Foliar Application of Iron Oxide-Graphene Oxide Nanocomposite and Magnetic Iron Nanoparticles Mitigates the Growth and Physiological Attributes of Calendula officinalis L. Exposed to Salinity Stress

Calendula officinalis L. is a medicinal and ornamental plant of the Compositae family with great importance in pharmaceutical industries. To investigate the effect of sodium chloride salinity stress (0, 75, and 150 mM) and foliar spraying of distilled water (control), and 100 mg/L iron oxide—graphene oxide nanocomposite (Fe3O4-GO NC) and magnetic Fe3O4 NP on the growth and some physiological traits of Calendula officinalis; a factorial experiment was conducted based on a completely randomized design. The results showed that the treatment without salinity stress increased the plant’s dry weight (76.6 g), flower diameter (4.9 cm), chlorophyll index (37.3), potassium (10.4 g/kg), zinc (7.2 mg/kg), and manganese (8.2 mg/kg) content, and potassium to sodium ratio. No saline conditions and 75 mM salinity stress enhanced plants’ calcium content, total phenolics content, and flower number. Exposure to the 75 and 150 mM salinity stress raised proline, Na, and malondialdehyde content and catalase activity. Foliar spraying with both compounds enhanced plant dry weight, flower number, chlorophyll index, catalase activity, total phenolics, proline, manganese, potassium content, and K/Na ratio in plants. Foliar treatments under non-saline conditions enhanced the nitrogen and phosphorus content of plants. Anthocyanins and flavonoids content was increased under no salinity, 75 mM NaCl salinity + foliar sprays. 150 mM Salinity without foliar spray elevated H2O2 content. Overall, the results revealed that salinity harmed the growth potential of plants. Otherwise, the foliar spray with both compounds promisingly improved growth traits and physiological responses of plants under stress conditions.