Effect of growth stage, drying method and distillation duration on the yield of essential oils in Salvia macrosiphon and Marrubium vulgare

Genetic and physiological characteristics, phenological stages, geographical habitat locations, drying methods, and extraction durations can all influence the yield of essential oils in medicinal plants. This study aimed to investigate the quantitative and qualitative changes in the essential oils of Salvia macrosiphon and Marrubium vulgare at different growth (phenological) stages, as well as to determine the most suitable drying method and extraction duration using water distillation. The study was designed as a completely randomized experiment with three treatments and three replications. Sampling was conducted randomly across three growth stages: vegetative growth, flowering, and seeding, in three habitats within the rangelands of the Delfard region in Jiroft city. Essential oils were extracted from the samples via water distillation, and the chemical components of these oils were analyzed using Gas Chromatography-Mass Spectrometry (GC/MS). The results showed that the highest yield of essential oil occurred during the flowering stage, while the lowest yield was recorded in the seeding stage (P<0.05). Changing the growth stage did not significantly affect the quality of the essential oils from either species (P>0.05). A total of 34 chemical compounds were identified in the essential oils of M. vulgare, and 20 compounds were identified in S. macrosiphon. The study found that shade-dried samples had the highest essential oil yield (0.069 mg), whereas samples dried in the oven had the lowest yield (0.02 mg). Specifically, the yield of essential oil from shade-dried samples was 33.3% and 62.3% higher than that from sun-dried and oven-dried samples, respectively. Additionally, the yield was highest from samples subjected to a 3-hour distillation (0.062 mg) and lowest from samples distilled for just 1 hour (0.01 mg) (P<0.05). In conclusion, the flowering stage (mid-May) is the optimal time to harvest the aerial parts of both species in the rangelands of the Delfard region in Jiroft city. The best drying method for these medicinal plants is shade drying, and the most effective distillation duration is 3 hours.