Effects of Exogenous Plant Growth Regulators on L-Ascorbic Acid Accumulation and Antioxidant Capacity of Loquat Leaves

Loquat leaves, a key raw material in traditional Chinese medicine, contain the antioxidant L-ascorbic acid (AsA) that significantly contributes to their medicinal efficacy. However, studies exploring how plant growth regulators (PGRs) affect AsA accumulation and antioxidant metabolism in loquat leaves remain limited. This study aimed to investigate the effects of exogenous foliar application of six PGRs, including indole-3-acetic acid (IAA), synthetic cytokinin (6-benzylaminopurine, 6-BAP), ethephon (2-chloroethylphosphonic acid, CEPA), gibberellin A3 (GA3), abscisic acid (ABA), and brassinolide (BL, representing brassinosteroid), on loquat leaves through systematic time-course and concentration-dependent experiments. Our results revealed that a 10-day treatment with 100 mg/L IAA, 6-BAP, GA3, ethephon, ABA, and 4 mg/L BL yielded optimal AsA accumulation in loquat leaves. In parallel, the treatments also significantly elevated the accumulation of multiple bioactive secondary metabolites with potent antioxidant properties, including flavonoids, triterpenoids, total phenols, and soluble polysaccharides. Furthermore, the total antioxidant capacity of loquat leaf extracts was evaluated via ferric reducing antioxidant power (FRAP), 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and 2,2-diphenyl-1-picrylhydrazyl (DPPH) assays. The results consistently demonstrated that foliar application of various PGRs effectively modulated antioxidant capacity, with 100 mg/L IAA and BL treatments showing the strongest activity. Collectively, this study demonstrates that foliar application of exogenous PGRs effectively modulates AsA content in loquat leaves and the antioxidant capacity of corresponding leaf extracts, which provides insights into the potential high-value industrial utilization of loquat leaf resources.