Zinc Oxide Nanopriming in Tomato Seeds: Image Analysis and Physiological Quality in Response to Stress Conditions

Seed priming with nanoparticles has emerged as a promising strategy to enhance seed performance under abiotic stress; however, phytotoxic effects remain poorly understood. This study evaluated the effects of zinc oxide (ZnO) nanopriming on the physical and physiological quality of tomato seeds (Solanum lycopersicum L.) exposed to water deficit (−0.3 MPa) and thermal stress (25 and 35 °C). Seeds were subjected to three treatments: non-primed control, osmopriming with polyethylene glycol (PEG), and PEG combined with ZnO nanoparticles. Water deficit reduced germination from 81% to 13% at 25 °C and from 68% to 4% at 35 °C, regardless of priming treatments. PEG-treated seeds exhibited greater seedling development, with root length reaching 14.1 mm, whereas ZnO-treated seeds showed reduced root growth (6.4 mm) and lower vigor indices. X-ray image analysis revealed higher internal tissue integrity in primed seeds, with more than double the density observed in the control. Conversely, nanopriming with ZnO at the tested concentration induced phytotoxic effects, compromising seedling growth and failing to mitigate water and thermal stress. Overall, osmopriming with PEG proved effective in enhancing seed vigor and stress tolerance, whereas ZnO nanopriming impaired physiological performance under the evaluated conditions.