Nitrate is one of the major sources of nitrogen taken up by plants. A large proportion of the nitrate acquired by plants is actively transported through nitrate transporters (NRT). To cope with low and high nitrate concentration in the soil, plants have developed high-affinity and low-affinity nitrate uptake systems, respectively. Genes of the NRT1 family are mainly low affinity nitrate transporters, and genes of the NRT2 family are high affinity transporters, whose functioning requires the participation of NRT3. Up to now, the molecular features and the regulation of nitrate transporters have been studied mostly in the model plant Arabidopsis thaliana. The molecular characterization of all NRT family genes has not yet been conducted in poplar, which is a model plant for genetic analysis in tree species. Earlier studies on poplar suggested that the response to N fertilization of poplar growth and wood properties were species-specific traits. We hypothesized that these traits also how intra-specific variation in response to N fertilization, because large differences in plant productivity were found among poplar genotypes. To investigate the role of NRT genes in poplar growth, four questions will be addressed in the present thesis.