Elevated atmospheric CO₂ concentration mitigates salt damages to safflower: Evidence from physiological and biochemical examinations

The physiological and biochemical responses of salt-stressed safflower to elevated CO₂ remain inadequately known. This study investigated the interactive effects of high CO₂ concentration (700 ± 50 vs. 400 ± 50 μmol mol^-1) and salinity stress levels (0.4, 6, and 12 dS m^-1, NaCl) on growth and physiological properties of four safflower (Carthamus tinctorius L.) genotypes, under open chamber conditions. Results showed that the effects of CO₂ on biomass of shoot and grains depend on salt stress and plant genotype. Elevated CO₂ conditions increased shoot dry weight under moderate salinity stress and decreased it under severe stress. The increased CO₂ concentration also increased the safflower genotypes' relative water content and their K⁺/Na ⁺ concentrations. Also enriched CO₂ increased total carotenoid levels in safflower genotypes and improved membrane stability index by reducing H₂O₂ levels. In addition, increased CO₂ level led to an increase in seed oil content, under both saline and non-saline conditions. This effect was particularly pronounced under severe saline conditions. Under conditions of high CO₂ and salinity, the Koseh genotype exhibited higher grain weight and seed oil content than other genotypes. This advantage is due to the higher relative water content, maximum quantum efficiency of photosystem II (Fv/Fm), and K⁺/Na⁺, as well as the lower Na⁺ and H₂O₂ concentrations. Results indicate that the high CO₂ level mitigated the destructive effect of salinity on safflower growth by reducing Na ⁺ uptake and increasing the Fv/Fm, total soluble carbohydrates, and membrane stability index. This finding can be used in safflower breeding programs to develop cultivars that can thrive in arid regions with changing climatic conditions.