Titanium dioxide nanoparticles potentially regulate the mechanism(s) for photosynthetic attributes, genotoxicity, antioxidants defense machinery, and phytochelatins synthesis in relation to hexavalent chromium toxicity in Helianthus annuus L.

Chromium (VI) is one of the hazardous heavy metal, heavily discharged into the soil and severely hampers the plants yield. The TiO2 NPs was selected due to its potential to alleviate the heavy metals toxicity. This manuscript unravels the mechanisms for Cr(VI) induced toxicity and how foliar application of TiO2 NPS potentially ameliorate the toxicity by regulating the photosynthetic attributes, DNA damage, antioxidants defense ma-chinery, and phytochelatins synthesis in Helianthus annuus L. Plants were exposed to Cr(VI) concentrations [0, 15, 30, and 60 mg Cr(VI) kg-1 of soil], and TiO2 NPS (15 mg L-1, 25 nm size) were foliar sprayed thrice to the plants at three days interval. The maximum accumulation of total chromium was recorded in root (12.53 mu g g- 1 DW) followed by shoot (5.67 mu g g- 1 DW) at 60 mg Cr(VI) treatment. The presence and localization of TiO2 NPs inside the plant leaf cells were confirmed by TEM-EDS analysis. The results revealed that Cr(VI) exposure had a dose-dependent inhibitory effects on photosynthetic attributes, structure of guard and epidermal cells, photo-synthetic pigments; inducing impacts on H2O2 and MDA productions, DNA damage, AsA-GSH cycle, and most importantly on PC2, and PC3 synthesis which is rarely reported. However, TiO2 NPs exposure minimized Cr(VI) induced toxicity through reduction of total chromium accumulation, H2O2 and MDA productions, thereby reducing DNA damage reported first time under combined treatment of Cr(VI)+ TiO2 NPs as evidenced through comet assay. It also positively regulate the photosynthetic pigments, AsA-GSH cycle, and modulates PC2 and PC3 synthesis which have crucial impacts on ROS quenching and Cr(VI) detoxification, respectively, and in turn, minimizes Cr(VI) toxicity in H. annuus L. Besides, this study strengthens the less acknowledged report that Cr(VI) is an inducer of PCs synthesis and also confirms that TiO2 NPs potentially counteract Cr(VI) toxicity.