Exogenous Melatonin Confers Salt-Alkali Tolerance in Fraxinus mandshurica by Orchestrating Resource Allocation and Activating Phenylpropanoid-Mediated Defenses

The physiological mechanism of melatonin in alleviating combined saline-alkali stress in <i>Fraxinus mandshurica</i> remains unclear. This study aimed to determine the efficacy of exogenous melatonin in enhancing salt tolerance and elucidate the underlying mechanisms through integrated physiological and multi-omics analyses. Seedlings were subjected to 400 mmol L^-1 saline-alkali stress and treated with foliar melatonin. We quantified key growth indicators (height, diameter, dry biomass) and measured the activities of antioxidant enzymes (SOD, POD). Melatonin significantly alleviated growth inhibition, increasing biomass and height by 29% and 13%, respectively, while enhancing net photosynthetic rate and antioxidant capacity. To uncover the systemic regulation, conjoint analysis of transcriptome (RNA-seq) and metabolome data was performed. This integrated approach revealed that melatonin specifically activated common KEGG pathways pivotal for stress adaptation, including plant hormone signal transduction, phenylpropanoid biosynthesis, and starch and sucrose metabolism, with coordinated upregulation of associated genes and metabolites. Collectively, our integrated data demonstrate that melatonin enhances <i>Fraxinus</i> tolerance by synergistically improving photosynthesis and antioxidant defense, underpinned by a reconfigured molecular network. This study provides a theoretical basis for using melatonin as an eco-friendly biostimulant to improve woody plant resilience in saline-alkali soils.