Titanium Dioxide Nanoparticles Require K and Hydrogen Sulfide to Regulate Nitrogen and Carbohydrate Metabolism During Adaptive Response to Drought and Nickel Stress in Cucumber

Crop plants face severe yield losses worldwide owing to their exposure to multiple abiotic stresses. The study described here, was conducted to comprehend the response of cucumber seedlings to drought (induced by 15% w/v polyethylene glycol 8000; PEG) and nickel (Ni) stress in presence or absence of titanium dioxide nanoparticle (nTiO). In addition, it was also investigated how nitrogen (N) and carbohydrate metabolism, as well as the defense system, are affected by endogenous potassium (K) and hydrogen sulfide (HS). Cucumber seedlings were subjected to Ni stress and drought, which led to oxidative stress and triggered the defense system. Under the stress, N and carbohydrate metabolism were differentially affected. Supplementation of the stressed seedlings with nTiO (15 mg L) enhanced the activity of antioxidant enzymes, ascorbate-glutathione (AsA-GSH) system and elevated N and carbohydrates metabolism. Application of nTiO also enhanced the accumulation of phytochelatins and activity of the enzymes of glyoxalase system that provided additional protection against the metal and toxic methylglyoxal. Osmotic stress brought on by PEG and Ni, was countered by the increase of proline and carbohydrates levels, which helped the seedlings keep their optimal level of hydration. Application nTiO improved the biosynthesis of HS and K retention through regulating Cys biosynthesis and H-ATPase activity, respectively. Observed outcomes lead to the conclusion that nTiO maintains redox homeostasis, and normal functioning of N and carbohydrates metabolism that resulted in the protection of cucumber seedlings against drought and Ni stress. Use of 20 mM tetraethylammonium chloride (K- channel blocker), 500 μM sodium orthovanadate (PM H-ATPase inhibitor), and 1 mM hypotaurine (HS scavenger) demonstrate that endogenous K and HS were crucial for the nTiO-induced modulation of plants' adaptive responses to the imposed stress.