Response of amino acids, phenolic acids, organic acids, and mineral elements to fulvic acid in spinach (Spinacia oleracea L.) under nitrate stress

Fulvic acid (FA) acid has many physiological activities, but the specific metabolic responses and changes in mineral element contents of spinach by FA in response to nitrate stress are unknown. Herein, we used liquid chromatography-mass spectrometry (LC-MS) and wet digestion using H₂SO₄-H₂O₂ to analyze the metabolic response and changes in the mineral element content of spinach to nitrate stress (150 mM NO₃^-) after FA (0.15%) foliar spray application. After 2 days of the stress treatment, FA was sprayed thrice (once every 7 days), sampled 4 days after the last spraying, and metabolites and mineral element contents were measured. FA treatment significantly increased organic acid contents (tartaric acid, malic acid, citric acid, and ascorbic acid) and amino acid contents (threonine, asparagine, valine, tyrosine, alanine, glutamate, serine, histidine, arginine, and glutamine) under nitrate stress. FA application also significantly improved mineral element contents (P, Na, Fe, and Zn) under nitrate stress. This study provides comprehensive insights into metabolite accumulation of metabolites and the improvement of nutritional quality in spinach through FA application under nitrate stress. Further research should focus on elucidating additional underlying molecular mechanisms of these metabolic responses for better utilization of this natural compound in agriculture.