Nano-Hydroxyapatite and ZnO-NPs Mitigate Pb Stress in Maize

Abstract

Heavy metals (HMs) stress, particularly lead (Pb) stress, is one of the most hazardous environmental stresses that can negatively affect plants’ growth, yield, and quality. Therefore, the effects of zinc oxide nanoparticles (ZnO-NPs; 50 mg L−1), nano-hydroxyapatite (HP-NPs; 50 mg kg−1), and their combination on growth, physiological, and yield traits of maize grown in soil contaminated with Pb (i.e., 100 mg kg−1) were investigated. The results showed that Pb stress significantly reduced plant leaf area by 50.9% at 40 days after sowing (DAS), 55.5% at 70 DAS, and 54.2% at 100 DAS in comparison to the unstressed plants (control). However, the combined application of ZnO-NPs (50 mg L−1) + HP-NPs (50 mg kg−1) reduced the adverse effects of Pb on plant growth in terms of increasing leaf area by 117.6% in plants grown in Pb-contaminated soil (100 mg kg−1). Similarly, the combined application of ZnO-NPs + HP-NPs resulted in increments in the total chlorophyll content by 47.1%, photosynthesis rate by 255.1%, and stomatal conductance by 380% in comparison to that obtained from maize stressed with Pb. On the other hand, antioxidants such as sodium dismutase (SOD; 87.1%), peroxidase (POX; 90.8%), and catalase (CAT; 146%), and proline content (116%) were significantly increased as a result of Pb stress compared to unstressed plants. Moreover, N, P, K, and Zn contents in the whole plant grown under Pb stress were decreased by 38.7%, 69.9%, 46.8%, and 82.1%, respectively, compared to those obtained from the control. Whereas the combined treatment of ZnO-NPs (50 mg L−1) + HP-NPs (50 mg kg−1) resulted in increased uptake of plant nutrients and, consequently, the highest values of ear weight, grain yield, and harvest index were obtained. Furthermore, the combined application of HP-NPs + ZnO-NPs in contaminated soil reduced Pb uptake in plant biomass by 77.6% and grains by 90.21% in plants exposed to Pb stress. In conclusion, the combined application of ZnO-NPs and HP-NPs significantly improved growth, physiological traits, antioxidants, and yield as well as elemental uptake of maize grown under Pb stress.