IImpacts of diurnal variation on essential oil properties and antioxidant activity of endemic Origanum dubium Boiss

Purpose: This study aimed to investigate the impact of diurnal variation on the essential oil (EO) yield, chemical composition, and antioxidant activity of Origanum dubium Boiss., an endemic and carvacrol-rich species naturally distributed in Türkiye. Method: The aerial parts of O. dubium were collected at five different harvest times during the flowering stage (06:00, 09:00, 12:00, 15:00, and 18:00) in Ermenek (Türkiye) in 2020. Essential oils were obtained by hydrodistillation using a Clevenger-type apparatus and analyzed by GC-FID/MS. Antioxidant activity was evaluated by the DPPH radical scavenging method. Findings: The EO content ranged between 6.4% (12:00) and 8.6% (18:00). Oxygenated monoterpenes (79.9–88.0%) were the dominant group, with carvacrol (72.0–83.4%) as the main compound. The highest carvacrol concentration was recorded at 09:00, whereas the lowest was at 12:00. Other major constituents were α-thujene (1.3–1.5%), γ-terpinene (1.9–6.3%), and p-cymene (4.0–5.4%). Antioxidant activity varied significantly depending on the harvest time, with radical scavenging activity ranging from 32.21% (06:00) to 63.90% (18:00). Conclusion: Diurnal variation significantly influences the EO yield, composition, and antioxidant activity of O. dubium. The results suggest that harvesting between 06:00–12:00 in the morning and 15:00–18:00 in the afternoon is optimal for obtaining higher oil yield, carvacrol content, and antioxidant potential.