Nitrogen sources and CO.sub.2 concentration synergistically affect the growth and metabolism of tobacco plants

The initial stimulation of photosynthesis under elevated CO.sub.2 concentrations (eCO.sub.2) is often followed by a decline in photosynthesis, known as CO.sub.2 acclimation. Changes in N levels under eCO.sub.2 can have different effects in plants fertilized with nitrate (NO.sub.3.sup.-) or ammonium (NH.sub.4.sup.+) as the N source. NO.sub.3.sup.- assimilation consumes approximately 25% of the energy produced by an expanded leaf, whereas NH.sub.4.sup.+ 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 eCO.sub.2 and fertigation with NO.sub.3.sup.- and/or NH.sub.4.sup.+ as N sources for tobacco (Nicotiana tabacum) plants. The results showed that the plants that received only NO.sub.3.sup.- as a source of N grew more slowly when exposed to a CO.sub.2 concentration of 760 [mu]mol mol.sup.-1 than when they were exposed to ambient CO.sub.2 conditions. On the other hand, in plants fertigated with only NH.sub.4.sup.+, eCO.sub.2 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 eCO.sub.2 depends on the form of inorganic N that is absorbed and assimilated.