Impacts of harvest time and water stress on the growth and essential oil components of horehound (Marrubium vulgare)

The study investigated the effect of water stress and harvesting frequency on the growth and essential oil (EO) content and components of Marrubium vulgare grown under Egyptian conditions. Two-month old seedlings of Marrubium vulgare were transplanted into pots and irrigation treatments (4, 8 and 12 days) were applied one month after transplanting. Growth and chemical component response variables were determined at flowering stage using plants harvested at 80, 150 and 210 days after transplanting. Essential oil was obtained by hydro-distillation and expressed as ml 100 g−1 fresh herb. Chemical composition of the EO was determined using liquid chromatography linked to mass spectrometry (GC–MS). The results showed that Marrubium plants irrigated every 4 days and harvested at 210 days gave the highest plant height, number of branches and fresh weight, whereas their lowest values were obtained when harvested at 210 days and irrigated every 12 days. On the other hand, the EO content of Marrubium plants irrigated every 12 days and harvested at 80 days was the highest, and the lowest was obtained when harvested at 210 days and irrigated every 4 days. Eighty compounds were identified in the essential oil of Marrubium vulgare. Thymol (29.6–60.7%), carvacrol (0.5–19.3%), m-cymene (1.0–14.2%), γ-terpinene (1.1–12.1%), thymol methyl ether (0.4–10.4%) and α-himachalene (0.0–10.3%) were the major marker compounds, whereas 14 compounds were minor and 60 compounds were considered as traces. In summary, water stress and harvesting frequency affected growth and caused quantitative changes in the essential oil components. It can be concluded that irrigating Marrubium vulgare plants every 4 days and harvesting at 210 days from the transplanting date is essential to maximize production, whereas irrigating every 12 days and harvesting 80 days after transplanting is recommended to maximize essential oil content.