Glycine Betaine and Salicylic Acid synergize to boost tomato saline-alkali tolerance via root exudate remodeling and antioxidant enhancement

Saline-alkali stress is a major abiotic constraint that severely impairs tomato ( Solanum lycopersicum L.) growth, development, and productivity worldwide. Exogenous glycine betaine (GB) and salicylic acid (SA) individually alleviate salt stress. However, their combined effects under composite saline-alkali conditions, particularly on root exudate metabolic reprogramming, remain poorly understood. This study investigated tomato seedling responses under five treatments: non-stress control (CK), saline-alkali stress alone (SS), SS + 10 mM GB (SGB), SS + 1 mM SA (SSA), and SS + 10 mM GB + 1 mM SA (SGA). Saline-alkali stress reduced dry weight by 37.5 ± 1.4% and increased the leaf Na⁺/K⁺ ratio approximately 30-fold compared to CK. Among ameliorative treatments, SGA showed the greatest improvements. Dry weight increased by 34.7 ± 1.1% relative to SS. MDA decreased by 29.1 ± 0.4%, and root activity was enhanced by 285.8 ± 2.9% relative to SS. Two-way ANOVA confirmed significant GB × SA non-additive interactions for root Na⁺/K⁺ ratio, POD activity, ethylene suppression, and ABA attenuation (all p < 0.001). The antioxidant defense system underwent a mechanistic transition from SOD/CAT-dominated to APX/POD-dominated scavenging pathways. SGA established a distinctive high-JA (+115.7 ± 6.3%) and low-ethylene (-48.2 ± 4.7%) hormonal signature. Non-targeted metabolomics identified 208 SGA-unique differentially accumulated metabolites (DAMs). Key metabolites in root exudates showed distinct accumulation patterns. Upregulated metabolites included jasmonate (log 2 FC = 1.1, VIP = 1.02, p < 0.01), N-acetyl- l -cysteine (NAC, log 2 FC = 1.19, VIP = 1.14, p < 0.001), and scopoletin (log 2 FC = 1.08, VIP = 1.08, p < 0.001). Sphingosine-1-phosphate (S1P, log 2 FC = -1.42, VIP = 1.23, p < 0.001) was significantly downregulated. Metabolite sets unique to SGA were enriched in branched-chain amino acid biosynthesis, arginine biosynthesis, and butanoate metabolism. Collectively, these findings establish an integrated physiological-metabolomic framework. They also demonstrate that co-application of GB and SA is a promising biostimulant strategy for enhancing tomato resilience and productivity in saline-alkali-affected agroecosystems.