Foliar L-arginine and seaweed extract mitigate salinity stress in Calendula officinalis L. by enhancing physiological and biochemical performance

BACKGROUND: Soil salinity poses a major constraint to the productivity of salt-sensitive species such as Calendula officinalis L., a high-value ornamental and medicinal plant. A greenhouse study was conducted entirely under severe salinity stress (soil EC = 8.64 dS m⁻¹) and investigated the individual and combined effects of foliar-applied L-arginine (L-Arg: 0, 150, 300 mg L⁻¹) and seaweed extract (SW: 0, 1, 2 g L⁻¹) on C. officinalis grown under severe salinity (soil EC 8.64 dS m⁻¹) using a 3 × 3 factorial completely randomized design. RESULTS: The co-application of 300 mg L⁻¹ L-Arg and 2 g L⁻¹ SW yielded the strongest responses, including increases in flower number by 87.5%, flower diameter by 67.0%, flower longevity by 52.0%, relative water content by 14.7%, chlorophyll a by 33.3%, chlorophyll b by 65.7%, and carotenoids by 73.9%, alongside decreases in electrolyte leakage by 34.6% and intercellular CO₂ concentration by 64.7% relative to the saline control. Main effects showed maximum increases in net photosynthetic rate (220.6% with 300 mg L⁻¹ L-Arg and 27.8% with 2 g L⁻¹ SW) and water use efficiency (279.2% with 300 mg L⁻¹ L-Arg and 71.6% with 2 g L⁻¹ SW). Catalase activity and total soluble protein rose by 98.7% and 54.9%, respectively, while root dry weight was maximized (88.2% increase) with 150 mg L⁻¹ L-Arg + 2 g L⁻¹ SW. CONCLUSION: The results suggest that foliar co-application of L-Arg and SW may interactively and complementarily mitigate salinity-induced osmotic, ionic, and oxidative stresses in C. officinalis under high saline conditions by enhancing biochemical defenses, photosynthetic performance, and morphological traits relative to the saline control, potentially offering a sustainable biostimulant strategy for cultivation in salt-affected regions.