RESPONSE OF POPLAR TO OZONE ALONE AND IN COMBINATION WITH NO2 AT DIFFERENT NITROGEN-FERTILIZATION LEVELS

Cuttings of hybrid poplar(Populus x euramericana var. ''Dorskamp'') we re exposed to ozone alone (mean concentration 60 nl l(-1)) and in comb ination with NO2 (mean concentration 100 nl l(-1)) during 16 weeks. Th ree different levels of nitrogen fertilization were applied, the lowes t causing growth limitation. Ozone fumigation induced premature leaf a bscision, regardless of the fertilization level. Whole-plant biomass p roduction was reduced as compared to control trees grown in filtered a ir. In trees under nitrogen limitation, biomass reduction by ozone occ urred in the roots rather than in the above-ground woody parts. The ro ots, however, maintained their uptake capacity for nitrogen, leading t o 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 hig her nitrogen concentrations in young leaves. As a consequence, these l eaves had a higher chlorophyll concentration, elevated CO2 assimilatio n rate and stomatal conductance, and a greater leaf area. In contrast, ozone-damaged older leaves exhibited chlorophyll degradation, lower C O2 assimilation rates and a poorer water use efficiency. Despite nitro gen limitation, this element was not mobilized from the leaves prior t o abscision, and therefore was lost for the plants. High nitrogen supp ly from the fertilizer suppressed nitrogen reallocation within the pla nts. The presence of atmospheric NO2 did not significantly affect the plant response to ozone. The induction of foliar nitrate reductase, wh ich is a key enzyme in nitrogen assimilation, suggested that NO2 was u sed as an additional nitrogen source. Nitrogen distribution within the poplar trees is discussed in the context of acclimation strategies to ozone stress.