Potential metabolic mechanisms for inhibited chloroplast nitrogen assimilation under high CO2

Improving photosynthesis is considered a major and feasible option to dramatically increase crop yield potential. Increased atmospheric CO₂ concentration often stimulates both photosynthesis and crop yield, but decreases protein content in the main C₃ cereal crops. This decreased protein content in crops constrains the benefits of elevated CO₂ on crop yield and affects their nutritional value for humans. To support studies of photosynthetic nitrogen assimilation and its complex interaction with photosynthetic carbon metabolism for crop improvement, we developed a dynamic systems model of plant primary metabolism, which includes the Calvin–Benson cycle, the photorespiration pathway, starch synthesis, glycolysis–gluconeogenesis, the tricarboxylic acid cycle, and chloroplastic nitrogen assimilation. This model successfully captures responses of net photosynthetic CO₂ uptake rate (A), respiration rate, and nitrogen assimilation rate to different irradiance and CO₂ levels. We then used this model to predict inhibition of nitrogen assimilation under elevated CO₂. The potential mechanisms underlying inhibited nitrogen assimilation under elevated CO₂ were further explored with this model. Simulations suggest that enhancing the supply of α-ketoglutarate is a potential strategy to maintain high rates of nitrogen assimilation under elevated CO₂. This model can be used as a heuristic tool to support research on interactions between photosynthesis, respiration, and nitrogen assimilation. It also provides a basic framework to support the design and engineering of C₃ plant primary metabolism for enhanced photosynthetic efficiency and nitrogen assimilation in the coming high-CO₂ world.

Simulations with a dynamic systems model of C₃ primary metabolism show that the decreased supply of reducing equivalent and 2-oxoglutaric acid cause decreased nitrogen assimilation under elevated CO₂.