Exogenous Ferulic Acid Mitigates Flooding Stress in Broccoli via GSH-Mediated Redox Homeostasis

Climate change is increasing flood frequency, exposing plants to severe stress. This study investigated the biostimulant-like effects of exogenous ferulic acid (FA; 1, 10, and 100 mg/L) on broccoli (Brassica oleracea var. cymosa) microgreens under regularly watered (RW) and flooded (F) conditions. Spectrophotometric, HPLC, and statistical analyses showed that all FA concentrations increased total phenolics and proanthocyanidins in flooded plants, while only 100 mg/L increased proanthocyanidins in RW plants. FA at 1 and 100 mg/L reduced soluble sugars in RW broccoli (18% reduction by both FA concentrations) and enhanced antioxidant capacity (measured by ferric reducing antioxidant power assay, FRAP) in flooded plants (8% and 11%, respectively). Only 10 mg/L FA lowered hydrogen peroxide in RW plants. Flooding significantly decreased glutathione (GSH) levels, but FA treatment doubled GSH concentration and restored its level in flooded broccoli, improving redox balance. FA also influenced individual polyphenols more strongly in RW plants, with notable increases in sinapic acid and kaempferol. Overall, FA enhanced antioxidant status and redox homeostasis under flooding stress, mainly by stimulating glutathione accumulation and phenylpropanoid metabolism. Its regulatory effects were strongly dependent on soil water conditions. These findings underscore the practical and agronomic potential of FA as an effective approach to enhance crop resilience under climate change.