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 x 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 H₂O 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 ¹⁵N-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.