Mechanistic Analysis of Physiological and Metabolic Responses in Non-Jointed Water Dropwort Under Phosphorus Stress

Background: Non-jointed water dropwort (Oenanthe javanica (Blume) DC.) is a widely cultivated aquatic vegetable with notable nutritional and pharmacological properties. Phosphorus (P) is a key nutrient affecting plant growth, photosynthesis, and metabolic balance, yet its role in water dropwort remains understudied. Methods: This study investigated the physiological and metabolic responses of non-jointed water dropwort under P-deficiency treatment (0 mg·L−1) and increasing P supply levels (5, 10, and 30 mg·L−1). Results: Moderate P supply (10 mg·L−1) significantly promoted plant growth, enhanced photosynthetic efficiency, and increased antioxidant enzyme activity, increasing by 55.9%, 20.2%, and 118%, respectively, compared with the P-deficiency treatment. High P levels (30 mg·L−1) inhibited growth and induced oxidative stress. Untargeted metabolomic analysis was conducted on root samples from CK (0 mg·L−1) and HP (30 mg·L−1) groups using UHPLC-MS. A total of 1274 metabolites were identified, with flavonoids, phenylpropanoids, fatty acid and conjugates being predominant. A total of 842 differential metabolites were screened under HP stress, with flavonoids (e.g., narcissin) showing the most significant upregulation. KEGG enrichment revealed key pathways including biosynthesis of amino acids, ABC transporters, and aminoacyl-tRNA biosynthesis, indicating metabolic reprogramming under HP stress. Notably, flavonoid and terpenoid pathways were upregulated, while certain lipid metabolism pathways, including fatty acid conjugates and phenylpropanoids, were downregulated. These findings suggest that non-jointed water dropwort adapts to high P stress by activating defense-related secondary metabolism and adjusting carbon–nitrogen allocation. Conclusions: This study provides a theoretical basis for P management and stress-resistant cultivar selection in non-jointed water dropwort.