The increasing use of cerium oxide nanoparticles (CeO₂-NPs) has raised concerns regarding their environmental fate and biological effects in soil–plant systems. This review uses cerium oxide nanoparticles (CeO₂-NPs) as a mechanistically well-characterized case study to examine relationships among dose, chemical speciation, and plant responses, with emphasis on Ce³⁺/Ce⁴⁺ redox cycling and condition-dependent, nanozyme-like activity. We discuss how particle size, surface charge, Ce³⁺/Ce⁴⁺ ratio, and coating interact with rhizosphere processes, including organic acid complexation, phosphate-mediated CePO₄ immobilization, and potentially microbially mediated transformation, to influence effective root-surface exposure. The resulting particulate, ionic, and secondary transformation-product pools affect root uptake, vascular transport, redox homeostasis, metabolism, and hormone-associated signaling. Major limitations include weak causal links between rhizosphere speciation and bioavailability, insufficient separation of particulate and ionic contributions, limited integration of multi-omics with speciation data, and uncertain extrapolation from hydroponic to soil systems. Standardized exposure protocols, integrated XANES, spICP-MS, and imaging workflows, and long-term soil studies are therefore required. Other rare-earth nanomaterials are discussed only briefly to define the material-specific boundaries of the CeO₂-centered framework.

Aronia melanocarpa puree is rich in anthocyanins and proanthocyanidins, but its pronounced tannin-derived astringency limits product acceptance. This study developed a non-removal astringency-modulation strategy using food-grade L-proline and evaluated its molecular basis, processing window, and quality effects. Ultraviolet–visible (UV–Vis) and Fourier-transform infrared (FT-IR) spectroscopic analyses suggested that L-proline altered the local microenvironment of procyanidin B2 (PC-B2) through hydrogen bonding, hydrophobic association, and molecular packing rearrangement, without evident disruption of the PC-B2 aromatic skeleton. In a PC-B2–bovine serum albumin model, an appropriate L-proline level reduced the protein precipitation rate from 45.3% to 31.2% and increased soluble phenolic retention, suggesting weakened polyphenol–protein precipitation. The strategy was then optimized in puree using machine learning-assisted multi-response analysis and Box–Behnken validation. The recommended condition was 150 mg/100 mL L-proline at 40 °C for 60 min, yielding a deastringency rate of 36.13%, with anthocyanin and vitamin C retention rates of 88.80% and 55.56%, respectively. The optimized treatment maintained red color, colloidal dispersion, and shear-thinning behavior; increased the anthocyanin digestion retention index from 50.0% to 87.4%; and improved overall sensory acceptance from 4.17 to 8.17. These findings support L-proline-mediated microenvironmental modulation as a mild processing approach for high-tannin cloudy berry products.