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 (gm ) and photosynthesis (A). The relative contributions of leaf anatomical traits in determining gm have been quantified; however, anatomical variabilities related to low gm 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 gm . 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 (Sc /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 gm variations, ascribing to an altered Sc /S. These results emphasize the important role of K on the regulation of gm by enhancing Sc /S and reducing cytoplasm resistance.