Synergistic Responses of Forage Pea in the Germination Stage to Saline–Alkali and Drought Stress at Phenotypic, Physiological, and Non-Targeted Metabolomic Levels

(1) Background: This study used Qingjian No. 1 forage pea (Pisum sativum L.) as a plant material to study its metabolic mechanisms in response to different stresses, given that saline–alkali stress and drought stress often occur simultaneously in natural environments and severely affect the growth and yield of forage pea, while the regulatory network underlying the adaptation of forage pea to combined stress remains poorly elucidated. (2) Methods: The metabolic mechanisms of forage pea in response to different stresses were elucidated by integrating phenotypic, physiological, and metabolomic analyses. (3) Results: The results show that compared to the control, all stress treatments significantly inhibited seed germination and seedling growth, with the combined saline–alkali and drought stress exhibiting the strongest inhibitory effect. In terms of physiological and biochemical responses, peroxidase (POD) activity increased with the complexity of the stress, with the highest POD activity observed under combined saline–alkali and drought stress, showing a 61.71% increase compared to the control (p < 0.05). Non-targeted metabolomic analysis revealed that isoflavone biosynthesis, nucleotide metabolism, and cutin–suberin–wax biosynthesis are the core responsive pathways. Correlation analysis revealed that isocorydine and phosphatidylinositol phosphate showed strong positive correlations with the vigor index, main root length, and superoxide dismutase (SOD) activity, and glycerophospholipid metabolites were positively correlated with ferric ion-reducing antioxidant capacity. (4) This study deepens understanding of the stress resistance mechanisms in forage peas and provides a theoretical basis for stress-resistant forage pea breeding.