Hormetic dose responses induced by nickel oxide nanoparticles (NiONPs) on growth, biochemical, and antioxidant defense systems of Dracocephalum kotschyi.

Abstract

The application of nickel oxide nanoparticles (NiONPs) in various fields leads to their release into soil and water and, consequently, interaction with plants. Unlike its bulk counterpart, the phytotoxic potential of NiONPs is relatively less studied, particularly in a hormesis framework. Hormesis is an interesting phenomenon characterized by low-dose stimulation and high-dose inhibition. Therefore, this study demonstrates the stimulatory and inhibitory effects of NiONPs on Dracocephalum kotschyi Boiss as a medicinal plant cultivated in a pot experiment carried out in a greenhouse for 3weeks. High bioaccumulation of nickel (Ni) in roots of treated plants relative to shoots indicates higher oxidative damage. NiONPs induced hormetic effects on photosynthetic pigments, as at low concentration of 50mg/L stimulated chlorophyll (2.8-46.7%), carotenoid (16%), and anthocyanin (5.9%) contents and at higher concentrations inhibited the content of these pigments. A hormetic response was observed in growth parameters, i.e., NiONPs induced shoot height (7.2%) and weight (33%) at 100mg/L, while inhibited shoot and root length (14.5-16.1% and 28.7-42.7%) and weight (46.8-48.1% and 37-40.6%), respectively, at 1000 and 2500mg/L. The treated plants declined the toxic effects and oxidative stress caused by NiONPs by activating non-enzymatic antioxidants (phenolic compounds and proline) and enzymatic antioxidants, i.e., increasing the levels of SOD, POD, CAT, and APX. Therefore, the present study investigated for the first time the different mechanisms and responses of D. kotschyi plants to NiONPs in a wide range of concentrations. The results suggest that NiONPs may act as an elicitor at lower concentrations in medicinal plants according to specific conditions. However, these NPs at higher concentrations induce oxidative stress and harmful effects on plants, so their use poses serious risks to human health and the environment.