Stable isotope analysis reveals differential effects of soil nitrogen and nitrogen dioxide on the water use efficiency in hybrid poplar leaves.

•  The effects of nitrogen dioxide (NO2 ) exposure are reported on the physiology, morphology and carbon partitioning of hybrid poplar clone cuttings (Populus ×euramericana) grown under high and low soil nitrogen supply. •  Plants were exposed to filtered air or NO2 -enriched air (80-135 nl l-1 ) over 12 wk growth in phytotrons. Stable isotope analysis, combined with CO2 and H2 O gas exchange measurements, biomass analysis and morphological development, was used to assess the integrated long-term effects of NO2 . •  NO2 had no toxic effects. A reduced 15 N-isotope ratio indicated incorporation of NO2 while nitrate reductase activity in leaves was stimulated. The two nitrogen sources had differential effects on water use efficiency (WUE): NO2 exposure increased long-term WUE; soil N supply decreased WUE; a result not detectable using growth and short-term gas exchange experiments. Plants benefited from airborne NO2 , increasing CO2 assimilation rate and biomass; both N sources increased shoot production at the expense of root growth. NO2 exposure induced leaf formation with reduced stomatal density and increased leaf area. •  NO2 exposure might be beneficial although the reduced root: shoot biomass could have a detrimental effect on nutrient balance and drought resistance.