Enhanced Cd Tolerance in Bamboo: Synergistic Effects of Nano-Hydroxyapatite and Fe₃O₄ Nanoparticles on Reactive Oxygen Species Scavenging, Cd Detoxification, and Water Balance

Nano-hydroxyapatite (n-HAP) and Fe₃O₄ NPs (Fe₃O₄ NPs) offer effective and economical approaches for reducing Cd toxicity, which presents considerable risks to both environmental and human health. We examined the mechanisms through which these NPs mitigate Cd toxicity in bamboo, <i>Pleioblastus pygmaeus</i>. The plants were exposed to Cd (0, 50, 100, and 150 mg L⁻¹) and received foliar sprays of 100 mg L⁻¹ n-HAP, 100 mg L⁻¹ Fe₃O₄ NPs, and a combination of both treatments. The findings indicated that Cd exposure led to oxidized molecules in bamboo, as evidenced by elevated levels of reactive oxygen species (ROS) and lipoperoxidation. Foliar treatments utilizing n-HAP and Fe₃O₄ NPs markedly diminished these effects. H₂O₂, O₂•−, malondialdehyde (MDA), and electrolyte leakage (EL) levels decreased by 56%, 71%, 65%, and 72%, respectively, compared to the controls. The application of n-HAP and Fe₃O₄ NPs significantly enhanced the enzymes, including superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), glutathione reductase (GR), and phenylalanine ammonia-lyase (PAL), with increases observed between 28% and 56%. Furthermore, there was an enhancement in proline accumulation, total phenolic content (TPC), flavonoids (TFC), nitric oxide levels, relative water content (RWC), chlorophyll concentration, and photosynthetic parameters. The combination of n-HAP and Fe₃O₄ NPs was most effective in improving bamboo tolerance to Cd, especially at moderate Cd concentrations of 50 and 80 mg L⁻¹. The results indicate that n-HAP and Fe₃O₄ NPs, particularly in combination, may mitigate Cd toxicity by decreasing Cd uptake, improving antioxidant capacity, and preserving plant water balance.