Biostimulant effects of titanium dioxide nanoparticles on germination and initial growth of tomato: evidence of hormesis.

Background: Titanium (Ti) is a beneficial element considered an inorganic biostimulant that may induce hormesis in plants. Supplied as titanium oxide nanoparticles (nTiO2) at appropriate concentrations, it may enhance seed germination, plant vigor, photosynthesis, and abiotic stress tolerance, while increasing nutrient uptake, crop yield and nutritional value. Nonetheless, applied at high concentrations, nTiO2 may trigger detrimental effects in plants. Methods: Tomato (Solanum lycopersicum L.) cv. Rio Grande seeds were imbibed in solutions containing 0.0, 52.2, 104.4, 156.6, or 208.8 µM nTiO2. The germination experiment lasted 16 days under controlled conditions in the laboratory (10 h natural light, 21 °C mean temperature, at saturated atmospheric conditions). During the experiment we estimated seed weight increase after imbibition; total germination percentage (TGP); germination speed coefficient (GSC); and the vigor indexes I and II (VI I and II). Seedlings were grown for 30 days after sowing and at the end we measured lengths of stems and roots, number of roots and leaves, moisture content, and fresh and dry biomass weight. In addition, the potential hormetic effect of Ti on length of roots and stems was estimated. Results: Applications of 156.6 and 208.8 µM nTiO2 significantly increased vigor index I, root length and total moisture content in leaves, while applying 208.8 µM nTiO2 significantly increased fresh biomass weight of roots. The hormetic analysis revealed that the application of 156.6 µM nTiO2 stimulated the length of roots and stems, with different dose-response curves. Conclusion: The application of nTiO2 to tomato seeds improved some germination and plant growth parameters during the initial growth stage, demonstrating its biostimulant effects.