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


Author for correspondence: Rolf Siegwolf Tel: +41 (0)56 310 27 86 Fax: +41 (0)56 310 45 25


  • • 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 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.