Non-Essential Element-Based Nanoparticles in Rice: Unraveling the Impacts of Yttrium Oxide and Zirconium Oxide Nanoparticles on Root Accumulation and Antioxidant Responses

Nanotechnology has attracted increasing attention in agricultural and environmental research, but the biological effects and potential risks of nanoparticles based on non-essential elements remain insufficiently understood. This study investigated the physiological and biochemical responses of rice (Oryza sativa L.) seedlings to yttrium oxide nanoparticles (Y2O3 NPs) and zirconium oxide nanoparticles (ZrO2 NPs) at 5, 25, and 100 mg/L under hydroponic conditions. The results showed that neither Y2O3 nor ZrO2 NPs significantly affected visible growth traits or SPAD-based leaf chlorophyll status, suggesting that seedling morphology and leaf greenness remained relatively stable during exposure. However, both nanoparticles induced distinct biochemical responses. Y2O3 NPs caused root-level stress-like responses, including increased malondialdehyde (MDA) accumulation and suppressed peroxidase (POD) and catalase (CAT) activities under specific exposure conditions. In contrast, ZrO2 NPs were more closely associated with the activation of antioxidant defenses, particularly through enhanced POD activity and increased root CAT activity. Inductively coupled plasma mass spectrometry (ICP-MS) analysis further showed that Y and Zr were mainly retained in roots, with root Y reaching 5014.12–11,255.05 mg kg−1 dry weight (DW) under Y2O3 NP exposure and root Zr reaching 189.68 mg kg−1 DW under high-concentration ZrO2 NP exposure. Bio-transmission electron microscopy (bio-TEM) supported the root-dominant localization of nanoparticle-associated electron-dense aggregates. These findings indicate that Y2O3 and ZrO2 NPs exert material-specific effects on rice seedlings, with root accumulation and antioxidant regulation serving as more sensitive indicators than visible growth traits. However, further research is needed to clarify the long-term environmental fate of Y2O3 and ZrO2 NPs and to assess their potential ecological and food safety risks in agricultural systems.