Leaf photosynthesis is mediated by the coordination of nitrogen and potassium: The importance of anatomical-determined mesophyll conductance to CO2 and carboxylation capacity.

Both nitrogen (N) and potassium (K) have been widely studied in maintaining efficient photosynthesis and plant growth. However, the mechanisms underlying the photosynthetic response to their interaction remain unclear. This study assessed the effects of N and K supply on photosynthetic limitations and the corresponding changes in anatomical structures in leaves of rice (Oryza sativa L.) plants, grown hydroponically under different levels of N and K in a greenhouse. Results revealed that a suitable leaf N/K ratio (2.99-3.10) maintain a high rate of photosynthesis (A). The A under N and/or K deficiency was primarily limited by mesophyll conductance (gm) and RuBP carboxylation in biochemical processes. The decline of gm in N- or K-starved leaves was mostly resulted from low surface area of chloroplasts exposed to intercellular airspaces (Sc) and high mesophyll cell wall thickness. Synergistic effects of N and K on gm were reflected in leaf anatomical structure, especially their coordinated roles in enhancing Sc. The enhanced photosynthesis in plants with coordinated supply of N and K was caused by the balance of RuBP carboxylation and regeneration. These results highlight the synergistic effect of N and K on leaf photosynthesis, which are mainly reflected in facilitating anatomical-determined gm and carboxylation capacity.