Yield, Essential Oil Content, and Quality Performance of <i>Lavandula angustifolia</i> Leaves, as Affected by Supplementary Irrigation and Drying Methods

In the present study, we investigated the irrigation of <i>L. angustifolia</i> plants and drying temperatures on the yield of dry leaves and lavender essential oil. Plants were irrigated using an on-surface system with drip lines. Plants without additional irrigation were the control object. Each dose of water consisted of 15 mm. The total amount of water used for irrigation in 2016 and 2017 was 90 L·m⁻². The plant raw material was dried using two methods: in natural conditions and convectively. Natural drying was performed in a shaded room at a temperature of 20–22 °C for five days. The convective drying process was carried out in a drying oven in a stream of air at 35 °C, flowing parallel to the layer being dried at 0.5 m·s⁻¹. Under the influence of irrigation, there was an increase in the yield of fresh and airdried leaves and a higher content of essential oil (EO) than in the cultivation without irrigation. The EO obtained from irrigated plants was characterized by higher contents of caryophyllene oxide (9.08%), linalool (7.87%), and β-caryophyllene (4.58%). In nonirrigated crops, α-muurolol (19.67%), linalyl acetate (15.76%), borneol (13.90%), γ-cadinene (8.66%), camphor (2.55%) had a higher percentage in the EO. After drying under natural conditions, the airdried herb yield and leaf yield of lavender were higher by 25% and 17%, respectively, as compared to the raw material dried at 30 °C. Higher drying temperatures (30 °C) increased the EO by 18% on average and total phenolic acid (TPA) by 50%. The plant material dried at 30 °C, with a larger amount of TPA, showed higher antioxidant activity (AA) in the 2,2-diphenyl-1-picrylhydrazyl (DPPH) tests. Linalyl acetate (15.76%) and linalool (7.87%) were predominant in the EO extracted from the oven-dried herb. Drying under natural conditions resulted in a decreased content of linalyl acetate (0.89%), β-caryophyllene (0.11%), linalool (1.17%), and camphor (1.80%) in comparison with thermal drying. Linalool, linalyl acetate, and β-caryophyllene had a higher percentage in the EO extracted from the raw material obtained from irrigated and oven-dried plants, whereas camphor was found to have a larger percentage in the case of the EO from nonirrigated plants. Our study reveals that there are prospects for the practical use of irrigation in lavender cultivation and of the raw material preservation method in order to modify the EO content and chemical composition.