Elevated CO₂ alters cotton leaf–root coordination under water stress and salt stress: Implications for crop water relations

As climate change intensifies drought and salinity, whether elevated CO₂ alters leaf–root coordination and whole-plant water relations in cotton remains unclear. Here, three cotton cultivars were grown under ambient CO₂ (a[CO₂], 400 ppm) and elevated CO₂ (e[CO₂], 800 ppm) under well-watered (WW), water stress (WS), and salt stress (SS) treatments. Leaf and root economic traits, hydraulic conductance, intrinsic water-use efficiency (iWUE), and non-structural carbohydrates were measured, and leaf–root coordination was assessed using principal component analysis and standardized major axis regression. WS and SS induced asymmetric leaf–root adjustment, with leaf economic strategy shifting towards conservatism and root economic strategy towards acquisitiveness. e[CO₂] reduced the magnitude of this stress-induced shift and modified stress effects on the relationships among economic strategy, hydraulic function, and iWUE. Under a[CO₂], stress more readily altered the relationship between leaf economic strategy and hydraulic function, whereas under e[CO₂], these effects were generally attenuated. Contrasting responses of soluble sugars and starch between leaves and roots (e.g., under a[CO₂], stress decreased leaf starch by 15.4% but increased root starch by 54.2% on average) further suggest that organ-level differences in carbon status may underlie shifts in leaf–root coordination. Overall, elevated CO₂ did not eliminate the effects of water and salt stress on cotton, but altered how stress influenced leaf–root coordination and associated functional linkages, highlighting the need to evaluate cotton responses from a whole-plant perspective.