Synergistic effects of fulvic acid and salicylic acid on mitigating salinity stress in Calendula officinalis: Physiological and biochemical responses

Soil salinity significantly constrains the productivity of salt-sensitive crops like Calendula officinalis L., a valuable medicinal and ornamental species. Although fulvic acid (FA) and salicylic acid (SA) are recognized biostimulants for mitigating salinity stress, their combined efficacy remains largely unexplored. A factorial completely randomized design was used to assess the individual and synergistic effects of foliar-applied FA (0, 1, 2, 3 g L⁻¹) and SA (0, 100, 200 mg L⁻¹) on C. officinalis under salinity stress (EC 8.64 dS m⁻¹) in a controlled greenhouse environment. The FA×SA interaction revealed significant synergistic effects across key parameters. The combination of FA (3 g L⁻¹) and SA (200 mg L⁻¹) markedly enhanced plant resilience, significantly increasing flower production, biomass, and photosynthetic efficiency while reducing water loss. It also elevated chlorophyll and carotenoid contents, bolstered antioxidant defenses via increased peroxidase activity and proline accumulation, and improved nutrient uptake and protein synthesis. This synergistic approach provides a sustainable strategy to enhance C. officinalis resilience and productivity, offering a promising solution for cultivating salt-sensitive crops in saline environments.