Defoliation Frequency and Intensity Modify the Antioxidant Activity and Bioactive Compound Concentration of Plantago lanceolata

Plantago lanceolata L. is a herb with diverse pharmacological properties attributed to its rich phytochemical composition, including iridoids, flavonoids, caffeic acid derivatives, and polyphenols with significant antioxidant potential. This study investigated how defoliation frequency (extended leaf length, ELL: 15, 25, and 35 cm) and intensity (residual height, RH: 5 and 8 cm) affect bioactive compound content and antioxidant activity in P. lanceolata extracts. Six defoliation combinations were evaluated in a 3 × 2 factorial arrangement (three pots per combination; n = 18 pots). Total phenolic content (TPC), antioxidant activity (DPPH radical scavenging), lipid content, the iridoids aucubin (AU) and catalpol (CAT), and the phenylpropanoid glycoside acteoside (AC) were determined in the aerial biomass, and high- and low-molecular-weight water-soluble carbohydrate (WSC) content in the aerial and root biomass. Functional group profiling was performed using Fourier transform infrared (FT-IR) spectroscopy; lipophilic compounds were putatively identified by gas chromatography–mass spectrometry (GC-MS) in pooled samples; and AU, CAT, and AC were quantified by high-performance liquid chromatography (HPLC). Total phenolic content depended on the ELL × RH interaction, while antioxidant activity was higher at 5 cm than at 8 cm RH (43.0 vs. 30.3%). Lipid content was not significantly affected by defoliation. GC-MS profiles were dominated by n-alkanes; putative neophytadiene exceeded the reporting threshold only at 15 cm ELL/5 cm RH, and tentatively identified α-tocospiro compounds (α-tocopherol oxidation products) showed their highest relative abundance at 35 cm ELL. Among the targeted bioactive compounds, acteoside was higher under the more frequent defoliations (15 and 25 cm ELL) and at 5 cm RH, catalpol remained unchanged across treatments, and aucubin was below the limit of quantification. High- and low-molecular-weight WSC contents in the aerial and root biomass accumulated most under the less frequent (25 and 35 cm ELL) defoliation regimes, driven mainly by greater herbage and root mass rather than by changes in concentration. Root low-molecular-weight WSC content was also higher at 8 than at 5 cm RH. Overall, more intense defoliation (5 cm RH) increased antioxidant activity and acteoside concentration, whereas less frequent defoliation increased WSC content, suggesting that defoliation management could be adjusted to balance the phytochemical quality and regrowth potential of P. lanceolata.