LAUSR

It is well known that NH4+ used as the sole N source is toxic, and that the degree of toxicity depends on environmental factors. However, far too little is known about the effect of the use of different NO3−/NH4+ ratios when the CO2 concentration [CO2] is high. Therefore, this work evaluates the extent to which the optimal form of the N-supply can increase growth at elevated CO2. Sweet pepper plants (Capsicum annuum L., cv. Melchor) were grown at ambient or elevated [CO2] (400 or 800 µmol mol−1) with a nutrient solution containing different NO3−/NH4+ ratios (concentration percentages of 100/0, 100/0 plus foliar urea (100/U), 90/10, 50/50, or 25/75). The results show that a low dose of NH4+ (90/10) in combination with the elevated [CO2] had beneficial effects on the plants. These plants had greater growth, root respiration rates, water-use efficiency, and chlorophyll fluorescence. Additionally, total phenolic compounds and ascorbate peroxidase activity were affected differentially, while the amino acid profile was also altered. This study reveals the strong interaction between the N form and the [CO2] in relation to the uptake of N, which requires further analysis to establish better nutritional strategies for the future.