Pollinisation

Résumé

vignette|upright=1.5|La reproduction est dite « entomophile » quand la pollinisation est faite par un insecte (ici un bourdon). La pollinisation correspond, chez les angiospermes et chez les gymnospermes, au transport du pollen des organes de reproduction mâle (étamines) vers le (ou les) organes de reproduction femelle (pistil) pour permettre la reproduction sexuée. La pollinisation est une étape préalable à la fécondation dans le cycle de vie de ces plantes. Ce transport a lieu soit à l'intérieur des fleurs (autopollinisation), soit par pollinisation croisée appelée aussi allopollinisation (le pollen d'une fleur se dépose sur les stigmates d'une autre fleur de la même espèce). Dans ce dernier cas, les vecteurs de pollinisation peuvent être biotiques (zoogamie assurée par les oiseaux, insectes…) ou abiotiques (les agents de transport du pollen sont le vent , l'eau …). Les plantes contribuent à l'alimentation des pollinisateurs en leur fournissant le pollen en excès ou nectar. Cette

Source officielle

https://fr.wikipedia.org/wiki/Pollinisation

À propos de ce résultat

Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.

Publications associées (11)

Publications associées

Chargement

Personnes associées

Chargement

Unités associées

Chargement

Concepts associés (97)

Concepts associés

Chargement

Cours associés (6)

Cours associés

Chargement

Séances de cours associées (10)

Séances de cours associées

Chargement

POLLEN CLUMPING AND WIND DISPERSAL IN AN INVASIVE ANGIOSPERM

Michael David Martin, Charles Vivant Ignacio Meneveau, Marc Parlange

Pollen dispersal is a fundamental aspect of plant reproductive biology that maintains connectivity between spatially separated populations. Pollen clumping, a characteristic feature of insect-pollinated plants, is generally assumed to be a detriment to wind pollination because clumps disperse shorter distances than do solitary pollen grains. Yet pollen clumps have been observed in dispersion studies of some widely distributed wind-pollinated species. We used Ambrosia artemisiifolia (common ragweed; Asteraceae), a successful invasive angiosperm, to investigate the effect of clumping on wind dispersal of pollen under natural conditions in a large field. Results of simultaneous measurements of clump size both in pollen shedding from male flowers and airborne pollen being dispersed in the atmosphere are combined with a transport model to show that rather than being detrimental, clumps may actually be advantageous for wind pollination. Initial clumps can pollinate the parent population, while smaller clumps that arise from breakup of larger clumps can cross-pollinate distant populations.

2009

Chargement

Relevant source area of pollen (RSAP) and pollen productivity for 11 key taxa characteristic of the pasture woodland landscape of the Jura Mountains, Switzerland, were estimated using pollen assemblages from moss polsters at 20 sites. To obtain robust pollen productivity estimates (PPEs), we used vegetation survey data at a fine spatial-resolution (1 x 1 m(2)) and randomized locations for sampling sites, techniques rarely used in palynology. Three Extended R value (ERV) submodels and three distance-weighting methods for plant abundance calculation were applied. Different combinations of the submodels and distance-weighting methods provide slightly different estimates of RSAP and PPEs. Although ERV submodel 1 using 1/d (d = distance in meters) best fits the dataset, PPE values for heavy pollen types (e.g. Abies) were sensitive to the method used for distance-weighting. Taxon-specific distance-weighting methods, such as Prentice's model, emphasize the intertaxonomic differences in pollen dispersal and deposition, and are thus theoretically sound. For the dataset obtained in this project, Prentice's model was more appropriate than other distance-weighting methods to estimate PPEs. Most of the taxa have PPEs equal to (Fagus, Plantago media and Potentilla-type), or higher (Abies, Picea, Rubiaceae and Trollius europaeus) than Poaceae (PPE = 1). Acer, Cyperaceae, and Plantago montana-type are low pollen producers. This set of PPEs will be useful for reconstructing heterogeneous, mountainous pasture woodland landscapes from fossil pollen records. The RSAP for moss polsters in this semi-open landscape region is ca. 300 m.

Springer Verlag2008

Chargement

Reduction and immobilization of uranium in the subsurface

Malgorzata Alicja Stylo

Decades of uranium (U) mining, milling and military use left a legacy of U contamination around the world. The radioactivity and chemical toxicity of U at contaminated sites pose an acute and long-term hazard to human health and the surrounding environment. In order to diminish the risk, in situ bioremediation methods, which contribute to contaminant immobilization, are proposed. Nevertheless, the reported prevalent formation of labile and non-crystalline U(IV) species as a result of microbial U(VI) reduction, in contrast to more stable and crystalline uraninite, undermines the effectiveness of the applied bioremediation. Therefore, a holistic understanding of the controls and mechanisms that govern the formation of non-crystalline U(IV) in the environment is at the core of this thesis. Presence of common groundwater solutes (sulfate, silicate and phosphate) were shown to induce the production of bacterial extracellular polymeric substances (biofilm matrix components), which in turn increases the formation of non-crystalline U(IV) as a result of microbial U reduction. In contrast, a field study suggested that non-crystalline U(IV) was a product of abiotic U reduction followed by the sequestration of U(IV) ions by the biofilm matrix. Those contrasting theories, motivated us to look for an indicator capable of differentiating between biotic and abiotic U reduction in the environment. Uranium isotope fractionation proved to be an excellent tool. Based on our results, the isotopic signature of biotic U reduction (accumulation of 238U in the reduced phase) is easily distinguishable from the abiotic U reduction signature (either no isotopic fractionation or fractionation in the opposite direction). When contrasted with U isotope signatures recorded in the sediments, the findings of this study indicated that biological activity contributed to the formation of many ancient and modern U(IV) deposits. Equipped with a tool capable of assessing the origin of the U(IV) product, and with an understanding of the relationship between the biofilm matrix and the formation of non-crystalline U(IV), we embarked on a project to validate the mechanism responsible for non-crystalline U(IV) formation in the subsurface. Results show that regardless whether U is reduced enzymatically or though abiotic agents, the formed U(IV) species are always non-crystalline if the biofilm matrix is present. However, according to our experiments, biological U reduction resulted in better immobilization and overall reduction of U. The mechanism of U reduction in our systems was shown to be controlled by U speciation, which in turn, is govern by the presence of bicarbonate. Under low carbonate concentrations, the majority of U(VI) adsorbs onto the mineral phase thus promoting abiotic U reduction mediated by redox active minerals. However, at higher carbonate concentrations, most of U(VI) is present as aqueous U(VI)-carbonate complexes precluding sorption and thus allowing for enhanced enzymatic U reduction. The results obtained here shed light on the processes occurring at U-contaminated sites and have several field implications. The effectiveness of applied bioremediation was shown to depend on the geochemical conditions at the field site. However, the injection of bicarbonate during in situ clean-up operation enhances microbial U reduction and by this, the overall reductive immobilization of U.

EPFL2015

Chargement

Reduction and immobilization of uranium in the subsurface

Malgorzata Alicja Stylo

EPFL2015