Subcellular Potassium Ion‐Mediated Structural Coordination Between Mesophyll Cells and Chloroplasts Determines CO 2 Mesophyll Conductance

ABSTRACT Potassium (K) homeostasis critically orchestrates photosynthetic performance through mesophyll conductance ( g m ) regulation—a key bottleneck limiting CO 2 assimilation under K deficiency. While K deprivation reduces g m via anatomical alterations, the mechanistic interplay between subcellular K + distribution and chloroplast‐level structural determinants remains elusive. Field experiments using two Brassica napus L. cultivars with contrasting K utilization efficiencies (KUtE) were employed to resolve how subcellular K + dynamics regulate g m through hierarchical structural modifications. Under mild K deficiency (50% of sufficient K supply), reduced vacuolar K + restricted mesophyll cell expansion, increasing cell density and reducing the intercellular airspace ( f ias ). Despite elevated gas‐phase CO 2 diffusion resistance, liquid‐phase conductance was maintained. Under severe K deficiency (K deprivation), reduced chloroplastic K + decreased chloroplast density and size, lowering the chloroplast surface area facing intercellular airspaces ( S c / S ). Cytosolic K + depletion further increased cytosolic CO 2 transport resistance. The g m reduction was stronger in the low‐KUtE cultivar, consistent with its sharper subcellular K + decline. Hydroponic experiments further confirmed that subcellular K + depletion reduced vacuolar metal ions but induced accumulations of organic acids and sugars, while Na + and sugars increased in chloroplasts, disrupting osmotic balance and aggravating structural impairment. These findings reveal that subcellular K + regulates g m by remodeling mesophyll and chloroplast structures.