Plasma-Activated Water Improves Seed Germination and Seedling Vigor in Hot Pepper (Capsicum annuum L.) via Redox-Mediated Hormetic Regulation

Plasma-activated water (PAW) is an eco-friendly seed priming agent, yet its efficacy depends on plasma activation conditions that remain insufficiently defined. This study characterized the physicochemical properties of PAW generated at different activation times (15–120 s) and evaluated their effects on germination and biochemical responses of hot pepper (Capsicum annuum L.) seeds, in comparison with non-primed and distilled water-primed controls. Increasing plasma activation time elevated electrical conductivity and reactive oxygen and nitrogen species (RONS; e.g., H2O2 and NO3−), while lowering pH, with NO2− peaking at intermediate durations. PAW priming induced a dose-dependent hormetic response, with PAW30 exhibiting the highest germination and seedling vigor. This improvement was associated with enhanced antioxidant enzyme activities, reduced oxidative damage, and increased α-amylase activity and soluble sugar accumulation, indicating improved metabolic activation. In contrast, longer activation times, particularly PAW120, increased oxidative stress markers and reduced the magnitude of germination and seedling vigor improvement relative to PAW30. Principal component analysis (PCA) further supported coordinated relationships among PAW properties, biochemical responses, and germination traits. Collectively, the results support a redox-associated hormetic response to PAW priming and identify PAW30 as the most effective activation condition for improving germination and early seedling growth in hot pepper.