Nitrogen sources and CO₂ concentration synergistically affect the growth and metabolism of tobacco plants

The initial stimulation of photosynthesis under elevated CO₂ concentrations (eCO₂) is often followed by a decline in photosynthesis, known as CO₂ acclimation. Changes in N levels under eCO₂ can have different effects in plants fertilized with nitrate (NO₃^-) or ammonium (NH₄⁺) as the N source. NO₃^- assimilation consumes approximately 25% of the energy produced by an expanded leaf, whereas NH₄⁺ requires less energy to be incorporated into organic compounds. Although plant-N interactions are important for the productivity and nutritional value of food crops worldwide, most studies have not compared the performance of plants supplied with different forms of N. Therefore, this study aims to go beyond treating N as the total N in the soil or the plant because the specific N compounds formed from the available N forms become highly engaged in all aspects of plant metabolism. To this end, plant N metabolism was analyzed through an experiment with eCO₂ and fertigation with NO₃^- and/or NH₄⁺ as N sources for tobacco (Nicotiana tabacum) plants. The results showed that the plants that received only NO₃^- as a source of N grew more slowly when exposed to a CO₂ concentration of 760 μmol mol^-1 than when they were exposed to ambient CO₂ conditions. On the other hand, in plants fertigated with only NH₄⁺, eCO₂ enhanced photosynthesis. This was essential for the maintenance of the metabolic pathways responsible for N assimilation and distribution in growing tissues. These data show that the physiological performance of tobacco plants exposed to eCO₂ depends on the form of inorganic N that is absorbed and assimilated.