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 (NO,) exposure are reported on the physiolo gy, morphology and carbon partitioning of hybrid poplar clone cuttings (Pop ulus 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)) o ver 12 wk growth in phytotrons. Stable isotope analysis, combined with CO2 and H2O gas exchange measurements, biomass analysis and morphological devel opment, was used to assess the integrated long-term effects of NO2. NO2 had no toxic effects. A reduced N-15-isotope ratio indicated incorporat ion of NO2 while nitrate reductase activity in leaves was stimulated. The t wo nitrogen sources had differential effects on water use efficiency (WUE): NO2 exposure increased long-term WUE; soil N supply decreased WUE; a resul t not detectable using growth and short-term gas exchange experiments. Plan ts benefited from airborne NO2, increasing CO2 assimilation rate and biomas s; both N sources increased shoot production at the expense of root growth. NO2 exposure induced leaf formation with reduced stomatal density and incr eased leaf area. NO2 exposure might be beneficial although the reduced root: shoot biomass c ould have a detrimental effect on nutrient balance and drought resistance.