Climate drivers of mesophyll conductance: temperature and water deficit alter CO2 diffusion and leaf anatomy in cotton.

Mesophyll conductance (gm) limits CO2 delivery to Rubisco, yet its responses to temperature, heat and water deficit are poorly resolved in cotton (Gossypium hirsutum). We characterised photosynthetic and gm 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 gm using simultaneous gas exchange and online stable carbon isotope discrimination (Δ, ‰) and measured leaf anatomy. Across species, photosynthesis and gm exhibited broad thermal optima (33-36°C). In G. hirsutum, water deficit at control temperature reduced gm and induced thicker, more porous mesophyll and thicker cell walls, while assimilation declined only under combined heat and water deficit. In G. bickii, gm and assimilation remained stable under combined stress, and gm was stimulated by elevated temperature without anatomical adjustment. gm 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 gm responses to environmental stresses and on the mechanisms underpinning these responses is required to inform model development and identify traits for climate resilient crops.