zero frequencies thereby yielding support for a reasoning by Mason and Weitz [Phys. Rev. Lett {\bf 74}, 1250 (1995)]. The dynamics transient to the structural relaxation is discussed with models incorporating short-time diffusion and hydrodynamic interactions for short times.Comment: 11 pages, 9 figures; to be published in Phys. Rev.

Biological basis of water quality assessment : the Kavango river, Namibia

Le fleuve Kavango prend sa source dnas les hauts plateaux d'Angola, se dirige vers le sud formant la frontière Angola-Namibie sur 415 km de direction Ouest-Est, et coule ensuite vers le sud pendant 65 km avant de former le delta de l'Okavango au Botswana. La portion moyenne de la rivière le long de la frontière est caractéristique des fleuves de faible pente des plaines d'inondation. Les crues sont d'environ 6 m au-dessus du niveau normal, avec un lit majeur de 5 km de large. Le fleuve est vital pour les populations locales dont 85% vivent à moins de 10 km du fleuve. Compte tenu des prévisions d'accroissement de la population, des besoins en eau, des programmes de redistribution des terres, de l'évolution des pratiques agricoles et d'une sécheresse chronique, le ministère des Pêches et des Ressources marines prépare les bases d'une gestion durable des ressources dont les populations locales sont directement dépendantes. C'est dans ce cadre qu'une recherche a été mise en place pour définir un protocole de suivi de la qualité des eaux en Namibie, protocole basé sur des critères biologiques. L'utilisation de critères biologiques pour suivre la qualité des cours des rivières dans les pays tempérés est commune. C'est moins le cas pour les pays tropicaux. L'indice d'intégrité biologique (IBI) est utilisé depuis plus de 10 ans en Europe et en Amérique du Nord. Il permet une estimation de la santé d'une rivière par l'analyse de son peuplement de poissons. Des données sur le peuplement de la portion namibienne du Kavango, obtenues en 1992, sont utilisées ici pour base d'un indice approprié. Les limites de l'approche sont analysées et des recommandations pour son extension à d'autres pays au sud du Sahara sont présentés. (Résumé d'auteur

Glasses in hard spheres with short-range attraction

We report a detailed experimental study of the structure and dynamics of glassy states in hard spheres with short-range attraction. The system is a suspension of nearly-hard-sphere colloidal particles and non-adsorbing linear polymer which induces a depletion attraction between the particles. Observation of crystallization reveals a re-entrant glass transition. Static light scattering shows a continuous change in the static structure factors upon increasing attraction. Dynamic light scattering results, which cover 11 orders of magnitude in time, are consistent with the existence of two distinct kinds of glasses, those dominated by inter-particle repulsion and caging, and those dominated by attraction. Samples close to the `A3 point' predicted by mode coupling theory for such systems show very slow, logarithmic dynamics.Comment: 22 pages, 18 figure

Evidence for Unusual Dynamical Arrest Scenario in Short Ranged Colloidal Systems

Extensive molecular dynamics simulation studies of particles interacting via a short ranged attractive square-well (SW) potential are reported. The calculated loci of constant diffusion coefficient $D$ in the temperature-packing fraction plane show a re-entrant behavior, i.e. an increase of diffusivity on cooling, confirming an important part of the high volume-fraction dynamical-arrest scenario earlier predicted by theory for particles with short ranged potentials. The more efficient localization mechanism induced by the short range bonding provides, on average, additional free volume as compared to the hard-sphere case and results in faster dynamics.Comment: 4 pages, 3 figure

A functional model for primary visual cortex

Many neurons in mammalian primary visual cortex have properties such as sharp tuning for contour orientation, strong selectivity for motion direction, and insensitivity to stimulus polarity, that are not shared with their sub-cortical counterparts. Successful models have been developed for a number of these properties but in one case, direction selectivity, there is no consensus about underlying mechanisms. This thesis describes a model that accounts for many of the empirical observations concerning direction selectivity. The model comprises a single column of cat primary visual cortex and a series of processing stages. Each neuron in the first cortical stage receives input from a small number of on-centre and off-centre relay cells in the lateral geniculate nucleus. Consistent with recent physiological evidence, the off-centre inputs to cortex precede the on-centre inputs by a small interval (~4 ms), and it is this difference that confers direction selectivity on model neurons. I show that the resulting model successfully matches the following empirical data: the proportion of cells that are direction selective; tilted spatiotemporal receptive fields; phase advance in the response to a stationary contrast-reversing grating stepped across the receptive field. The model also accounts for several other fundamental properties. Receptive fields have elongated subregions, orientation selectivity is strong, and the distribution of orientation tuning bandwidth across neurons is similar to that seen in the laboratory. Finally, neurons in the first stage have properties corresponding to simple cells, and more complex-like cells emerge in later stages. The results therefore show that a simple feed-forward model can account for a number of the fundamental properties of primary visual cortex