Zinc oxide and titanium dioxide nanoparticles influence heat stress tolerance mediated by antioxidant defense system in wheat

In this study, the growth, antioxidant potential and membrane stability parameters in root and shoot of two wheat (Triticum aestivum L.) cultivars (Unnat PBW 343 and HD 296) treated with 1.5 ppm and 10 ppm zinc oxide (ZnO) and titanium dioxide (TiO2) nanoparticles under high temperature (32 degrees C) were studied in twelve day old seedlings. Shoot, root length and biomass activities of superoxide dismutase (SOD) and guaiacol peroxidase (GPX) increased more than catalase, with application of ZnO and TiO2 nanoparticles especially at 32 degrees C. Apparently, phenols and flavonols showed dramatic increase in parallel with higher free radical scavenging activities as measured by 2,2-diphenyl-1-picrylhydrazyl and ferric reducing antioxidant potential indicating stress tolerance by scavenging free radicals. Both ZnO and TiO2 nanoparticles had promoting effect at lower concentration (1.5 ppm) compared with higher concentration. Interestingly, TiO2 nanoparticles though improved seedling performance slightly better than ZnO nanoparticles; however, antioxidant defense response was greater with ZnO nanoparticles. Nonetheless, compared to the HD 2961 cultivar, the Unnat PBW 343 cultivar had lower malondialdehyde (MDA) and hydrogen peroxide (H2O2) contents along with higher upregulation of antioxidant defense system under heat stress conditions treated with nanoparticles owing to the better expression of antioxidants. Nanoparticles uptake was authenticized using scanning electron microscope-energy-dispersive X-ray spectroscopy. Results demonstrated different shapes and size of nanoparticles such as bunch of irregular particles, round shaped and small particles. Therefore, pretreatment by ZnO and TiO2 nanoparticles significantly improved resistance against heat stress through increase in more SOD and GPX activity, causing further lowering of H2O2 levels and thereby stabilizing membrane as evidenced by 1.5 times lower MDA content.