Trade-offs between photosynthetic capacity, mesophyll conductance stability and leaf anatomy shape heat and water deficit resilience in Gossypium

Summary Mesophyll conductance ( g m ) governs CO 2 diffusion to Rubisco and is a key determinant of photosynthetic performance, yet the mechanisms underlying its sensitivity to heat and water stress remain unresolved. We quantified g m temperature responses across diverse Gossypium species and examined anatomical drivers of g m plasticity in cultivated cotton ( G. hirsutum ) and the wild Australian species G. bickii under elevated temperature and soil water deficit. Species exhibited contrasting g m strategies: G. hirsutum exhibited high g m and carbon assimilation near thermal optima but showed greater sensitivity under combined heat and water deficit, whereas G. bickii maintained comparatively stable g m and photosynthesis across stress conditions. Under water deficit, structural adjustments in G. hirsutum (increased leaf porosity, cell wall thickness and mesophyll surface exposure to intercellular airspaces) were insufficient to sustain g m , suggesting that liquid-phase resistances impose dominant constraints on CO 2 diffusion under extreme climatic stress. These results identify g m as a dynamic, multi-component trait and a key physiological vulnerability in cotton, shaped by coordinated anatomical characteristics and potentially cell wall properties and membrane-associated processes, with major implications for mechanistic photosynthesis modelling and improving climate resilience in cotton and other C 3 species.