Les activités humaines agricoles, urbaines et industrielles, sans cesse croissantes, sont à l'origine d'une contamination de notre environnement par les métaux lourds. Alors que de nombreuses molécules organiques peuvent être dégradées, les métaux lourds ne le peuvent pas et leur concentration augmente régulièrement dans les sols et les eaux. Ceci expose les plantes à des concentrations croissantes de métaux lourds. L'accumulation de métaux lourds dans les plantes présente un risque toxique pour l'Homme, car les plantes cultivées sont le point d'entrée dans la chaîne alimentaire. Par comparaison aux autres espèces, la laitue «Lactuca sativa» a tendance à présenter des teneurs élevées en cadmium, un métal lourd très toxique dont la concentration croît régulièrement dans les sols cultivés pour des raisons environnementales. En termes de sécurité alimentaire, il est donc important de produire des variétés accumulant des teneurs en cadmium réduites, et la laitue est un bon modèle pour débuter ce type d'approche. L'objectif de notre thèse était de caractériser aux niveaux physiologique, génétique, et moléculaire, les déterminants majeurs contrôlant l'accumulation du cadmium chez la laitue. Notre objectif a été décliné en différentes opérations. Tout d'abord une analyse de diversité nous a permis de décrire la capacité d'accumulation de cadmium de 18 génotypes de laitue et de sélectionner des génotypes présentant des performances extrêmes vis-à-vis du cadmium. Ainsi, des génotypes présentant des performances extrêmes du point de vue de la tolérance au cadmium, de l'accumulation de cadmium et de la capacité de translocation du cadmium des racines vers les parties aériennes ont été sélectionnés. Dans une deuxième étape, nos variétés extrêmes pour les caractères d'accumulation de cadmium et de translocation de cadmium des racines vers la partie aérienne ont été utilisées pour développer une approche physiologique. L'objectif de cette approche était de montrer l'origine de leur variabilité afin de comprendre les déterminants physiologiques contrôlant l'accumulation du cadmium chez la laitue. Dans le cadre de cette approche, des expériences d'efflux faites en utilisant du cadmium radioactif (109Cd) ont montré que la variété la moins accumulatrice de cadmium se caractérise par un efflux de cadmium marqué plus important par comparaison aux autres variétés. Cette variété possède vraisemblablement un mécanisme d'efflux plus efficace par comparaison aux autres variétés qui pourraient être le déterminant majeur permettant d'obtenir une plus faible accumulation de cadmium chez cette variété. Notre étude a permis de montrer une forte corrélation positive entre teneur en cadmium et teneur en zinc. Ce qui a permis d'émettre l'hypothèse d'un transport très associé entre le cadmium et le zinc. / Cadmium is an extremely toxic pollutant causing a great number of diseases. It is a widespread metal which concentration rises in cultivated soils, thus exposing food or feed crops to it and ultimately causing potential major sanitary problems as a consequence of its entry in the food chain. Among the vegetable species used for food, lettuce shows one of the highest capacities of accumulating cadmium. Our objectives are to characterize mechanisms controlling cadmium accumulation in this species and identify the corresponding genetic determinants, ultimately aiming at breeding lettuce for cadmium under-accumulation.A phenotypical analysis of cadmium accumulation was carried out on 18 lettuce genotypes representing the genetic diversity of the species, revealing a great variability of response, both with respect to cadmium accumulation and cadmium translocation from roots to shoot. The lettuce genotypes displayed independent variations for both traits, and also between these traits and cadmium tolerance. In contrast, a very strong positive correlation linked cadmium and zinc accumulation. Increasing calcium and iron concentration in the culture medium had a protective effect against cadmium accumulation, however this characteristic did not discriminate the lettuce genotypes presenting extreme cadmium accumulation phenotypes. Interestingly, experiments measuring 109Cd influx in roots, 109Cd efflux from roots and 109Cd translocation from roots to shoots revealed that the genotype displaying the least cadmium accumulation could be discriminated from the genotype displaying the highest one by its markedly increased ability to efflux cadmium from the roots to the culture medium. Progeny analysis from crosses between the genotypes displaying extreme performances for cadmium accumulation, cadmium translocation from roots to shoots and cadmium tolerance revealed that none of these traits was supported by a single genetic determinism. QTL mapping is under development. Surprisingly, the genetic analysis demonstrated that the ability to limit cadmium accumulation (and thus probably to increase cadmium efflux from the roots) was recessive.In conclusion, phenotypic analysis of cadmium accumulation in a large set of lettuce genotypes revealed that a major determinant limiting cadmium accumulation in these species is the ability to efflux cadmium from the root to the culture medium.

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Dust accumulation on photovoltaic (PV) modules and its effect on their performance are of high concern for regions with a high rate of dust, low frequency and intensity of rain. In this thesis, the effect of dust on PV modules is investigated with respect to dust concentration and spectral transmittance. The measured spectral transmittance of the dust sample shows spectral attenuation effect that varies at different wavelengths. This effect is explained by the particle size distribution of the dust samples: At shorter wavelengths more light is scattered due to the effect of the smaller particles. This effect has a major impact on the PV module as it affects PV technologies with a wider band-gap more than those of a narrower band-gap. The effect of dust is accumulative, i.e. PV module performance is reduced by increasing deposition over time or until it's cleared manually or by rain. The tilt angle of the PV installation plays a major role in the amount of dust accumulated on the devices, where higher tilt angles result in decreased dust concentrations. This effect is demonstrated in outdoor measurements where tilted modules had lower losses in daily as well as total array yield. It is also shown that tilted modules benefit from precipitation more than horizontal modules. However over the exposure period the modules did not show any clear aging effect caused specifically from dust accumulation or exhibit any seasonal variation. Different tilt angles can produce varying non-uniform dust patterns on the device surface. This effect and its pattern over long and short periods of exposure are investigated by means of spatial three dimensional modelling. The simulations compare two dust accumulation patterns that represent a short exposure to a single dusty day (one day) and a long exposure of dust (3 months). Out of the two patterns, the long exposure patterns showed higher losses of 19.4% in comparison to 14.8% for the short exposure. The simulation also showed that dust accumulation that promotes high concentration of dust at the bottom of the PV modules where it covers a full cell has a high risk of triggering hot spots and thus risks permanent module damage. A dust correction model for energy prediction is developed. The model takes into consideration dust concentration, spectral attenuation effect of dust, PV technology, and various meteorological variables. The modified spectral transmittances of the dust were incorporated into the model in the form of pre-measured data. This means in this work samples collected in Kuwait were measured and used to generate the input. The model is compared against the outdoor measured data and a good agreement between measurements iv and simulations is demonstrated. Using this model two procedures were developed. The first evaluates the uncertainties associated with dust over long periods of time. The second is to find the optimised cleaning schedule and frequency of cleaning based on acceptable yield loss margins over the simulated period of time. The optimisation of the cleaning schedule showed that for Kuwait setting the daily energy losses in PV modules at less than 10% will set the cost of cleaning higher than the cost of energy lost due to dust.

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We make a first-order calculation of accumulation rates in the catchment of Thwaites Glacier (TG), West Antarctica using the Nye and Daansgard-Johnson methodologies. Both formulations compute accumulations as a function of the age-depth relationship, including a thinning correction due to ice flow. For this purpose, I track and firn-correct two continuous, shallow ice layers obtained from radio echo soundings surveyed during the 2004-05 AGASEA expedition. The layers range from 60 to 700 meters depth between the ice divide and the coast. Dating of layers come from the ice core WDC06A, located on the West Antarctic Ice Sheet (WAIS) ice divide, which have ages 548 and 725 years, respectively. We compare our accumulation results with four independent datasets: 1)IceBridge snow radar (2009-2010), optimized for tracking near-surface layers; 2) a contemporary model of snowfall precipitation, 3) an interpolation of ice core data using satellite passive microwave; 4) ice cores data. We test the hypothesis that accumulation rates have increased since the beginning of the industrial era, a change that has not been observed. Indeed, I find that observations indicate that accumulation rates in the TG catchment have not changed during the past ~700 years. From here I assess the mass balance of the system and analyze what it tells about the history of the glacier. / text

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