<p>This study aimed to evaluate the comparative effects of free and nanoparticle-conjugated red kidney bean extract (RKE) on parsley physiology and antioxidant responses. RKE exhibited high phenolic (520.4 mg GAEg⁻¹) and flavonoid (82.4 mg QE g⁻¹) contents, with strong dose-dependent antioxidant activity (up to 82.8% at 1000 µg mL⁻¹). HPLC analysis of RKE revealed major phenolics, including chlorogenic and gallic acids. A visible color shift to ruby red initially indicated the formation of Se-NPs + RKE. It was further supported by TEM, DLS analysis, and UV–Vis spectroscopy, showing an absorption peak shift from 380 to 395 nm upon conjugation. On the other hand, a field experiment on parsley was conducted using five treatments (control, RKE 500 µg mL⁻¹, RKE 1000 µg mL⁻¹, Se-NPs + RKE 500 µg mL⁻¹, and Se-NPs + RKE 1000 µg mL⁻¹), and the results showed that both RKE and Se-NPs + RKE significantly enhanced parsley growth compared to the control. RKE at 1000 µg mL⁻¹ produced the tallest plants (41 cm) and highest fresh weight (4.52 g), relative to 31 cm and 1.20 g in controls. Se-NPs + RKE at 500 µg mL⁻¹ also promoted growth (39 cm; 2.77 g), although higher doses reduced plant height. Photosynthetic pigments were markedly improved, with chlorophyll a peaking at 2.38 mg g⁻¹ under RKE 1000 µg mL⁻¹, while carotenoids were maximized at 0.70 mg g⁻¹ with Se-NPs + RKE 500 µg mL⁻¹. Antioxidant enzymes (catalase, peroxidase, SOD) were significantly elevated, reaching up to 33.61 units for catalase under Se-NPs + RKE 1000 µg mL⁻¹. Proline content also increased, with RKE treatments reaching \ ~ 0.42 mg g⁻¹ FW and Se-NPs + RKE up to 0.90 mg g⁻¹ FW. These results highlight the potential of RKE and Se-NPs + RKE as eco-friendly biostimulants for enhancing parsley productivity, supporting sustainable agriculture. Nevertheless, the dose-dependent responses, particularly with Se-NPs + RKE, underline the need for careful optimization to minimize potential toxicity at higher concentrations.</p>