agriculture Article Yield, Essential Oil Content, and Quality Performance of Lavandula angustifolia Leaves, as Affected by Supplementary Irrigation and Drying Methods Andrzej Sałata , Halina Buczkowska * and Renata Nurzy ´nska-Wierdak Department of Vegetable and Medicinal Plants, University of Life Sciences in Lublin, 20-950 Lublin, Poland; [email protected] (A.S.); [email protected] (R.N.-W.) * Correspondence: [email protected]; Tel.: +48-81-445-6964 Received: 29 October 2020; Accepted: 17 November 2020; Published: 29 November 2020 Abstract: In the present study, we investigated the irrigation of L. angustifolia 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. Keywords: Lavandula angustifolia; essential oil constituents; irrigation; drying methods; DPPH radical scavenging activity 1. Introduction Narrow-leaved lavender (L. angustifolia) belongs to the family Lamiaceae and is an aromatic plant that is widely grown for essential oil production or as an ornamental plant. The lavender oil is extracted, at an amount of about 3% [1], by steam distillation mainly from flowers, but also from leaves [2]. Agriculture 2020, 10, 590; doi:10.3390/agriculture10120590 www.mdpi.com/journal/agriculture Agriculture 2020, 10, 590 2 of 19 Lavender oil can contain more than 100 various constituents, predominantly terpene compounds. The main compounds found in the oil distilled from flowers are as follows: linalyl acetate, linalool, and γ-cadinene [3,4]. In the lavender oil obtained from leaves, on the other hand, the following are predominant: p-cymen-8-ol, borneol, lavandulol, o-cymene, bornyl acetate, (E)-caryophyllene, eucalyptol, and γ-cadinene [5]. The literature reveals that lavender exhibits antimicrobial activity [6–9]. The lavender oil is used in medicine, including in the treatment of digestive disorders, migraine, arthritis, skin diseases, airway infections, and as a sedative [10,11]. Moreover, it stimulates bile secretion and has analgesic and relaxant effects [12]. Lavender oil content and composition depend on many factors: differences between individual varieties and their hybrids, agronomic factors, and the processing and storage of raw plant materials. Broad research has been conducted to determine yield, yield components, and essential oil content and composition [13–15], as well as fertilization and crop density under different organic conditions [16]. Only a few scientific publications have dealt with the irrigation of lavender plants [17]. In the light of existing research, plant response to water deficit-induced stress is a very complex phenomenon. Plant response to drought stress largely depends on plant resistance to drought, which is a species-specific or even cultivar-specific trait, and also on environmental conditions. Many papers indicate that, under soil water deficit conditions the essential oil content in various Lamiaceae species usually tends to decrease: Mentha arvensis [18], Salvia officinalis [19], and Ocimum basilicum [20,21]. Water deficit decreases the oil yield of Rosmarinus officinalis [22,23] and mboxO. basilicum [22,24]. Okwany et al. [25] reported that deficit irrigation usually entails the risk of a negative impact on crop yield and product quality. A mild water deficit, in turn, can lead to increased essential oil content, which has been observed in Salvia. officinalis [26], Satureja hortensis [27], and O. basilicum [22,28]. The highest yield of the herb O. basilicum was obtained when irrigation treatment increased to 125% FC, but the highest essential production was found in 50% FC [22]. The benefits flowing from irrigation of herbal crops have long been documented in the literature [28,29]. The basil essential oil yield was higher in irrigated than nonirrigated crops in the first harvest [30]. Fresh herbal materials are perishable due to their high water content (70–80%). Drying, as a method of preservation of herbal raw materials, inhibits the growth of microorganisms and prevents biochemical changes [31]. The drying process can contribute to a decreased amount of essential oil and to changes in its composition, as has been demonstrated in numerous studies on various species: Laurus nobilis L. [32], L. angustifolia [5,33], O. basilicum [34], R. officinalis [35], S. officinalis [36,37], Thymus daenensis [38], Melissa officinalis [39], T. vulgaris [33], Artemisia dracunculus [40], and Mentha. longifolia [41]. Changes have been observed to occur in the chemical composition and proportions of individual oil constituents in different species after drying—for example, eugenol in L. nobilis leaves [32,42] and thymol in the herb of T. vulgaris [33,38]. In most cases of essential oil plant species, the maintenance of temperature below 30–35 ◦C during the drying process results in the preservation of a larger number of aromatic compounds [43,44]. In medicine, the oil isolated from lavender flowers is only used [45]. Modern research reveals that oil can also be extracted from lavender leaves, which are treated as production waste in industrial essential oil production. It has been confirmed that the essential oil distilled from lavender leaves exhibits unique biological activity despite containing terpene compounds at a lower concentration. For instance, Łyczko et al. [5] report that a high percentage of camphor in the essential oil distilled from lavender leaves is an important characteristic of its quality. In the present study, we investigated the effect of supplementary irrigation and drying method on the yields and quality characteristics of L. angustifolia EO distilled from the leaves. Agriculture 2020, 10, 590 3 of 19 2. Materials and Methods 2.1. Description of the Station’s Location Agronomic experiences were conducted in 2016–2017 at a research station of the University of Life Sciences in Lublin located in southeastern Poland (51.23◦ N, 22.56◦ E). Determination of the chemical composition was made at the Department of Vegetable and Herb Crops, University of Life Sciences in Lublin. 2.2. Experimental Design and Management Practices The experimental material consisted of the lavender (L. angustifolia Mill.) variety “Hidcote Blue Strain.” Seeding material was obtained from PNOS (O˙zarów Mazowiecki, Poland). The experiment investigating the effect of irrigation on fresh herb yield was a single-factor one. The experimental factor was crop irrigation with a drip line, while crops grown without additional irrigation were the control treatment. The experiment regarding the yield of airdried herb (without inflorescences), as well as the chemical composition of raw material and its EO content, was a two-factor one. The experimental factors were crop irrigation (crops without additional irrigation were the control treatment) and the drying method of lavender: in natural conditions or convective drying in a drying oven. The two-factor experiment was set up as a split-plot design with four replicates. The area of each plot was 8.0 m2 (2.0 m 4.0 m). Lavender was grown from transplants at a × spacing of 45 cm 45 cm. Forty lavender plants were grown per replicate in each treatment. × Crops were grown on luvisol derived from medium silty loam, which contained, in the 0–20 cm layer (in %): sand, 35.2; clay, 25.8; loam, 39; organic matter, 1.6; Ca, 4.5; total N, 0.68; P, 1.2; K, 1.8; and Mg, 0.9. The pH in KCl was 6.7. To produce transplants, seeds were sown in a greenhouse in plug trays filled with peat substrate (the volume of a single pot was 90 cm3) in the first 10 days of April in 2016 and 2017.