Effect of green synthesized ZnO nanoparticles for growth promotion in Pennisetum glaucum (L.) R. Br. validated through physio-biochemical and molecular analysis

Zinc is essential for plant metabolism and physiological function. Zinc activates a variety of plant enzyme functions includes protein synthesis, auxin production & regulation, cellular membrane integrity preservation, glucose metabolism, and pollen formation. In this study, zinc oxide nanoparticles (ZnO NPs) at varying concentrations (0, 50, 100, 150, 200, and 500 mg kg⁻¹) were used to observe the effects on Pennisetum glaucum over a 30-day period. The crop plant P. glaucum has been taken as an experimental plant due to its significant place in the food industry. Nanoparticles are green synthesized from Azadirachta indica and investigated using Ultraviolet–visible spectroscopy (UV–VIS), Fourier-transform infrared spectroscopy (FTIR), Scanning electron microscope (SEM), Dynamic light scattering (DLS), and Zeta potential. The synthesized particles had an optimum size of 76.8 nm. Upon treatment of ZnO nanoparticles alpha-amylase, shoot and root length, biomass and photosynthetic pigments of plants increased slightly at 100 mg kg⁻¹ and 150 mg kg⁻¹.The total chlorophyll content of 150 mg kg⁻¹ ZnO NPs treated plants to that of control is around 45.33 % increased after 10 days and 47.75 % increased after 30 days. Upon ZnO NPs treatment, plants also showed a significant increase in antioxidant activity in P. glaucum at 100 mg kg⁻¹ and 150 mg kg⁻¹, whereas, the decline in the percentage at 200 and 500 mg kg⁻¹. Zinc oxide nanoparticles treatment indicated that at higher concentrations, nanoparticles are showing negative effects on free radical scavenging activity. At concentrations of 100 mg kg⁻¹ and 150 mg kg⁻¹ treatment, showed the best free radical scavenging activity and quenched DPPH free radicals by 31.39 % and 42.61 % after 10 days, 48 % and 55.65 % after 20 days and 54.69 % and 55.78 % after 30 days respectively. At the molecular level the expression of Transcription Factors (TF) genes; ERF2 and ERF3 was higher, their activity increased by 7 and 5 folds respectively. The results indicate that 100 mg kg⁻¹ and 150 mg kg⁻¹ application of zinc oxide nanoparticle on pearl millet can enhance crop productivity and at the same time being less harmful for the environment. This study highlights how nanotechnology can be implemented effectively in agriculture at an optimum concentration.