Effect of foliar application of cerium oxide nanoparticles on growth, photosynthetic pigments, electrolyte leakage, compatible osmolytes and antioxidant enzymes activities of Calendula officinalis L.

The nanoparticles influence on plants' biological processes has not been clearly understood. Increase of nanoparticles in the environment during advent of industrial technologies can be a significant threat to ecosystems, and their interaction with plants is very important. The aim of this study was to investigate the effects of cerium oxide nanoparticles (CeO2 NPs) on biochemical and physiological parameters of marigold (Calendula officinalis L.). Seeds were planted and after 2 weeks, seedlings were treated with different concentrations of CeO2 NPs (50, 100, 200, 400, 800, 1600 and 3200 mg L-1) with foliar spray method. The hydrogen peroxide content and membrane electrolyte leakage were respectively increased after 100 and 200 mg L-1 CeO2 NPs treatment. Except for glutathione reductase (GR), the activities of antioxidant enzymes e.g. catalase (CAT), superoxide dismutase (SOD), ascorbate peroxidase (APOX), guaiacol peroxidase (GPOX), glutathione S-transferase (GST) and polyphenol oxidase (PPO) were induced by high concentrations of CeO2 NPs. Application of CeO2 NPs increased the contents of proline and soluble sugars after 200 mg L-1 and glycine betaine (GB) after 400 mg L-1. The growth parameters and chlorophyll contents were increased at low concentrations of CeO2 NPs (50 and 100 mg L-1) and decreased at higher ones. The results suggested that low concentrations of CeO2 NPs had a positive induction effect on C. officinalis but higher ones caused oxidative stress and toxicity responses. It seemed that induction of defense system including antioxidant enzymes activities and accumulation of osmolytes were not sufficient for reactive oxygen (ROS) detoxification.