Subcellular Potassium Ion-Mediated Structural Coordination Between Mesophyll Cells and Chloroplasts Determines CO2 Mesophyll Conductance.

Potassium (K) homeostasis critically orchestrates photosynthetic performance through mesophyll conductance (gm) regulation-a key bottleneck limiting CO2 assimilation under K deficiency. While K deprivation reduces gm 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 gm 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 (fias). Despite elevated gas-phase CO2 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 (Sc/S). Cytosolic K+ depletion further increased cytosolic CO2 transport resistance. The gm 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 gm by remodeling mesophyll and chloroplast structures.