Elevated CO2 improves antioxidant capacity, ion homeostasis, and polyamine metabolism in tomato seedlings under Ca(NO3)2-induced salt stress

With the expansion of greenhouse vegetable production, secondary soil salinization and CO2 deficits have emerged as major handicaps that seriously affect the yield and quality of tomatoes. To explore the effects of CO2 enrichment on Ca(NO3)2 that generates the major cations (Ca2+) and anions (NO3–) responsible for secondary salinization in protected vegetable production, the tomato cultivar 'Zhong za 9′ was exposed to 800 μmol/mol CO2 and 80 mmol/L Ca(NO3)2 for seven days, and their effects on the growth and physiological characteristics were examined. Enrichment with CO2 significantly promoted the net photosynthetic rate and biomass accumulation, alleviated the oxidative damage caused by salt stress, and improved the ion homeostasis under Ca(NO3)2 stress. CO2 enrichment also reduced the electrolyte leakage, malondialdehyde, superoxide anion, and hydrogen peroxide concentrations by increasing the activity of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT). Elevated CO2 also differentially modulated the free amino acid concentrations and endogenous polyamine inter-conversion under salt stress, which improved the tolerance of plants to Ca(NO3)2 stress. Our results suggest that CO2 enrichment can effectively alleviate the damage of tomato seedlings caused by Ca(NO3)2 stress, and it can be considered as a feasible strategy to manage secondary salinization in protected vegetable production.