Abstract <p>Current research shows that pre-sowing seed treatment with metal oxide nanoparticles can have both stimulating and inhibitory effects on germination and plant development, depending on their type, concentration, and shape. However, the mechanisms and long-term effects of nanoparticles remain poorly understood, especially at low concentrations. Although most studies use relatively high doses, there is growing evidence that stable colloidal solutions of nanoparticles can be effective at much lower levels. In this study, the effects of colloidal TiO₂, Fe₃O₄, and NiO nanoparticles produced via a two-phase electric arc method were tested on barley seedlings (<i>Hordeum vulgare</i> L. cv. ‘Vakula’). After sterilization, seeds were soaked for 12&#xa0;h in the dark with aeration, then germinated under light for 7–10 days. Fe₃O₄ and NiO at 0.035 and 0.0035 ppm respectively stimulated early germination (days 3–4), but the effects were transient and had little impact on seedling morphology. The most pronounced and stable results were observed with TiO₂ at 0.35 ppm, which enhanced root development by 30%, increased water content, and intensified leaf transpiration. No toxic effects were detected for any treatment, as confirmed by pigment, phenol (GAE), and antioxidant (TEAC) analyses. Our findings highlight the potential of low-dose TiO₂ colloidal nanoparticles as a nano-priming agent for improving early plant vigor and underscore the importance of nanoparticles form and concentration in sustainable agricultural applications.</p>