The objective of this work was the examination of chemical composition of rooted cuttings (root and stem) of three poplar clones P. x euramericana cl. Pannonia, P. deltoides cl. B-81, (P. nigra x P. maximowitzii) x P. nigra var. »Italica« cl. 9111/93, grown under different nitrate concentration in nutrient solution during investigation of their phytoremediation potential. It was found that the most amount of nitrate was acummulated in the plant’s roots, and that is reason of significant increasing of ash content in root and stem. Because the analysis of chemical composition is carried out with very young plants, cellulose contents is very low, and lignine content is very high, compared to common values for poplar wood.