Could ammonium nutrition increase plant C-sink strength under elevated CO₂ conditions?

Atmospheric carbon dioxide (CO₂) is increasing, and this affects plant photosynthesis and biomass production. Under elevated CO₂ conditions (eCO₂), plants need to cope with an unbalanced carbon-to-nitrogen ratio (C/N) due to a limited C sink strength and/or the reported constrains in leaf N. Here, we present a physiological and metabolic analysis of ammonium (NH₄⁺)-tolerant pea plants (Pisum sativum L., cv. snap pea) grown hydroponically with moderate or high NH₄⁺ concentrations (2.5 or 10 mM), and under two atmospheric CO₂ concentrations (400 and 800 ppm). We found that the photosynthetic efficiency of the NH₄⁺ tolerant pea plants remain intact under eCO₂ thanks to the capacity of the plants to maintain the foliar N status (N content and total soluble proteins), and the higher C-skeleton requirements for NH₄⁺ assimilation. The capacity of pea plants grown at 800 ppm to promote the C allocation into mobile pools of sugar (mainly sucrose and glucose) instead of starch contributed to balancing plant C/N. Our results also support previous observations: plants exposed to eCO₂ and NH₄⁺ nutrition can increase of stomatal conductance. Considering the C and N source-sink balance of our plants, we call for exploring a novel trait, combining NH₄⁺ tolerant plants with a proper NH₄⁺ nutrition management, as a way for a better exploitation of eCO₂ in C3 crops.