Distinct Fatty Acid and Phenolic Signatures Define Serotonin and Melatonin Seed Priming Effects in Capsicum annuum Under Non-Stress Conditions

Seed priming is an effective pre-sowing strategy to reprogram plant metabolism, which may contribute to stress resilience. While serotonin (SER) and melatonin (MEL) are recognized antioxidants and growth regulators, their comparative impacts on metabolic reprogramming in chili ( Capsicum annuum ) seed priming remain largely unexplored. This work demonstrated that SER and MEL played distinct, complementary roles: SER (10 μM) primarily accelerates early germination and reducing sugar mobilization, whereas MEL (100 μM) significantly enhances shoot elongation, yield, and chlorophyll content (+92.19%). Antioxidant defense profiles also differed; MEL strongly activated multiple enzymes (SOD, CAT, and POX), while SER primarily boosted SOD activity. Both treatments improved non-enzymatic antioxidant capacity as measured by DPPH (2,2-Diphenyl-1-picrylhydrazyl) and FRAP (Ferric Reducing Antioxidant Power) activities. Metabolic profiling revealed unique chemical signatures: MEL markedly increased salicylic (+124%) and myristic acids (+121.5%), while SER promoted catechin (+146%) and alpha-linolenic acid (+108.8%) accumulation. Interestingly, MEL-induced increases in MDA and electrolyte leakage may suggest a transient redox shift during metabolic reprogramming. These findings offer novel insights into the distinct regulatory functions of SER and MEL, providing a viable strategy to enhance the metabolic readiness and potential field resilience of chili crops. • Melatonin (MEL) and Serotonin (SER) priming enhance chili seedlings growth and stress resilience. • Melatonin significantly increases shoot elongation and chlorophyll content. • Serotonin aids germination and sugar mobilization, supporting early seedling. • Melatonin upregulated Superoxide dismutase (SOD) and Catalase (CAT) activity. • Distinct metabolic shifts were observed under Serotonin and Melatonin priming.