Acid Electrolyzed Water Priming Induces Phenylpropane Metabolism and Antioxidant Enzyme System to Promote Seed Germination of Celery

Seed germination of celery (Apium graveolens L.) is notoriously slow and asynchronous, which severely constrains uniform seedling establishment and crop yield. Seed priming is an effective technique to improve germination, and acidic electrolyzed water, characterized by low pH and high oxidation–reduction potential, has emerged as a novel priming agent. However, the effect of acid electrolyzed water priming (EWP) on celery seed germination and the underlying mechanisms still need to be explored. The present study aimed to investigate the physiological and molecular mechanisms by which EWP promotes celery seed germination, with a focus on the roles of the phenylpropane metabolism and the antioxidant enzyme system. Celery seeds were treated with EWP, hydro-priming (HYD), and untreated (CK). It was found that the EWP treatment significantly enhanced germination characteristics compared to both CK and HYD. Transcriptome analysis revealed that EWP triggered more extensive transcriptional reprogramming than HYD, and EWP specifically enriched “Phenylpropanoid biosynthesis” and “Flavonoid biosynthesis” pathways, downregulating upstream genes (PAL, 4CL) while upregulating downstream genes (CCR, CHI, F3H) in the phenylpropane pathway. Physiologically, EWP significantly increased CHI activity and the contents of total phenols and flavonoids at all sampling time points, and enhanced the activities of SOD, POD, CAT, and APX. Consequently, the DPPH and FRAP free radical scavenging capacities were significantly strengthened in EWP-treated seeds. In conclusion, it is believed that EWP activation promotes celery seed germination by coordinating the phenylpropane pathway and antioxidant enzyme system, ensuring effective radical scavenging activities and cell protection. These findings provide a theoretical basis for the application of EWP and highlight the potential as a novel priming technology for celery and other horticultural crops.