Zinc (Zn) is an indispensable micronutrient for plant growth and metabolism, yet its bioavailability in soil is often limited. Recent advances in nanotechnology offer potential strategies to improve nutrient delivery through green-synthesized nanoparticles (NPs). The present study investigates the cytological, enzymatic and other metabolic responses of Vicia faba plants treated with green-synthesized zinc oxide NPs (ZnO NPs) compared with bulk zinc sulfate (ZnSO4). First, ZnO NPs were green synthesized from Sorghum bicolor leaves extract giving a size of 23.6 nm. Germination bioassay and pot experiment were conducted using different concentrations of ZnO NPs and ZnSO4 (25, 50, and 100 mg L−1 ). Concerning the germination bioassay, cytological observations revealed differential effects on mitotic index, chromosomal behavior and number of aberrant cells indicating ZnO NPs-mediated modulation of cell division while high concentrations of both ZnO NPs and ZnSO4 caused cell abnormalities. Both ZnO NPs and ZnSO4 primarily boosted protein and carbohydrate content, and enzyme activities of amylase and protease in the V. faba seedlings. In the pot experiment, all growth, photosynthetic and gas exchange parameters were increased with Zn supplements except for the higher doses and the more enhancement for ZnO NPs. Biochemical assays demonstrated enhanced activities of carbonic anhydrase, nitrate reductase and antioxidant enzymes, including superoxide dismutase, catalase, and peroxidase, alongside altered levels of proline due to ZnO NPs application compared to bulk ZnSO4. Zn determination in shoots and roots revealed that Zn absorption was higher in plants treated with ZnSO4 at all tested concentrations which was accompanied by elevated levels of electrolyte leakage and lipid peroxidation. Metabolic changes further reflected ZnO NP-induced regulation of oxidative stress responses and cellular homeostasis by maintaining membrane stability and water content. Overall, the findings suggest greater phytotoxicity in plants subjected to ZnSO4 compared with ZnO NPs, likely due to the accumulation of dissolved Zn ions absorbed into plant tissues. These findings emphasize the role of green-synthesized ZnO NPs as sustainable growth regulators, offering novel insights into their cytological and biochemical impact on higher plants.