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

Potassium (K) homeostasis critically orchestrates photosynthetic performance through mesophyll conductance (g_m) regulation-a key bottleneck limiting CO₂ 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 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 (S_c/S). Cytosolic K⁺ depletion further increased cytosolic CO₂ 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.