Divergent Biochemical Strategies and Organ-Specific Metabolic Adjustments in Spinach Mediated by Exogenous Amino Acids Under Salt Stress.

Understanding the physiological and biochemical responses of spinach to salt stress through amino acid supplementation is crucial for improving crop resilience under increasing soil salinity conditions. Salt stress represents one of the most severe abiotic constraints limiting vegetable crop productivity worldwide, yet comprehensive studies examining organ-specific metabolic reprogramming across entire plant systems remain limited. However, knowledge about how different amino acids mediate these responses through distinct metabolic pathways is limited. We investigated mineral nutrition, antioxidant defense systems, and secondary metabolite profiles of spinach supplemented with three amino acids (asparagine, phenylalanine, and tryptophan) under salt stress conditions. Amino acid type, salt treatment, and organ significantly influenced all measured parameters (p ≤ 0.0001). Asparagine consistently demonstrated comprehensive protective effects, excluding toxic ions while promoting calcium uptake, maintaining photosynthetic capacity, and dramatically enhancing flavonol biosynthesis (quercetin and rutin accumulation increased several-fold) under combined stress compared to other treatments. Phenylalanine excelled in ionic homeostasis restoration, achieving superior Na/K ratio reduction and enhanced phenylpropanoid pathway activation through elevated cinnamic acid biosynthesis. Tryptophan uniquely triggered exceptional divalent cation accumulation (6-10-fold increases in magnesium, calcium, and phosphorus) and maximally enhanced antioxidant enzyme activities, though with notable protein synthesis trade-offs. Organ-specific accumulation patterns revealed leaves as primary sites for photosynthetic pigments and phenolic compounds, roots as storage organs for specialized flavonoids and catechins, and petioles showing exceptional rutin accumulation. These findings demonstrate that amino acid selection fundamentally reshapes metabolic priorities in salt-stressed spinach through divergent yet complementary biochemical strategies. We conclude that amino acid selection significantly influences spinach resilience to salt stress through divergent metabolic reprogramming strategies, with each amino acid offering distinct advantages depending on cultivation priorities. Differential metabolic responses between amino acids provide insights for precision agriculture applications, while the quantitative biochemical patterns identified offer valuable parameters for optimizing amino acid supplementation strategies under saline conditions.