Potassium availability influences the mesophyll structure to coordinate the conductance of CO 2 and H 2 O during leaf expansion.

Leaf growth relies on photosynthesis and hydraulics to provide carbohydrates and expansion power; in turn, leaves intercept light and construct organism systems for functioning. Under potassium (K) deficiency stress, leaf area, photosynthesis and hydraulics are all affected by alterations in leaf structure. However, the connection between changes in leaf growth and function caused by the structure under K regulation is unclear. Consequently, the leaf hydraulic conductance (K leaf ) and photosynthetic rate (A) combined with leaf anatomical characteristics of Brassica napus were continuously observed during leaf growth under different K supply levels. The results showed that K leaf and A decreased simultaneously after leaf area with the increasing K deficiency stress. K deficiency significantly increased longitudinal mesophyll cell investment, leading to a reduced volume fraction of intercellular air-space (f ias ) and decreased leaf expansion rate. Furthermore, reduced f ias decreased mesophyll and chloroplast surfaces exposed to intercellular airspace and gas phase H 2 O transport, which induced coordinated changes in CO 2 mesophyll conductance and hydraulic conductance in extra-xylem pathways. Adequate K supply facilitated higher f ias through smaller palisade tissue cell density (loose mesophyll cell arrangement) and smaller spongy tissue cell size, which coordinated CO 2 and H 2 O conductance and promoted leaf area expansion.