Anatomical variation of mesophyll conductance under potassium deficiency has a vital role in determining leaf photosynthesis

Leaves exposed to potassium (K) deficiency usually present decreased mesophyll conductance (g_m) and photosynthesis (A). The relative contributions of leaf anatomical traits in determining g_m have been quantified; however, anatomical variabilities related to low g_m under K starvation remain imperfectly known. A one‐dimensional model was used to quantify anatomical controls of the entire CO₂ diffusion pathway resistance within a leaf on two Brassica napus L. cultivars in response to K deficiency. Leaf photosynthesis of both cultivars was significantly decreased under K deficiency in parallel with down‐regulated g_m. The mesophyll conductance limitation contributed to more than one‐half of A decline. The decreased internal air space in K‐starved leaves was associated with the increase of gas‐phase resistance. Potassium deficiency reduced liquid‐phase conductance by decreasing the exposed surface area of chloroplasts per unit leaf area (S_c/S), and enlarging the resistance of the cytoplasm that can be interpreted by the increasing distance of chloroplast from cell wall, and between adjacent chloroplasts. Additionally, the discrepancies of A between two cultivars were in part because of g_m variations, ascribing to an altered S_c/S. These results emphasize the important role of K on the regulation of g_m by enhancing S_c/S and reducing cytoplasm resistance.