S-ABA Enhances Rice Salt Tolerance by Regulating Na⁺/K⁺ Balance and Hormone Homeostasis

In order to explore the regulating role and the physiological and biochemical mechanisms of trans-abscisic acid (hereinafter referred as S-ABA) in the process of rice growth and development under salt stress, we took Chaoyou 1000 and Yuxiangyouzhan as materials and set up three salt concentration treatments, CK0 (Control treatment), N1 (50 mmol L^-1 NaCl), and N2 (100 mmol L^-1 NaCl), in potted trials; we aimed to study the mechanism of rice's response to salt stress from the perspective of agricultural traits and physiological biochemicals and to improve rice's resistance to salt stress through exogenously applying the regulating technology of S-ABA. The following results were obtained: Under salt stress, the growth of rice was significantly suppressed compared to CK0, exhibiting notable increases in agricultural indicators, photosynthesis efficiency, and the NA⁺ content of leaves. However, we noted a significant decrease in the K⁺ content in the leaves, alongside a prominent increase in NA⁺/K⁺ and a big increase in MDA (malondialdehyde), H₂O₂ (hydrogen peroxide), and O₂^- (superoxide anion). This caused the cytomembrane permeability to deteriorate. By applying S-ABA under salt stress (in comparison with salt treatment), we promoted improvements in agronomic traits, enhanced photosynthesis, reduced the accumulation of NA⁺ in leaves, increased the K⁺ content and the activity of antioxidant enzymes, and reduced the active oxygen content, resulting in a sharp decrease in the impact of salt stress on rice's development. The application of S-ABA decreased the endogenous ABA (abscisic acid) content under salt stress treatment but increased the endogenous GA (gibberellin) and IAA (indole acetic acid) contents and maintained the hormonal homeostasis in rice plants. To summarize, salt stress causes damage to rice growth, and the exogenous application of S-ABA can activate the pouring system mechanism of rice, suppress the outbreak of active oxygen, and regulate NA⁺/K⁺ balance and hormone homeostasis in the blades, thus relieving the salt stress.