Leaf and canopy photosynthesis of C₃ plants at elevated CO₂ in relation to optimal partitioning of nitrogen among photosynthetic components: Theoretical prediction

Effects of changes in the organization of photosynthetic components on leaf photosynthesis under contrasting atmospheric CO₂ conditions (35 and 70 Pa) are evaluated using an optimization model, in which the photosynthetic rate is limited either by the capacity of ribulose bisphosphate carboxylase (RuBPCase) to consume ribulose bisphosphate (RuBP) or by the capacity of RuBP regeneration. The nitrogen cost of photosynthetic components to carry out each process is calculated for the optimal partitioning of nitrogen among the components. The model predicts that nitrogen allocation to the components carrying out RuBP regeneration should be increased with reduction in allocation to RuBPCase to maximize daily photosynthesis at 70 Pa CO₂. At a temperature of 25°C, doubling the current CO₂ level increases daily photosynthesis by 60% with optimal reallocation of the nitrogen partitioning while the increase without reallocation of nitrogen is 40%. However, at lower growth irradiance, the advantage in daily photosynthesis due to the reallocation decreases with increasing nitrogen content. The ratio of photosynthesis at 70 Pa to that at 35 Pa increases with increasing temperature. The effects of CO₂ levels on photosynthesis of a canopy in which nitrogen is optimally allocated among leaf layers are also examined. At 25°C, canopy photosynthesis at the doubled CO₂ level is predicted to increase 60 and 40% with and without the optimization of nitrogen partitioning among photosynthetic components, respectively. Doubling the CO₂ level does not affect the optimal nitrogen distribution among leaf layers in the canopy irrespective of optimization of nitrogen partitioning among photosynthetic components.