Optimizing NPK fertilization rate and ratio improves growth, flowering, nutrient uptake, and stress tolerance in Nymphaea micrantha

Water lilies (Nymphaea spp.) are highly valued aquatic ornamental plants. Among them, Nymphaea micrantha is particularly prized for its intense floral fragrance, making it an important species for cut flowers and essential oil production. However, scientific and systematic guidance on precision fertilization during its flowering stage is lacking, which limits yield and quality improvement. This study aimed to determine the optimal compound fertilizer application rate and to elucidate the individual and interactive effects of nitrogen (N), phosphorus (P), and potassium (K) on the growth, flowering, nutrient uptake, and physiological stress responses of N. micrantha. Two independent container experiments were conducted. The first tested compound fertilizer (N-P₂O₅-K₂O = 15-5-25) rates at 0 (CK), 30, 100, and 200 g plant⁻¹. The second examined N, P, and K deficiencies and single-element supplies. Growth parameters, flowering traits, leaf physiological indicators (chlorophyll, malondialdehyde (MDA), proline, peroxidase (POD) activity, and hydrogen peroxide (H₂O₂)), and leaf N, P, and K contents were measured. The results showed that the 100 g plant⁻¹ balanced fertilizer treatment consistently outperformed all other rates, increasing biomass, maximum leaf width, leaf number, flower diameter, and flower count by 348.69%, 141.60%, 78.96%, 57.65%, and 84.86%, respectively, relative to the unfertilized control. Phosphorus (PD) and potassium (KD) deficiencies were less detrimental but still significantly reduced reproductive traits. Single-element N-only outperformed P-only and K-only but remained inferior to balanced FN. Physiological data revealed that balanced fertilization enhanced antioxidant enzyme (POD) activity and reduced oxidative damage, while nutrient stress, particularly N and K deficiency, elevated stress markers. Leaf nutrient analysis showed that fertilization also markedly increased leaf N, P, and K contents; the 100 g plant⁻¹ treatment raised these by 202.98%, 194.43%, and 327.77%, respectively, relative to controls, while deficiency or single‑element supplies failed to significantly enhance nutrient accumulation. We conclude that a balanced application of 100 g plant⁻¹ of compound fertilizer during flowering optimally promotes growth, floral traits, antioxidant defense, and nutrient homeostasis in N. micrantha , with adequate nitrogen supply as the foundation and sufficient potassium as the key to improving flower quality and stress resilience. This study provides a theoretical and practical framework for precision nutrient management in water lily cultivation, promoting high-yield and high-quality production.