Drought is certainly one of the main abiotic stresses that our forests will have to face to in the near future, especially in the Mediterranean basin where climatic change forecasts predict a 1.5 to 4°C increase in temperatures. Compared to what is known in annuals, relatively few are known regarding the molecular responses of perennials to water deficit. Natural populations of Maritime pine, with ecotypes ranging from very dry to wet areas, constitute an excellent system to study forest tree adaptation to drought. Exploring the molecular plasticity of two ecotypes contrasting in their edapho-climatic regimes should highlight molecular mechanisms that have been shape by natural selection to provide them the highest fitness. In this study, we used a factorial design crossing two contrasted ecotypes by different watering conditions (control, 6h, 48h, 3 weeks of stress). Water deficit was applied using PEG as osmoticum in hydroponics on 8 weeks old seedlings. Physiological and metabolic characterizations of the plants (roots and leaves) were first carried out to evaluate the intensity of the imposed stress. Then, expression profiles in roots were obtained using a 7.2K maritime pine cDNA micro-array. Our primary interest was for gene showing contrasted reaction norms between the two ecotypes, among which we found genes involved in cell wall composition, osmoregulation, terpene biosynthesis, general stress response and regulation of transcription. Whether these genes really matter is validated by population geneticists who describe their landscape of nucleotide diversity and in particular molecular signatures of natural selection.