Cuttings of hybrid poplar(Populus x euramericana var. ''Dorskamp'') were exposed to ozone alone (mean concentration 60 nl l^-1) and in combination with NO₂ (mean concentration 100 nl l^-1) during 16 weeks. Three different levels of nitrogen fertilization were applied, the lowest causing growth limitation. Ozone fumigation induced premature leaf abscision, regardless of the fertilization level. Whole-plant biomass production was reduced as compared to control trees grown in filtered air. In trees under nitrogen limitation, biomass reduction by ozone occurred in the roots rather than in the above-ground woody parts. The roots, however, maintained their uptake capacity for nitrogen, leading to similar whole-plant nitrogen content in fumigated and control trees. Trees under nitrogen limitation responded to ozone with reallocation of nitrogen from the roots to the developing foliage, resulting in higher nitrogen concentrations in young leaves. As a consequence, these leaves had a higher chlorophyll concentration, elevated CO₂ assimilation rate and stomatal conductance, and a greater leaf area. In contrast, ozone-damaged older leaves exhibited chlorophyll degradation, lower CO₂ assimilation rates and a poorer water use efficiency.
Despite nitrogen limitation, this element was not mobilized from the leaves prior to abscision, and therefore was lost for the plants. High nitrogen supply from the fertilizer suppressed nitrogen reallocation within the plants. The presence of atmospheric NO₂ did not significantly affect the plant response to ozone. The induction of foliar nitrate reductase, which is a key enzyme in nitrogen assimilation, suggested that NO₂ was used as an additional nitrogen source. Nitrogen distribution within the poplar trees is discussed in the context of acclimation strategies to ozone stress.