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.