Climate drivers of mesophyll conductance: temperature and water deficit alter CO 2 diffusion and leaf anatomy in cotton

Summary Mesophyll conductance ( g m ) limits CO 2 delivery to Rubisco, yet its responses to temperature, heat and water deficit are poorly resolved in cotton ( Gossypium hirsutum ). We characterised photosynthetic and g m temperature responses in five Gossypium species, then examined cultivated G. hirsutum and the Australian native Gossypium bickii under combined elevated temperature and soil water deficit to link physiological responses with mesophyll anatomy. Plants were grown under controlled glasshouse conditions. We quantified g m using simultaneous gas exchange and online stable carbon isotope discrimination (Δ, ‰) and measured leaf anatomy. Across species, photosynthesis and g m exhibited broad thermal optima (33–36°C). In G. hirsutum , water deficit at control temperature reduced g m and induced thicker, more porous mesophyll and thicker cell walls, while assimilation declined only under combined heat and water deficit. In G. bickii , g m and assimilation remained stable under combined stress, and g m was stimulated by elevated temperature without anatomical adjustment. g m emerged as a stress‐sensitive trait in cultivated cotton, whereas G. bickii exhibited stress resilience, with responses incompletely explained by anatomical acclimation. Further research on g m responses to environmental stresses and on the mechanisms underpinning these responses is required to inform model development and identify traits for climate resilient crops.