Exogenous Nitric Oxide Donor Mitigates Drought Stress in Pot Marigold (Calendula officinalis) Via Enhanced Osmoregulation and Oxidative Stress Management

Abstract Elucidating the physiological and biochemical responses of ornamental plants to drought stress and nitric oxide-mediated amelioration strategies is essential for developing sustainable production systems under climate change scenarios. However, comprehensive assessments of sodium nitroprusside (SNP) effects on morpho-physiological traits, oxidative stress responses, and metabolic pathways in drought-stressed ornamentals remain limited. We evaluated the effects of foliar SNP applications at 0, 50, 100, and 150 µM on Calendula officinalis L. grown under three irrigation regimes: 100%, 70%, and 40% of field capacity (FC), followed by assessment of morphological parameters, water relations, photosynthetic pigments, oxidative stress markers, antioxidant enzyme activities, osmoprotectant accumulation, and GABA metabolism. Stress treatment and SNP concentration significantly influenced all measured parameters ( p ≤ 0.01). Particularly, 100 µM SNP consistently demonstrated superior ameliorative effects, maintaining higher plant height, shoot biomass, leaf area, and flower production under both well-watered and drought conditions. Under severe drought (40% FC), 150 µM SNP reduced RWC decline from 24.6% to 11.2% and effectively mitigated reductions in total chlorophyll (63.1%) and carotenoids (55.3%) compared to untreated controls. Moreover, SNP enhanced proline accumulation up to 3-fold and substantially reduced oxidative stress markers (H₂O₂, MDA, and EL) by up to 50%. Although drought stress upregulated antioxidant enzymes (CAT, APX, POD, and SOD) by 2.2–4.3 times, SNP application moderated these elevations, suggesting reduced oxidative burden. GABA metabolism also responded markedly, with SNP enhancing glutamate decarboxylase (GAD) activity and GABA content to 0.68 U mg⁻¹ protein and 2.47 µmol g⁻¹ FW, respectively, while downregulating GABA transaminase (GABA-T) activity to 0.13 U mg⁻¹ protein. Principal component analysis revealed that 96% of total trait variance was explained by the first two components, clearly separating treatments by stress level and SNP concentration. We conclude that SNP significantly enhances drought tolerance in pot marigold through coordinated modulation of water relations, antioxidant systems, and GABA shunt pathway activation. The consistent superiority of specific SNP concentrations across multiple functional traits provides insights for optimizing foliar bio-stimulant applications, while the quantitative response patterns identified offer valuable parameters for developing stress-resilient ornamental production systems under changing climatic conditions.