Dual roles of TiO₂ nanoparticles under cadmium stress: Alleviating oxidative damage while exacerbating growth inhibition in rice (Oryza sativa L.)

Cadmium (Cd) pollution poses a serious threat to agricultural ecosystems and food security. Titanium dioxide nanoparticles (TiO₂ NPs) have been proposed as a strategy to alleviate heavy metal stress, yet their effects under Cd exposure remain controversial. In this study, rice seedlings were exposed to Cd (0 or 30 mg/kg) with or without TiO₂ NPs (anatase or rutile; 0, 20, 50, or 100 mg/kg) for 40 days. Compared with the blank, Cd exposure alone did not significantly affect root length, root dry weight, shoot length, shoot dry weight, or chlorophyll content (ANOVA, p  > 0.05). In contrast, under Cd stress, TiO₂ NP treatment induced significant growth inhibition relative to the blank, with shoot length, root dry weight, and chlorophyll content reduced by 22–29 %, 27–47 %, and 29–41 %, respectively ( p  < 0.05). With respect to oxidative stress indicators, Cd exposure reduced antioxidant enzyme activities (e.g., peroxidase) and increased malondialdehyde levels, while co-treatment with TiO₂ NPs substantially reversed these alterations, restoring values close to those of the blank. Multi-omics analyses revealed that under Cd stress, anatase TiO₂ NPs alleviated oxidative stress by modulating flavonoid metabolism but impaired growth through energy disruption, whereas rutile exerted its effects via thiamine metabolism, amino acid metabolism, and the TCA cycle. These findings suggest that under Cd stress, crystal structure was not the primary determinant of TiO₂-mediated effects, as both anatase and rutile activated distinct molecular pathways yet converged on comparable physiological and growth outcomes. Overall, our results support a defense-growth trade-off model, in which TiO₂ NPs alleviate Cd-induced oxidative stress while concurrently exacerbating growth inhibition. The paradoxical role of TiO₂ NPs highlights the need for comprehensive safety assessments prior to their use in nano-enabled agriculture on heavy metal-contaminated soils.