Sargent, D.; Conaty, W.; Chapman, K.; Dubey, G.; George, L.; Lindsay, S.; Wuhrer, R.; von Caemmerer, S.; Evans, J.; Sharwood, R.

- Mesophyll conductance (gm) governs CO2 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 gm temperature responses across diverse Gossypium species and examined anatomical drivers of gm plasticity in cultivated cotton (G. hirsutum) and the wild Australian species G. bickii under elevated temperature and soil water deficit. - Species exhibited contrasting gm strategies: G. hirsutum exhibited high gm and carbon assimilation near thermal optima but showed greater sensitivity under combined heat and water deficit, whereas G. bickii maintained comparatively stable gm 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 gm, suggesting that liquid-phase resistances impose dominant constraints on CO2 diffusion under extreme climatic stress. - These results identify gm 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 C3 species.