MODULATION OF CARBON AND NITROGEN-METABOLISM, AND OF NITRATE REDUCTASE, IN UNTRANSFORMED AND TRANSFORMED NICOTIANA-PLUMBAGINIFOLIA DURING CO2 ENRICHMENT OF PLANTS GROWN IN POTS AND IN HYDROPONIC CULTURE

Transformed plants of Nicotiana plumbaginifolia Viv. constitutively ex pressing nitrate reductase (35S-NR) or beta-glucuronidase (35S-GUS) an d untransformed controls were grown for two weeks in a CO2-enriched at mosphere. Whereas CO2 enrichment (1000 mu l. 1(-1)) resulted in an inc rease in the carbon (C) to nitrogen (N) ratio of both the tobacco line s grown in pots with vermiculite, the C/N ratio was only slightly modi fied when plants were grown in hydroponic culture in high CO2 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 a ccumulation was similar in both types of plant when hydroponic or pot- grown material, grown in air or high CO2, were compared. Shoot dry mat ter accumulation was primarily related to the presence of stored carbo hydrate (starch and sucrose) in the leaves. In the pot-grown tobacco, growth at elevated CO2 levels caused a concomitant decrease in the N c ontent of the leaves involving losses in NO3- and amino contrast, the N content and composition were similar in all plants grown in hydropon ic culture. The 35S-NR plants grown in air had higher foliar maximum e xtractable NR activities and increased glutamine levels (on a chloroph yll or protein basis) than the untransformed controls. These increases were maintained following CO2 enrichment when the plants were grown i n hydroponic culture, suggesting that an increased flux through nitrog en assimilation was possible in the 35S-NR plants. Under CO2 enrichmen t the NR activation state in the leaves was similar in all plants. Whe n the 35S-NR plants were grown in pots, however, foliar NR activity an d glutamine content fell in the 35S-NR transformants to levels similar to those of the untransformed controls. The differences in NR activit y between untransformed and 35S-NR leaves were much less pronounced in the hydroponic than in the pst-grown material but the difference in t otal extractable NR activity was more marked following CO2 enrichment. Foliar NR message levels were decreased by CO2 enrichment in all grow th conditions but this was much more pronounced in pot-grown material than in that grown hydroponically. Since beta-glucuronidase (GUS) acti vity and message levels in 35S-GUS plants grown under the same conditi ons of CO2 enrichment (to test the effects of CO2 enrichment on the ac tivity of the 35S promoter) were found to be constant, we conclude tha t NR message turnover was specifically accelerated in the 35S-NR plant s as well as in the untransformed controls as a result of CO2 enrichme nt. The molecular and metabolic signals involved in increased NR messa ge and protein turnover are not known but possible effecters include N O3-, glutamine and asparagine. We conclude that plants grown in hydrop onic culture have greater access to N than those grown in pots. Regard less of the culture method, CO2 enrichment has a direct effect on NR m RNA stability.