Elevated CO₂-conferred sodium homeostasis is linked to abscisic acid-mediated stomatal closure and signalling pathway in tomato

Soil salinity hampers plant performance. Elevated atmospheric CO₂ (e[CO₂]) could alleviate the detrimental effect of salinity on plants but whether abscisic acid (ABA) is involved in this process is unclear. To address this issue, three tomato (Solanum lycopersicum) genotypes with varying endogenous ABA concentrations (wild-type AC, ABA-deficient mutant flacca and ABA-overproduction line SP5) were grown in pots under ambient (400 μmol · mol^–1) or elevated (800 μmol · mol^–1) CO₂ with or without the addition of 100 mmol · L⁻¹sodium chloride (NaCl). The results showed that e[CO₂] favored ion homeostasis by decreasing root-to-shoot delivery of Na⁺, which was mainly attributed to lowered transpiration rate rather than altered xylem-sap Na⁺ concentration. In AC and SP5, the low transpiration rate of e[CO₂]-plants under salinity was accompanied by enhanced endogenous ABA levels, which might play a role in upregulating the abundance of specific transcripts related to Na⁺ homeostasis (i.e., SALT OVERLY SENSITIVE) under salt stress. In flacca, e[CO₂]-induced Na⁺ homeostasis was abolished, which could be ascribed to the low and unaltered ABA levels, albeit the ethylene biosynthesis was enhanced in flacca under salt stress, indicating an antagonistic relationship between ABA and ethylene. Furthermore, e[CO₂] inhibited ethylene biosynthesis under salt stress in all three genotypes. The results enrich our comprehension of the fundamental processes of e[CO₂]-conferred salt tolerance in tomato.