Transformed plants of Nicotiana plumbaginifolia Viv. constitutively expressing nitrate reductase (35S-NR) or b-glucuronidase (35S-GUS) and untransformed controls were grown for two weeks in a CO 2enriched atmosphere. Whereas CO 2 enrichment (1000 ll á l A1) resulted in an increase in the carbon (C) to nitrogen (N) ratio of both the tobacco lines grown in pots with vermiculite, the C/N ratio was only slightly modi®ed when plants were grown in hydroponic culture in high CO 2 compared to those grown in air. Constitutive nitrate reductase (NR) expression per se did not change the C/N ratio of the shoots or roots. Biomass accumulation was similar in both types of plant when hydroponic or pot-grown material, grown in air or high CO 2 , were compared. Shoot dry matter accumulation was primarily related to the presence of stored carbohydrate (starch and sucrose) in the leaves. In the potgrown tobacco, growth at elevated CO 2 levels caused a concomitant decrease in the N content of the leaves involving losses in NO À 3 and amino acid levels. In contrast, the N content and composition were similar in all plants grown in hydroponic culture. The 35S-NR plants grown in air had higher foliar maximum extractable NR activities and increased glutamine levels (on a chlorophyll or protein basis) than the untransformed controls. These increases were maintained following CO 2 enrichment when the plants were grown in hydroponic culture, suggesting that an increased ¯ux through nitrogen assimilation was possible in the 35S-NR plants. Under CO 2 enrichment the NR activation state in the leaves was similar in all plants. When the 35S-NR plants were grown in pots, however, foliar NR activity and glutamine content fell in the 35S-NR transformants to levels similar to those of the untransformed controls. The dierences in NR activity between untransformed and 35S-NR leaves were much less pronounced in the hydroponic than in the pot-grown material but the dierence in total extractable NR activity was more marked following CO 2 enrichment. Foliar NR message levels were decreased by CO 2 enrichment in all growth conditions but this was much more pronounced in potgrown material than in that grown hydroponically. Since b-glucuronidase (GUS) activity and message levels in 35S-GUS plants grown under the same conditions of CO 2 enrichment (to test the eects of CO 2 enrichment on the activity of the 35S promoter) were found to be constant, we conclude that NR message turnover was speci®cally accelerated in the 35S-NR plants as well as in the untransformed controls as a result of CO 2 enrichment. The molecular and metabolic signals involved in increased NR message and protein turnover are not known but possible eectors include NO À 3 , glutamine and asparagine. We conclude that plants grown in hydroponic culture have greater access to N than those grown in pots. Regardless of the culture method, CO 2 enrichment has a direct eect on NR mRNA stability.