• The effects of N-availability and elevated atmospheric CO 2 partial pressure ( pCO2 ) on growth, allometry and N-metabolism of poplar plants are reported here. • Poplar plants were grown hydroponically at deficient and sufficient N-supply under ambient and elevated pCO2 . The N-fluxes within the plants were estimated by comparing the fate of newly acquired 15 N-NO3 - in plants either severely N-limited or with sufficient N-supply. • At deficient N-supply, plants accumulated less biomass and exhibited an increased root : shoot ratio compared with sufficient N-supply; a larger fraction of newly acquired 15 N was allocated to the youngest leaves immediately after exchange of the nutrient solution. Increasing the external N-supply from deficient to sufficient shifted the site of nitrate reduction from roots to leaves. • Elevated pCO2 increased total biomass and the root : shoot ratio at deficient N-supply, but had no effect at sufficient N-supply. Elevated pCO2 decreased rates of N-uptake in both treatments. Increased root : shoot ratio at deficient N-supply coincided with enhanced nitrate reduction in the root and elevated pCO2 also enhanced the allocation of newly acquired 15 N to the youngest leaves. Root nitrate reduction as a possible factor controlling the root : shoot ratio and N-allocation is discussed.
Keywords: N-uptake; Populus; elevated CO2; nitrate reduction; root : shoot ratio.