Effect of seasonal variations on the secondary metabolites and antioxidant activities of Bridelia ferruginea, Lippia multiflora, and Azadirachta indica leaves

The aim of the study was to investigate the effect of the annual seasonal variations in Ghana on the phytochemical composition and antioxidant properties of the leaves of Bridelia ferruginea (BF), Lippia multiflora (LM), and Azadirachta indica (AI). The medicinal plants were collected at the end of each quarter from September 2021 to June 2022, representing four seasons. The phytochemical constituents of the samples were screened and quantified. The antioxidant properties of the samples were evaluated using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging activity. Thin layer chromatography and infrared profiling of the extracts were also analyzed, serving as fingerprint. From the results, total phenolic content (TPC) of BF, LM, and AI leaves were all highest in season 1. The total flavonoid, alkaloid and saponin contents in BF leaves were highest in season 4. The total flavonoid and saponin contents were highest in season 1 and season 3, respectively for LM, but highest in season 2 and season 4, respectively for AI. The antioxidant activity of BF, LM and AI were strongest for samples collected in season 1(45.70 mg/ml AAE), season 3 (20.39 mg/ml AAE) and season 4 (62.67 mg/ml AAE), respectively. All three samples (BF, LM and AI leaves) collected in season 2 showed the lowest antioxidant activity: 54.07 ± 2.92, 60.23 ± 4.14, and 115.15 ± 13.55 mg/ml AAE, respectively. From the study, the rainy seasons (major or minor) appear to generally favor the accumulation of the phytochemical constituents screened in this study, thus recommended as the suitable harvesting period for high yield of the phytochemical constituents.