Drought severity and frequency are increasing with climate changes threatening plant survival. Perennials species such as poplar trees are concerned through their water dependence and economical impact. Phenotypic plasticity is considered as a key lever in plant response to environmental fluctuation but little is known concerning transcriptional plasticity in water fluctuating conditions. This project explored relationships between ecophysiological and transcriptional responses to identify genes involved in drought tolerance. Six Populus nigra genotypes, contrasted by their geographical origin, were submitted to water withholding up to a severe drought fixed thanks to predawn leaf water potential measurements (-2 MPa). Hydraulic, physiological and architectural leaf measurements revealed genotype-specific drought responses and plasticity. One genotype was particularly interesting because of its low sensibility to drought while keeping biomass production. RNAseq was used to analyze the transcriptome of leaf grown during the stress. At least 30% of total leaf transcripts were differentially expressed. Among them, genes families related to water flux regulation (aquaporins) and transcription factors were identified. Along with ecophysiological plasticity, transcriptional plasticity was genotype specific and showed particular transcripts involved in plasticity maximization. Such transcripts were also correlated to ecophysiological traits within networks. These results highlighted the importance to integrate plasticity in improvement of tree breeding.