Seed Priming with Iron Oxide Nanoparticles Modulate Antioxidant Potential and Defense-Linked Hormones in Watermelon Seedlings

In the present study, onion extract was used for green synthesis of iron oxide nanoparticles (Fe-NPs). The X-ray diffraction, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy characterizations demonstrated that the biosynthesized Fe-NPs were mainly composed of low-crystalline or amorphous Fe2O3 with diameter of 19–30 nm. The effect of seed priming with different concentrations of Fe-NPs (20, 40, 80, and 160 mg L–1) on seedling growth parameters, photosynthetic pigments, antioxidant potential, metabolites, and hormonal profiles of diploid and triploid watermelon (Citrullus lanatus (Thunb.) Matsum. and Nakai) varieties were studied on the third and eighth days of seedling development. Seed priming with Fe-NPs was safer compared to its bulk counterparts (FeCl3 and Fe2O3) and had no toxic impact on seed germination, seedling development, and chlorophyll biosynthesis at studied concentrations. Untargeted metabolomics studies showed that different Fe-NPs priming treatments distinctly altered the metabolome of diploid and triploid watermelon seedlings. Uniquely, we found that different Fe-NPs priming treatments significantly modulate the 12-oxo phytodienoic acid (OPDA) level in diploid and triploid watermelon seedlings. In conclusion, seed priming with nontoxic Fe-NPs can be applied sustainably to increase nonenzymatic antioxidant potential as well as to prime or induce jasmonates-linked defense responses in watermelon seedlings.