Dose-Dependent Impacts of Nano-Sized Ceria (CeO2) on Seed Germination, Early Growth and Physiological Parameters of Marigold Seedling

Marigold is widely used as an ornamental-medicinal plant. Interaction between nanoparticles (NPs) and biological systems is one of the most promising areas of research in modern nanoscience and technology. Researchers have reported the uptake of cerium oxide or ceria (CeO2) NPs by plants. The aim of this investigate was to determine the impacts of nanoceria on seed germination, growth and biochemical characteristics of 9-day-old seedling of marigold (Calendula officinalis L.). Seeds were germinated in Petri dishes containing eight various dosages of nanoceria (0, 0.05, 0.1, 0.2, 0.4, 0.8, 1.6, and 3.2 g/L). After 9 days, early growth and biochemical parameters were measured. Results showed that seed germination, fresh and dry weights of seedling, and lengths of radicle, plumule, and seedling stimulated at 0.05 and/or 0.1 g/L of nanoceria but retarded at higher dosages (after 0.2 or 0.4 g/L) of NPs. The contents of H2O2, malondialdehyde (MDA) and lipoxygenase (LOX) activity incremented after 0.2 g/L of nanoceria. The activities of antioxidant enzymes and protein content were incremented at higher dosages of nanoceria. Also, the activity of phenylalanine ammonialyase (PAL), phenol content, and antioxidant capacity stimulated at 0.8 to 3.2 g/L of nanoceria. The proline content improved at 0.2-3.2 g/L of nanoceria. Altogether, the results confirmed the inducive oxidative stress of nanoceria that was accompanied by plant defense system include antioxidant enzymes, phenolic compounds and compatible osmolytes such as proline. These results showed that nanoceria at the low dosages (0.05 and/or 0.1 g/L) caused a positive induction on marigold germination and seedling growth but by increasing in its dosage (more than 0.2 g/L), the results was reversed and showed toxicity that forced the plant to activate its defense systems.