Elevated CO₂ concentrations were found to cause early senescence during leaf development in sunflower (Helianthus annuus L.) plants, probably by reducing nitrogen availability since key enzymes of nitrogen metabolism, including nitrate reductase (NR); glutamine synthetase (GS) and glutamate dehydrogenase (GDH), were affected. Elevated CO₂ concentrations significantly decreased the activity of nitrogen assimilation enzymes (NR and GS) and increased GDH deaminating activities. Moreover, they substantially rose the transcript levels of GS1 while lowering those of GS2. Increased atmospheric CO₂ concentrations doubled the CO₂ fixation and increased transpiration rates, although these parameters decreased during leaf ontogeny. It can be concluded that elevated atmospheric CO₂ concentrations alter enzymes involved in nitrogen metabolism at the transcriptional and post-transcriptional levels, thereby boosting mobilization of nitrogen in leaves and triggering early senescence in sunflower plants.