Both Cu deficiency and excess impair plant growth, complicating Cu fertilization and raising concerns regarding environmental safety. CuO engineered nanoparticles (ENPs) have been proposed as alternative Cu delivery systems; however, their benefits and potential hazards depend strongly on transformation processes occurring in environmental compartments. Here, we investigated how chemical (sulfidation), biological (protein corona formation), and combined transformations modify the behavior, bioavailability, and plant impacts of pristine CuO ENPs (1-50 mg Cu L−1) in Hordeum vulgare L. Plant Cu accumulation, mineral nutrient homeostasis, photosynthetic pigment content, and oxidative stress markers were assessed. Both pristine and transformed CuO ENPs increased root Cu concentrations by 5–200-fold, with sulfidation reducing and protein corona formation enhancing Cu uptake. Shoot Cu accumulation was highest for sulfidized CuO ENPs (1.3-4-fold higher than pristine CuO), but remained substantially lower than for ionic Cu, indicating distinct uptake and translocation pathways. Transformations that enhanced ENP dissolution induced stronger perturbations of mineral nutrient balance and elevated reduced glutathione levels (1.2-3.5-fold), reflecting increased metal stress. In contrast, less soluble ENPs preferentially stimulated photosynthetic pigment accumulation and lipid peroxidation (1.4-5-fold), suggesting indirect oxidative responses associated with altered Cu availability rather than direct toxicity. Importantly, protein corona formation on sulfidized CuO ENPs attenuated excessive Cu2+ release and moderated nanoparticle-plant interactions, resulting in reduced physiological disruption. Overall, these findings demonstrate that transformation-dependent processes govern Cu bioavailability, plant responses, and hazard potential of CuO ENPs, underscoring the need to incorporate realistic transformation scenarios into environmental risk assessment of nano-enabled agrochemicals.