Dose- and Application-Dependent Effects of Biogenic Selenium Nanoparticles on Germination, Growth, and Antioxidant Response of Capsicum annuum L.

Selenium nanoparticles (SeNPs) synthesized through green routes have emerged as promising nanobiostimulants in sustainable agriculture due to their ability to enhance plant growth and antioxidant defense. The aim of this study was to evaluate the biostimulant effect of SeNPs on Capsicum annuum at two stages of crop development to characterize the response to SeNP exposure and identify concentration-dependent effects and application methods. Physiological indicators, including growth, photosynthetic pigment content, and antioxidant activity, were evaluated. Different concentrations of SeNPs were tested during germination, and dosage and two types of application were compared during the vegetative phase in a hydroponic experiment. SeNPs at concentrations of 1.25, 2.5, 5, 10, 20, 40, and 80 µM were applied to chili seeds for 20 days. The plants were exposed to SeNPs concentrations ranging from 1.25 to 80 µM, applied through the roots and leaves. Germination parameters were not significantly affected except for the seed vigor index, which increased at all concentrations, particularly at 20 µM. Low to moderate doses (1.25–20 µM) acted as biostimulants, enhancing plant height, root length, biomass accumulation, photosynthetic pigment content, and phenolic and flavonoid compound synthesis. Conversely, high doses (80 µM) induced phytotoxic effects, especially via root exposure, reflected by growth inhibition, and reduced chlorophyll content. Foliar application demonstrated a systemic biostimulant response, improving root growth and photosynthetic activity without toxicity symptoms. Antioxidant assays (DPPH and ABTS) revealed dose-dependent modulation of redox balance, suggesting adaptive responses to SeNP-induced oxidative conditions. These findings highlight the potential of SeNPs as biostimulants that improve physiological performance in chili plants, while emphasizing the importance of an optimal dosing and application method for sustainable nanotechnology-based crop management.