Manganese Oxide Nanoparticles as Safer Seed Priming Agent to Improve Chlorophyll and Antioxidant Profiles in Watermelon Seedlings

The use of nanoscale nutrients in agriculture to improve crop productivity has grown in recent years. However, the bioefficacy, safety, and environmental toxicity of nanoparticles are not fully understood. Herein, we used onion bulb extract to synthesize manganese oxide nanoparticles (MnO-NPs). X-ray diffraction, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy were used for the structural and morphological characterization of synthesized MnO-NPs. The MnO-NPs were oval shape crystalline nanoparticles of Mn₂O₃ with sizes 22-39 nm. In further studies, we assessed the comparative toxicity of seed priming with MnO-NPs and its bulk counterparts (KMnO₄ and Mn₂O₃), which showed seed priming with MnO-NPs had comparatively less phytotoxicity. Investigating the effect of seed priming with different concentrations of MnO-NPs on the hormonal, phenolic acid, chlorophyll, and antioxidant profiles of watermelon seedlings showed that treatment with 20 mg·L^-1 MnO-NPs altered the chlorophyll and antioxidant profiles of seedlings. At ≤40 mg·L^-1, MnO-NPs had a remarkable effect on the phenolic acid and phytohormone profiles of the watermelon seedlings. The physiological outcomes of the MnO-NP seed priming in watermelon were genotype-specific and concentration-dependent. In conclusion, the MnO-NPs were safer than their bulk counterparts and could increase crop productivity.