Elevated CO₂ mitigates NaCl stress associated with physiological and anatomical adaptations in maize (Zea mays L.)

Atmospheric CO₂ concentrations are rising and projected to reach ∼800 μmol mol^-1 by 2100, while soil salinity is expanding globally, yet their combined impact on crops remains unclear. In particular, the interactive effects of elevated CO₂ and salinity on maize (Zea mays L.) physiology and development are poorly understood. The physiological and anatomical responses of maize to elevated CO₂ (800 μmol mol^-1) and NaCl stress (0-150 mmol L^-1) were investigated through a controlled factorial experiment. Results showed that elevated CO₂ increased net photosynthetic rate, water-use efficiency, and biomass accumulation, partially mitigating salt-induced reductions in growth and photosynthesis under moderate NaCl stress, while mitigating salt-induced growth inhibition through three key mechanisms: (1) reinforcement of antioxidant defenses and improve water use efficiency by upregulating superoxide dismutase and peroxidase, (2) osmotic adjustment via leaf non-structural carbon accumulation especially under moderate salinity, and (3) high salinity caused structural deteriorations such as thinner leaves, lower stomatal density, and reduced vascular bundle size, while elevated CO₂ counteracted salinity-induced structural degradation, maintaining leaf thickness and vascular bundle integrity while modulating stomatal patterning and opening. However, growth and physiological function were still markedly hindered under severe salinity. Our multi-trait analysis demonstrates that rising CO₂ may partially compensate for salinity impacts in maize, providing critical insights for predicting C₄ crop stress resistance under future climate scenarios.