Physiological and transcriptional regulation of cadmium stress tolerance in ornamental pepper by silicon nanoparticles and melatonin.

Nanotechnology has been established as a promising approach to improve heavy metal stress tolerance in horticultural crops. Melatonin (MT) is an emerging biostimulant for inducing stress resistance in plants. In this study, we evaluated the individual and combined effects of silicon nanoparticles (SiNPs; 50 mg L-1) and MT (25 mg L-1) on ornamental pepper (Capsicum annuum L., cv. Aladdin) grown in Cd-spiked soil (50 mg Cd kg-1). Morpho-physiological traits (biomass, photosynthetic pigments, leaf gas exchange, and Cd concentration), biochemical traits (proline, soluble sugars, electrolyte leakage, malondialdehyde, hydrogen peroxide, and superoxide), antioxidant enzyme activities, and the expression of defense genes were analyzed. Cd stress induced oxidative damage, inhibited photosynthesis-related parameters, and suppressed plant growth. Application of SiNPs and MT reduced tissue Cd accumulation, sustained photosynthesis, and improved plant growth, with the combined treatment showing the strongest overall mitigation. These improvements were associated with lower electrolyte leakage, hydrogen peroxide, superoxide, and malondialdehyde levels, together with higher antioxidant enzyme activities and defense-gene expression. The results support a combined Cd-stress mitigation in ornamental pepper; however, the study does not demonstrate soil Cd immobilization or statistically verified synergy. Further dose-response, long-term, and molecular experiments are mandatory to elucidate the interaction between SiNPs and MT under Cd stress.