Principaux facteurs de la germination de Heracleum sphondylium L : importance de l'oxygène

Heracleum sphondylium est une ombellifère de friches qui ne supporte pas un travail du sol intensif. Cette sensibilité s'explique non seulement par la biologie d'hémicryptophyte mais aussi par les caractères germinatifs; la levée s'effectue de façon très homogène au début du printemps, aussi bien en surface qu'en profondeur, si bien qu'aucun stock semencier ne peut persister en terrain perturbé. La régularité des germinations à la sortie de l'hiver est la conséquence d'une levée de dormance par le froid et l'humidité, conditions nécessaires à la réduction des inhibitions tégumentaires mais, surtout, à la maturation de l'embryon; aussi bien la levée de dormance primaire que le remplissage de l'embryon ne se déroulent qu'à une température inférieure à 10 °C. Cette exigence thermique s'explique simplement par un besoin en oxygène relativement élevé, satisfait à une température où la diffusion de ce gaz est facilitée : meilleure solubilisation, limitation des réactions de fixation. Les enveloppes (téguments et substances de réserve) ont, ainsi, un rôle complexe de régulation de la quantité d'oxygène et de la qualité d'éléments nutritifs disponibles. Après une phase d'hypoxie empêchant la germination à contre-saison, l'embryon dispose en fin d'hiver du flux idéal pour son développement, correspondant à une atmosphère environnante riche de 10-15% d'oxygène.Principal factors in the germination of Heracleum sphondylium L : importance of oxygenation. Heracleum sphondylium is an umbellifer found on uncultivated soil which cannot tolerate intensive cultivation. This sensitivity may be explained not only by the biology of the hemicryptophyte, but also by its germination characteristics: the seedlings emerge nearly simultaneously at the beginning of spring, near the surface as well as from deeper levels, in such a way that no live seeds can persist if the soil has been disturbed. The regularity of germination at the end of winter is the consequence of dormancy breaking by cold and moisture, necessary conditions for the reduction of tegumentary inhibition, but more especially for the maturation of the embryo. Thus dormancy breaking as well as embryo swelling do not take place unless the temperature is below 10 °C. This thermal requirement may be explained simply by the need for a relatively high level of dissolved oxygen around the seed, which may be satisfied at a temperature at which the diffusion of this gas is facilitated, making it more soluble and limiting its fixation reactions. The seed envelopes (seed coats and endosperm) thus play a complex role in regulating the availability quantity of oxygen and nutritive elements. After a phase of hypoxia preventing germination in the off-season, there is an ideal flow rate for the development of the embryo by the end of winter, ie is when the surrounding atmosphere reaches a partial pressure of 10 to 15% oxygen

Self-cleaning and wear-resistant polymer nanocomposite surfaces

Superhydrophobic self-cleaning and wear-resistant nanocomposite surfaces were produced by mimicking the hierarchical structure of the lotus leaf using a combination of rapid self-assembly and a UV nano-imprint lithography (UVNIL) process with a silicone master. Two different acrylate formulations containing acrylated silica nanoparticles and an acrylated silicone surfactant were used. The presence of the silicone master did not suppress the spontaneous migration of the surfactant to the polymer surface, which increased its hydrophobic character. Adding acrylated silica particles considerably increased the viscosity of the acrylate suspensions and led to a shear-thinning behavior. However the particles did not prevent the fast migration process of the surfactant and further increased the hydrophobicity of the material, due to increased nanoscale roughness of the nanocomposite surface. The largest increase of hydrophobicity was achieved for the UVNIL printed lotus surfaces using the acrylate formulation with lowest viscosity. These surfaces became superhydrophobic for the highest investigated concentration of silica. These nanocomposite lotus surfaces were, in addition, very hard with a microhardness above 400 MPa and particularly wear-resistant, and were self-cleaning with respect to hydrophobic contamination

Protein secretion by the principal cells of mouse epididymis evidenced by in vitro incorporation of tritiated leucine

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Assessment of the allelochemical activity of Ostreopsis cf. ovata and the ovatoxins towards competitive benthic microalgae

Recurrent blooms of the toxic dinoflagellate Ostreopsis cf. ovata are frequently reported in the Northwestern Mediterranean Sea. The impact of these proliferations on other microalgal species inhabiting the same habitats is of interest from an ecological prospective. In vitro experiments were carried out to investigate the influence of O. cf. ovata on the growth of the co-occurring benthic diatoms Licmophora paradoxa, Navicula arenaria and the benthic dinoflagellates Prorocentrum lima and Coolia monotis. Overall, O. cf. ovata exhibited weak allelopathic effects towards these microalgal species, with a reduction in the cell abundance for L. paradoxa and P. lima only. Interestingly, dead cells of L. paradoxa and N. arenaria were observed embedded in the thick mucus surrounding O. cf. ovata cells, suggesting that the mucous layer could act as a toxic phycosphere, especially for non-motile cells. All competitors were further exposed for 24 h to ovatoxins, the major toxins produced by O. cf. ovata, and the maximum quantum yield efficiency of L. paradoxa, N. arenaria and P. lima was affected at a minimum concentration of 10 µg mL−1. We then hypothesized that the diffusion of solubilized ovatoxins in the culture medium affects only moderately the competitors’ growth, whereas their accumulation in the mucus would yield deleterious effects. More precisely, the competitors’ sensitivity to ovatoxins was enhanced in their stationary phase of growth and resulted from a rapid inhibition of an uncharacterized photosynthetic step downstream photosystem II. Altogether, these results emphasize the predominant role of the O. cf. ovata’s mucus in driving ecological interactions and suggest that it can affect the growth of several benthic microalgae by accumulating the potent ovatoxins