Effect of Harvest Time on the Biofunctional Properties of <i>Schizonepeta tenuifolia</i> Briq

<i>Schizonepeta tenuifolia</i> Briq (<i>S. tenuifolia</i>), a perennial plant from the Lamiaceae family, has been used in herbal medicine to treat diseases. Herein, we investigate the biofunctional properties of <i>S. tenuifolia</i> at different harvesting time points. Biofunctional compounds such as rosmarinic acid, pulegone, and menthone from <i>S. tenuifolia</i> extracts were quantified by using high-performance liquid chromatography and gas chromatography-mass spectrometry. The antioxidant activities of <i>S. tenuifolia</i> extracts were measured in vitro using 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2-azinobis-(3-ethyl-benzothiazoline-6-sulfonic acid) (ABTS), and ferric reducing antioxidant power (FRAP) assays. The total phenolic content was measured in vitro. The <i>S. tenuifolia</i> extracts showed the highest ABTS, FRAP, and total phenolic compound at the full bloom stage (S4) and the highest DPPH activity at the fruiting stage (S6). Rosmarinic acid (RA) peaked at S5 (676.87 μg/g dry weight), with strong correlation to antioxidant activity. The pulegone content was the highest at the spike formation stage (S3), whereas the menthone content peaked at S6. Considering that the antioxidant activity and number of functional compounds in <i>S. tenuifolia</i> vary with harvest time, the optimal harvest periods identified were during the S4 and S5 (fruiting) stages, which maximize the bioactivity, including antioxidant activity and product quality.