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

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.

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].

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 [6,7,8,9]. 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 [13,14,15], 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 O. 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.

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 (Ożarów 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. Plants were fertilized twice with a 0.1% solution of Florovit. Lavender plants were planted in the field on 5 May 2016 and 8 May 2017.

Before the start of the experiment, the macronutrient content in the field was replenished to the following levels (in mg·dm⁻³): 120 N; 80 P; 200 K; 60 Mg. During the growing season, necessary crop management operations were carried out (several manual weed removals) and the crops were fertilized twice, with nitrogen applied as ammonium nitrate with 34% N (a single dose of about 7 kg N·ha⁻¹). No crop protection chemicals were used during the cultivation period.

Plants were irrigated using an on-surface system with drip lines (T-Tape 508-20-400), placed next to plant rows. In the period of water scarcity, a drip line with a capacity of 4.0 dm³·m·h⁻¹, at a working pressure of 1.5 bar, was used. Irrigation was applied when the value of the soil water potential at a depth of 25–30 cm was equal to or less than −20 kPa. The value of water potential in the soil was measured using a tensiometer (Irrometer Company Inc., Riverside, CA, USA). Each dose of water consisted of 15 mm. The total amount of water used for irrigation in 2016 and 2017 year was 90 L·m².

2.3. Raw Material Collection and Post-Harvest Treatments

Plant material was collected from one-year-old plants. The raw material was harvested once from plants irrigated additionally with a drip line and from nonirrigated ones. Over the experimental period, the lavender (leaves after harvest inflorescence for another experiment) was collected on 12 September, at the beginning of plant flowering (in the experiment, 45% of all plants produced inflorescence stems). From the plants that had produced inflorescence stems, leaves were collected separately (from five randomly selected plants). Fresh herb yield (kg·m⁻²) was calculated based on the weight of the leaves. The lavender raw material was collected by hand using a knife, cutting the herb 3 cm above ground level. Immediately after harvest, drying samples were prepared, separately for irrigated plants and for those without additional irrigation, maintaining the separation between fresh herb yield. On the basis of the weight of herb from five plants, after they had been dried, the yield of airdried herb (g·m⁻²) was calculated. Having been dried, the stems were rubbed through sieves to separate leaves from stems. Based on the weight of airdried leaves, the leaf yield was calculated (g·m⁻²).

The plant raw material was dried using two methods: 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. During drying, the leaves took up 2–2.5 kg·m⁻² in area. The drying process was carried out in complete darkness. After drying, the leaves contained 12–14% water in five successive measurements. Drying of the raw material consisted of gradually increasing the temperature by 5 °C each time, to finish at 35 °C, with the fans open. The conditioning process lasted 24 h in order to get rid of the water residue, with the fans closed.

Next, 0.25-kg samples were made from airdried leaves and, after grinding them for laboratory analysis, the plant material was kept in airtight containers. In airdried lavender leaves, the content of total phenolic acids, expressed as caffeic acid equivalents [46], as well as the content of LEO and its composition [47], was determined. The antioxidant activity of the compounds was also determined 2,2-diphenyl-1-picrylhydrazyl (DPPH).

2.4. Sample Preparation and Analyses

2.4.1. Extract Preparation

Three samples (1 g) of airdried leaves from each treatment were milled with 10 mL methanol (80%) [48]. The extract was then centrifuged (Rotofix 32 A, Hettich Zentrifugen, Kirchlengern, Germany) for 30 min, 5000 rev./min. at 4000× g. After centrifugation, the supernatant was transferred to a 15-mL falcon tube, and stored at 4 °C until further analyses for phenolic and total antioxidant activity.

2.4.2. Total Phenolic Acid Content

To a 10 mL measuring test-tube, 1.0 mL of water extract was added, as well as 1 mL of hydrochloric acid (18 g L⁻¹), 1 mL of Arnov’s reagent, and 1 mL of sodium hydroxide (40 g L⁻¹), and the mixture was topped up with water to 10 mL (solution A). Then the solution absorbance was measured at 490 nm, applying a mixture of reagents without the extract as a reference. The content of phenolic acids (%) was determined in conversion to caffeic acid (C₉H₂O₄), assuming absorbability $a\frac{l\%}{lcm}=285$, according to the formula: $X=\frac{A\times 3.5087}{m}$, where A means absorbance of solution A, and m is a weighted sample of raw material in g.

2.4.3. DPPH Radical Scavenging Activity Assay

DPPH radical scavenging activity was expressed as % of DPPH inhibition. The determination was performed according to the method given by Yen and Chen [49], and the calculation of DPPH inhibition was made according to the formula given by Rossi et al. [50]: % DPPH = $100-|\frac{At}{Ar}\times 100|$. To prepare a reagent containing a solution of radicals, 0.012 g DPPH (2,2’-diphenyl-1-picrylhydrasyl) was weighed out, transferred to a measuring flask with a capacity of 100 mL, filled up with methanol (100%), and then dissolved in an ultrasound washer for 15 min. The blind assay (Ar) was prepared as follows: 1 mL of distilled water was measured out into a test tube (pH > 5), as well as 3 mL of methanol (100%) and 1 mL of DPPH solution. After 10 min of stirring, it was read on a spectrophotometer at 517 nm, against methanol (100%). To perform the examined assay (At), 1 mL of a sample was diluted in methanol and 3 mL of methanol (100%) were added, as well as 1 mL of DPPH solution. The sample was stirred and after 10 min it was read on a spectrophotometer at 517 nm, against methanol (100%).

2.4.4. Essential Oil Distillation

The dried plant material, after samples had been weighed out (20 g each), was placed in glass flasks with a capacity of 1 dm³, covered with 400 mL of water, and set for distillation in Clevenger-type apparatuses for 3 h, counting from the moment when the contents of the flask started to boil and the first drop was distilled. The intensity of heating was regulated in such a way that 3–4 mL of liquid fell into the receiver per minute. After distillation had finished, cooling was switched on, and after 30 min the result was read.

2.4.5. Essential Oil Composition

The quantitative and qualitative composition of lavender oil obtained from the leaves, buds, and flowers was determined with the use of the gas chromatography and mass spectrometry methods (GC-MS). For our studies we used a Varian 4000 MS/MS apparatus with VF-5 m column (an equivalent of DB-5), a registered range of 40–1000 m/z, and a scanning speed of 0.8 s/scan. The carrier gas was helium, at a steady flow of 0.5 mL min⁻¹. The temperature of the batcher was 250 °C and the temperature gradient of 50 °C was applied for 1 min, then increased to 250 °C at a speed of 4 °C min⁻¹ and 250 °C for 10 min. Split 1:1000 m/z, 1 μL of solution was dosed (10 μL of assay in 1000 μL of hexane). Nonisothermal Kovacs’ retention indexes were determined on the basis of a range of alkanes C₁₀–C₄₀. The qualitative analysis was carried out on the basis of the MS Spectral Library (2008). The identity of the compounds was confirmed by their retention indices, taken from the literature [51] and our own data.

2.5. Statistical Analysis

The results were statistically analyzed with two-way analysis of variance (ANOVA), based on a factorial combination of irrigation × drying methods. Means were separated by the least significance difference (LSD) test, when the F-test was significant. Data were evaluated by HSD Tukey test at p < 0.05. All calculations and analyses were performed using Statistica 10.0 PL software (StatSof Inc., Tulsa, OK, USA).

3. Results

A significant decrease in yield was found in lavender crops without irrigation compared to plants grown using supplementary irrigation (

Table 1. Effect of irrigation on the yield of fresh herbs and water content.

Treatments		* YFH a (kg·m2)	% Water Content
Irrigation	WI	1.37 ± 0.46 a	78.62 ± 6 a
	NI	0.69 ± 0.20 b	66.53 ± 5 b
Year	2016	1.20 ± 0.16 a	76.59 ± 4 a
	2017	0.86 ± 0.25 b	71.87 ± 4 b
Irrigation (I) × Year (Y)
WI × 2016 WI × 2017 NI × 2016 NI × 2017		1.68 ± 0.48 a	81.25 ± 6 a
		1.07 ± 0.15 b	74.58 ± 5 b
		0.72 ± 0.24 bc	65.56 ± 8 c
		0.66 ± 0.16 c	67.73 ± 4 c
Mean		2.03 ± 42	72.28 ± 5

^a YFH = yield of fresh herbs; WI = with irrigation; NI = no irrigation; * Different letters within each column and main factor indicate significant differences (p < 0.05).

). In 2016 a higher herb yield and a higher airdried leaf yield were obtained than in 2017.

The year 2016 was characterized by a greater number of sunshine hours and a relatively low amount of rainfall during the period of intensive growth of lavender plants (

Table 2. Climatic conditions during experiment in 2016–2017.

Year	Month	Temperature (°C)			Precipitation (mm)	Total Insolation (h)
		Average Maximum	Average Minimum	Average Diurnal
2016	May	19.2	8.2	14.3	38	222
	June	22.4	13.0	18.6	43	205
	July	22.0	14.7	18.4	130	170
	August	24.5	13.4	18.8	71	202
	September	22.1	12.1	15.2	11	169
	Average/Total	22.0	12.3	17.1	59/293	194/968
2017	May	20.6	8.5	14.2	29	198
	June	24.1	13.3	18.6	28	222
	July	23.9	14.5	19.0	108	185
	August	24.5	13.6	20.0	48	201
	September	21.3	10.2	14.0	77	103
	Average/Total	22.9	12.0	17.2	58/290	182/909

). The use of supplementary irrigation in 2016 caused a significant increase in herb yield and leaf yield, but had no effect on the water content in the herb (Table 1).

The raw material drying method had a much greater (50.0%) impact on lavender herb yield than irrigation (31.8%) (

Table 3. Mean square per source of variation (percentage of total) resulting from analysis of variance.

Source of Variation	Degrees of Freedom	YDH a	YDL b	EO c	TPA d	DPPH e
Irrigation (I)	1	31.8 *	36.8 *	28.0 *	4.4 *	48.7 *
Drying (D)	1	50.0 *	46.7 *	14.3 *	85.1 *	36.8 *
Year (Y)	1	12.6 *	13.1 *	1.4 *	1.1 *	10.5 *
Y × I	2	0.1 NS	0.5 NS	0.3 *	1.3 *	0.5 *
Y × D	2	0.7 NS	0.0 NS	0.7 *	1.1 NS	0.1 NS
I × D	2	1.4 NS	2.8 *	55.2 *	7.1 *	3.1 *
Y × I × D	3	3.4 *	0.1 NS	0.0 NS	0.0 NS	0.4 *
Total mean square		217,055	44,157	2.8	1.8	9448

^a YDH = yield of dry herb; ^b YDL = yield of dry leaves; ^c EO = essential oil; ^d TPA = total phenolic acids; ^e DPPH = antioxidant activity by DPPH inhibition; * indicates significance at p < 0.05; NS, not significant.

).

Under water deficit conditions (without supplementary irrigation), the EO content was found to decrease by 25% on average compared to the treatment with irrigation (

Table 4. Effect of irrigation and drying method on the yield of airdried herbs, yield of airdried leaves, and chemical constituents of lavender leaves and its antioxidant activity (AA).

Treatments *		** YDH a (g·m2)	YDL b (g·m2)	EO c (mg 100 g−1)	TPA d (%)	AA by DPPH e Inhibition (%)
Irrigation	WI	293 ± 5 a	190 ± 25 a	0.88 ± 0.22 a	0.466 ± 0.19 a	57 ± 6 a
	NI	231 ± 6 b	160 ± 20 b	0.67 ± 0.09 b	0.399 ± 0.14 b	41 ± 5 b
Drying method	O	223 ± 5 b	158 ± 16 b	0.85 ± 0.26 a	0.580 ± 0.11 a	56 ± 7 a
	N	301 ± 4 a	192 ± 25 a	0.70 ± 0.05 b	0.284 ± 0.06 b	42 ± 8 b
Year	2016	281 ± 7 a	184 ± 27 a	0.80 ± 0.19 a	0.441 ± 0.16 a	53 ± 10 a
	2017	242 ± 5 b	166 ± 25 b	0.75 ± 0.20 b	0.423 ± 0.18 a	45 ± 10 b
Irrigation (I) × Year (Y)
WI × 2016		315 ± 4 a	201 ± 24 a	0.89 ± 0.24 a	0.493 ± 0.18 a	60 ± 5 a
WI × 2017		272 ± 5 b	180 ± 23 b	0.87 ± 0.06 a	0.438 ± 0.21 ab	55 ± 7 b
NI × 2016		249 ± 5 b	167 ± 19 bc	0.71 ± 0.22 b	0.389 ± 0.13 b	46 ± 10 c
NI × 2017		213 ± 6 b	153 ± 20 c	0.63 ± 0.10 c	0.409 ± 0.15 b	37 ± 9 d
Drying method (D) × Year (Y)
O × 2016		238 ± 7 c	167 ± 19 c	0.89 ± 0.24 a	0.573 ± 0.13 a	60 ± 5 a
O × 2017		208 ± 3 c	150 ± 14 c	0.81 ± 0.28 b	0.588 ± 0.08 a	53 ± 8 b
N × 2016		325 ± 3 a	202 ± 26 a	0.71 ± 0.05 c	0.310 ± 0.06 b	46 ± 10 c
N × 2017		277 ± 4 b	183 ± 23 b	0.70 ± 0.04 c	0.259 ± 0.06 b	39 ± 10 d
Irrigation (I) × Drying method (D)
WI × O		261 ± 4 b	170 ± 13 b	1.10 ± 0.02 a	0.657 ± 0.02 a	62 ± 3 a
WI × N		325 ± 2 a	212 ± 15 a	0.66 ± 0.02 c	0.275 ± 0.04 c	53 ± 5 b
NI × O		186 ± 4 c	148 ± 13 c	0.60 ± 0.06 d	0.504 ± 0.11 b	50 ± 6 b
NI × N		276 ± 5 b	173 ± 19 b	0.75 ± 0.03 b	0.294 ± 0.084 c	33 ± 5 c
Year (Y) × Irrigation (I) × Drying method (D)
2016 × WI × O 2016 × WI × N 2016 × NI × O 2016 × NI × N 2017 × WI × O 2017 × WI × N 2017 × NI × O 2017 × NI × N		288 ± 5 ab	179 ± 8 c	1.13 ± 0.01 a	0.670 ± 0.03 a	64 ± 3 a
		314 ± 2 a	223 ± 5 a	0.66 ± 0.01 d	0.316 ± 0.01 c	56 ± 2 bc
		189 ± 4 d	155 ± 10 de	0.66 ± 0.03 d	0.475 ± 0.11 b	55 ± 2 c
		308 ± 4 a	180 ± 17 c	0.76 ± 0.03 c	0.303 ± 0.08 c	36 ± 2 e
		234 ± b cd	159 ± 9 d	1.08 ± 0.01 b	0.644 ± 0.01 a	60 ± 2 ab
		310 ± 11 a	200 ± 12 b	0.66 ± 0.02 d	0.233 ± 0.01 c	48 ± 2 d
		183 ± 4 d	140 ± 9 e	0.54 ± 0.01 e	0.533 ± 0.09 b	45 ± 2 d
		244 ± 4 bc	166 ± 18 cd	0.73 ± 0.03 c	0.284 ± 0.08 c	28 ± 2 f
Mean		262 ± 5	175 ± 27	0.77 ± 0.02	0.432 ± 0.17	49 ± 11

* WI = with irrigation; NI = no irrigation; O = oven; N = natural; ^a YDH = yield of dry herb; ^b YDL = yield of dry leaves; ^c EO = essential oil; ^d TPA = total phenolic acids; ^e AA = antioxidant activity. ** Different letters within each column and main factor indicate significant differences (p < 0.05).

). The decrease in EO content was attributable to a reduction in the dry weight under soil water deficit conditions. The lavender plant material contained total phenolic acid (TPA) at a concentration of 0.23–0.67%. More TPA was found in the herb obtained from plants grown with irrigation. The higher TPA content was associated with the high AA and reducing activity in DPPH tests. Irrigation affected the amount and composition of EO to a small degree (28%; Table 3). The application of supplementary irrigation resulted in an increased percentage of linalool and β-caryophyllene in the EO, but did not affect linalyl acetate (

Table 5. Effects of irrigation and drying method on the linalool, borneol, camphor, and linalyl acetate contents of essential oils obtained from L. angustifolia leaves.

Treatments *		Linalool **	Borneol	Camphor	Linalyl Acetate
Irrigation	WI	4.51 ± 0.26 a	9.72 ± 0.90 b	1.88 ± 0.13 b	7.35 ± 0.67 b
	NI	3.23 ± 0.17 b	11.98 ± 1.88 a	2.11 ± 0.33 a	7.54 ± 0.71 a
Drying method	O	5.95 ± 0.13 a	9.66 ± 0.86b	2.20 ± 0.22 a	14.26 ± 0.97 a
	N	1.79 ± 0.04 b	12.04 ± 1.83 a	1.78 ± 0.11 b	0.63 ± 0.16 b
Year	2016	4.07 ± 0.28 a	11.24 ± 1.75 a	2.05 ± 0.26 a	7.86 ± 0.72 a
	2017	3.68 ± 0.16 b	10.46 ± 1.90 b	1.94 ± 0.28 b	7.04 ± 0.66 b
Irrigation (I) × Year (Y)
WI × 2016		4.81 ± 0.32 a	10.29 ± 0.48 c	1.92 ± 0.15 c	7.88 ± 0.72 a
WI × 2017		4.22 ± 0.18 b	9.15 ± 0.87 d	1.84 ± 0.09 c	6.83 ± 0.63 d
NI × 2016		3.33 ± 0.22 c	12.19 ± 2.04 a	2.18 ± 0.28 a	7.83 ± 0.74 b
NI × 2017		3.14 ± 0.12 d	11.76 ± 1.75 b	2.04 ± 0.37 b	7.26 ± 0.70 c
Drying method (D) × Year (Y)
O × 2016		6.71 ± 1.12a	10.07 ± 0.48 c	2.25 ± 0.21 a	14.98 ± 0.63 a
O × 2017		5.21 ± 0.88b	9.24 ± 0.96 d	2.16 ± 0.25 b	13.55 ± 0.67 b
N × 2016		1,44 ± 0.26d	12.41 ± 1.78 a	1.84 ± 0.10 c	0.73 ± 0.14 c
N × 2017		2.15 ± 0.26c	11.67 ± 1.85 b	1.73 ± 0.09 d	0.53 ± 0.12 c
Irrigation (I) × Drying method (D)
WI × O		6.99 ± 0.97 a	9.10 ± 0.78 c	1.99 ± 0.07 b	13.94 ± 0.99 b
WI × N		2.04 ± 0.37 c	10.22 ± 0.49 b	1.77 ± 0.05 c	0.76 ± 0.11 c
NI × O		4.92 ± 0.62 b	10.34 ± 0.50 b	2.42 ± 0.11 a	14.59 ± 0.85 a
NI × N		1.55 ± 0.38 c	13.73 ± 0.75 a	1.80 ± 0.15 c	0.50 ± 0.09 c
Year (Y) × Irrigation (I) × Drying method (D)
2016 × WI × O		7.94 ± 0.13 a	9.85 ± 0.16 d	2.06 ± 0.03 b	14.89 ± 0.25 a
2016 × WI × N		1.68 ± 0.03 g	10.73 ± 0.19 c	1.77 ± 0.03 d	0.87 ± 0.01 d
2016 × NI × O		5.47 ± 0.32 c	10.30 ± 0.60 cd	2.43 ± 0.14 a	15.07 ± 0.88 a
2016 × NI × N		1.18 ± 0.05 h	14.08 ± 0.66 a	1.92 ± 0.09 c	0.59 ± 0.02 d
2017 × WI × O		6.05 ± 0.05 b	8.34 ± 0.07 e	1.92 ± 0.01 c	13.00 ± 0.11 c
2017 × WI × N		2.39 ± 0.09 e	9.95 ± 0.41 d	1.77 ± 0.07 d	0.66 ± 0.02 d
2017 × NI × O		4.35 ± 0.15 d	10.15 ± 0.36 cd	2.40 ± 0.08 a	14.10 ± 0.50 b
2017 × NI × N		1.91 ± 0.10 f	13.38 ± 0.71 b	1.68 ± 0.08 d	0.41 ± 0.02 d
Mean		3.87 ± 0.38	10.85 ± 1.85	1.99 ± 0.28	7.45 ± 0.68

* WI = with irrigation; NI = no irrigation; O = oven; N = natural. ** Different letters within each column and main factor indicate significant differences (p < 0.05).

).

In lavender crops without supplementary irrigation, from the group of monoterpenes more borneol, camphor, and linalyl acetate were determined in the lavender EO, but less linalool, whereas as far as the group of sesquiterpenes is concerned, more γ-cadinene, caryophyllene oxide, and α-muurolol were found, but less β-caryophyllene (Table 5 and

Table 6. Effect of irrigation and drying method on the β-caryophyllene, γ-cadinene, caryophyllene oxide, and α-muurolol of essential oils obtained from L. angustifolia leaves.

Treatments *		β-Caryophyllene **	γ-Cadinene	Caryophyllene Oxide	α-Muurolol
Irrigation	WI	2.32 ± 0.17 a	4.87 ± 0.64 b	5.96 ± 0.78 b	13.94 ± 3.20 b
	NI	1.27 ± 0.08 b	7.13 ± 0.18 a	6.39 ± 0.62 a	15.25 ± 4.34 a
Drying method	O	3.39 ± 0.09 a	5.18 ± 0.95 b	5.79 ± 0.67 b	10.93 ± 0.48 b
	N	0.19 ± 0.00 b	6.82 ± 0.14 a	6.57 ± 0.70 a	18.26 ± 1.48 a
Year	2016	1.82 ± 0.09 a	6.24 ± 0.62 a	6.97 ± 0.32 a	14.77 ± 4.10 a
	2017	1.76 ± 0.08 b	5.76 ± 0.30 b	5.39 ± 0.81 b	14.43 ± 3.62 b
Irrigation (I) × Year (Y)
WI × 2016		2.36 ± 0.23 a	5.02 ± 0.57 c	7.24 ± 0.75 a	13.92 ± 3.58 c
WI × 2017		2.28 ± 0.20 b	4.71 ± 0.68 d	4.69 ± 0.24 d	13.96 ± 2.88 c
NI × 2016		1.28 ± 0.12 c	7.46 ± 0.14 a	6.69 ± 0.59 b	15.61 ± 4.50 a
NI × 2017		1.25 ± 0.09 c	6.80 ± 0.84 b	6.09 ± 0.50 c	14.86 ± 4.28 b
Drying method (D) × Year (Y)
O × 2016		3.54 ± 0.11 a	5.30 ± 0.90 c	5.88 ± 0.43 b	10.87 ± 0.60 c
O × 2017		3.25 ± 0.10 b	5.05 ± 0.02 d	5.70 ± 0.85 b	11.00 ± 0.34 c
N × 2016		0.10 ± 0.00 d	7.17 ± 0.16 a	8.05 ± 0.94 a	18.66 ± 1.46 a
N × 2017		0.28 ± 0.00 c	6.46 ± 0.11 b	5.08 ± 0.65 c	17.86 ± 1.43 b
Irrigation (I) × Drying method (D)
WI × O		4.46 ± 0.01 a	4.26 ± 0.21 d	5.21 ± 0.34 c	10.82 ± 0.41 c
WI × N		0.18 ± 0.00 c	5.47 ± 0.19 c	6.36 ± 0.33 b	17.06 ± 0.63 b
NI × O		2.32 ± 0.01 b	6.09 ± 0.29 b	6.71 ± 0.29 a	11.05 ± 0.54 c
NI × N		0.21 ± 0.01 c	8.17 ± 0.73 a	6.42 ± 0.83 b	19.46 ± 1.05 a
Year (Y) × Irrigation (I) × Drying method (D)
2016 × WI × O		4.62 ± 0.07 a	4.46 ± 0.07 e	5.54 ± 0.09 d	10.45 ± 0.17 d
2016 × WI × N		0.10 ± 0.00 f	5.57 ± 0.10 d	8.94 ± 0.16 a	17.40 ± 0.32 c
2016 × NI × O		2.46 ± 0.14 c	6.15 ± 0.36 c	6.21 ± 0.36 c	11.30 ± 0.57 d
2016 × NI × N		0.11 ± 0.00 f	8.77 ± 0.41 a	7.17 ± 0.33 b	19.93 ± 0.93 a
2017 × WI × O		4.31 ± 0.03 b	4.07 ± 0.03 f	4.89 ± 0.04 e	11.20 ± 0.10 d
2017 × WI × N		0.25 ± 0.00 e	5.36 ± 0.22 d	4.49 ± 0.18 f	16.72 ± 0.69 c
2017 × NI × O		2.18 ± 0.07 d	6.03 ± 0.21 c	6.51 ± 0.23 c	10.80 ± 0.38 d
2017 × NI × N		0.31 ± 0.00 e	7.56 ± 0.40 b	5.56 ± 0.30 d	18.99 ± 1.00 b
Mean		1.78 ± 0.08	6.00 ± 1.48	6.18 ± 1.34	14.60 ± 3.84

* WI = with irrigation; NI = no irrigation; O = oven; N = natural. ** Different letters within each column and main factor indicate significant differences (p < 0.05).

). The use of supplementary irrigation in growing lavender crops had a significant effect on the percentage of borneol (39.5%) and γ-cadinene (60.8%) in the lavender EO (

Table 7. Mean square per each source of variation (percentage of total) resulting from analysis of variance of linalool, borneol, camphor, and linalyl acetate.

Source of Variation	Degree of Freedom	Linalool	Borneol	Camphor	Linalyl Acetate
Irrigation (I)	1	7.8 *	39.5 *	18.3 *	0.0 *
Drying (D)	1	82.2 *	43.9 *	61.7 *	99.2 *
Year (Y)	1	0.7 *	4.7 *	3.7 *	0.3 *
Y × I	2	0.1 *	1.0 *	0.1 NS	0.0 *
Y × D	2	5.8 *	0.0 NS	0.0 NS	0.2 *
I × D	2	3.0 *	9.9 *	13.4 *	0.1 *
Y × I × D	3	0.1 *	0.7 *	2.5 *	0.0 *
Total mean square		379.1	231.6	5.1	3369.4

* indicate significance at p < 0.05; NS, not significant.

and

Table 8. Mean square per source of variation (percentage of total), resulting from analysis of variance.

Source of Variation	Degree of Freedom	β-Caryophyllene	γ-Cadinene	Caryophyllene Oxide	α-Muurolol
Irrigation (I)	1	8.8 *	60.8 *	2.6 *	3.0 *
Drying (D)	1	81.3 *	31.9 *	8.6 *	93.9 *
Year (Y)	1	0.0 *	2.7 *	35.8 *	0.2 *
Y × I	2	0.0 NS	0.3 *	13.7 *	0.2 *
Y × D	2	0.4 *	0.6 *	28.1 *	0.3 *
I × D	2	9.3 *	2.2 *	7.5 *	2.0 *
Y × I × D	3	0.0 NS	1.2 *	3.5 *	0.1 NS
Total mean square		226.7	151.3	125.2	1028.1

* indicates significance at p < 0.05; NS, not significant.

).

GC/MS analysis allowed for identifying 98.09–99.63% of constituents in the EO oil (

Table 9. Effect of irrigation and drying method on the chemical fraction of the essential oils from L. angustifolia plants depending on irrigation and drying method (%).

No.	Compound	RI **	WI *		NI
			O	N	O	N
1	Cumene	926	0.14	0.13	0.10	0.07
2	α-Pinene	933	1.40	0.78	1.39	0.58
3	Camphene	950	0.83	0.97	0.80	0.87
4	Verbenene	968	0.29	0.27	0.20	-
5	Sabinene	973	0.57	0.44	0.27	0.34
6	β-Pinene	978	3.50	1.77	3.70	1.97
7	1-Octen-3-ol	980	0.13	0.25	0.13	0.25
8	3-Octanone	986	0.24	-	0.18	-
9	Myrcene	990	0.55	0.31	0.50	0.28
10	2-δ-Carene	1009	1.85	1.51	1.20	1.01
11	Hexyl acetate	1012	0.11	-	0.19	-
12	p-Cymene	1019	0.88	0.83	0.98	1.13
13	ortho-Cymene	1023	1.61	1.87	1.91	2.07
14	β-Phelandrene	1026	-	0.65	-	0.15
15	Limonene	1027	1.44	3.15	1.74	3.75
16	1,8-Cineole	1030	2.97	5.90	1.97	3.90
17	(Z)-β-Ocimene	1033	0.70	0.18	0.50	0.08
18	(E)-β-Ocimene	1042	0.24	0.08	0.14	0.08
19	trans-Linalool oxide	1064	-	0.22	-	1.22
20	cis-Linalool oxide	1079	0.17	-	1.17	-
21	Linalool	1093	7.87	1.71	5.72	1.17
22	1-Octen-3-yl acetate	1095	0.40	-	1.40	-
23	2Z-Heptenyl acetate	1098	-	0.20	-	-
24	trans-p-Mentha-2,8-dien-1-ol	1100	0.07	0.11	0.17	0.21
25	3-Octanol acetate	1111	-	0.13	-	0.07
26	cis-p-Menth-2-en-1-ol	1113	-	0.26	-	0.16
27	allo-Ocimene	1121	0.11	0.12	0.21	0.08
28	cis-Limone oxide	1123	0.13	0.15	0.23	0.07
29	cis-p-Mentha-2,8-dien-1-ol	1127	-	0.20	-	0.10
30	trans-Sabinol	1132		0.40	-	0.30
31	cis-Sabinol	1136	0.48	0.18	0.58	-
32	Camphor	1143	2.05	1.80	2.55	1.90
33	Pinocarvone	1158	0.99	0.79	0.59	0.59
34	Borneol	1172	9.77	10.90	10.77	13.90
35	Cryptone	1188	1.17	4.57	2.27	3.57
36	γ-Terpineol	1197	2.08	1.09	1.58	1.29
37	Verbenone	1211	0.44	0.67	0.59	1.17
38	cis-Carvenol	1223	-	0.62	-	0.92
39	Nerol	1228	-	0.10	-	1.10
40	Isobornyl formate	1228	-	0.40	-	1.40
41	trans-Chrysanthenyl acetate	1232	0.30	0.10	0.40	-
42	Cumin aldehyde	1247	0.46	2.13	0.26	1.13
43	Carvone	1248	0.28	0.26	0.48	0.26
44	Linalyl acetate	1254	14.76	0.89	15.76	0.59
45	Geraniol	1255	-	0.18	-	0.58
46	Thymoquinone	1269	-	0.06	-	0.06
47	Neryl formate	1284	3.84	2.17	2.04	1.17
48	p-Cymen-7-ol	1297	0.35	0.79	0.25	0.39
49	γ-Terpinen-7-al	1310	0.12	0.21	0.22	0.11
50	Piperitenone	1313	0.31	-	0.21	-
51	trans-Verbenyl acetate	1333	-	0.19	-	0.49
52	Neryl acetate	1361	0.36	1.86	0.56	2.96
53	Linalyl isobutanoate	1381	1.90	-	1.50	-
54	7-epi-Sesquithujene	1389	0.13	-	0.13	-
55	α-Santalene	1420	-	3.16	-	1.16
56	β-Caryophyllene	1425	4.58	0.11	2.58	0.11
57	α-trans-Bergamotene	1439	0.31	0.34	0.21	0.54
58	(Z)-β-Farnesene	1458	1.11	0.42	2.11	0.42
59	α-Humulene	1465	0.06	0.50	-	0.70
60	Germacrene D	1494	0.27	0.12	0.17	0.15
61	(Z)-α-Bisabolene	1516	0.12	0.13	0.42	0.15
62	β-Bisabolene	1520	0.14	0.08	0.34	-
63	γ-Cadinene	1525	4.43	5.66	6.43	8.66
64	trans-Calamenene	1528	0.76	0.85	0.46	-
65	β-Sesquiphellandrene	1534	-	0.74	-	0.94
66	epi-Longipinalol	1559	1.06	1.56	1.56	1.06
67	Caryophyllene oxide	1587	5.50	9.08	6.50	7.08
68	1,10-di-epi-Cubenol	1621	0.88	1.73	0.58	1.73
69	α-Muurolol	1653	10.36	17.67	11.36	19.67
70	Himachalol	1665	-	0.61	-	0.11
71	14-hydroxy-9-epi-(E)-Caryphylene	1670	0.27	0.26	0.17	0.16
72	(Z)-α-Santalol	1687	0.24	0.72	0.14	0.62
73	cis-14-nor-Muurol-5-en-4-one	1703	1.38	1.95	0.78	1.95
Total Identified			98.09	99.13	99.28	99.63
Monoterpenes Hydrocarbons			13.99	13.16	13.36	12.63
Oxygenated Monoterpenes			51.87	39.39	52.05	40.86
Sesquiterpenes Hydrocarbons			12.97	13.67	14.41	13.89
Oxygenated Sesquiterpenes			19.37	33.03	19.67	32.33
Others			0.74	1.01	0.14	1.01

* WI = with irrigation; NI = no irrigation; O = oven; N = natural; ** RI = nonisothermal Kovats retention indices (from temperature programming, using the definition of Van den Dool and Kratz (1963) for the series of n-alkanes (C6–C40)).

). In total, 57 compounds were identified in the raw material from irrigated and oven-dried plants, while in those dried under natural conditions the number was 70. In the treatment without irrigation, 56 compounds were determined in the oven-dried plant material and 60 compounds in the naturally dried raw material. In the EO from the herb dried in the oven, the following compounds were not identified: β-phelandrene, trans-linalool oxide, 3-octanol acetate, cis-p-menth-2-en-1-ol, cis-p-mentha-2,8-dien-1-ol, trans-sabinol, cis-carvenol, nerol, isobornyl formate, geraniol, thymoquinone, trans-verbenyl acetate, α-santalene, β-sesquiphellandrene, and himachalol. Classification of the individual EO constituents showed that the percentage of OM was highest, in the range of 39.39–52.05%, while that of OS was 19.37–33.03%. The percentage of compounds from the HM and HS groups was at a similar level, 12.63–13.99% and 12.97–14.41%, respectively. The EO obtained from plants grown without irrigation contained more OM and less HM than that extracted from nonirrigated plants. The main constituents of the monoterpene group found in the lavender EO were (in decreasing order): borneol (9.77–13.90%), linalyl acetate (0.59–15.76%), linalool (1.17–7.87%), and 1.8-cineole (1.97–5.90%); the percentage of cryptone, β-pinene, limonene, camphor, and neryl formate were in the range 1.17–4.57%. As regards the sesquiterpene group, α-muurolol had the highest percentage (10.36–19.67%), followed by caryophyllene oxide (5.50–9.08%) and γ-cadinene (4.43–8.66%).

In the case of drying under natural conditions, the airdried herb yield and leaf yield of lavender were higher by 25% and 17%, respectively, in comparison with the thermal method of raw material preservation (Table 1). In 2016 and 2017, higher airdried herb and leaf yields were obtained in the treatment with irrigation and drying under natural conditions, while a lower one was obtained from nonirrigated plants dried at a temperature of 35 °C.

The raw material preserved under natural conditions contained 18% less EO on average and 50% less TPA in comparison to oven drying (Table 4). The plant material dried at 30 °C, with a larger amount of TPA, showed higher AA in the DPPH tests.

In the EO distilled from the raw material harvested from irrigated and nonirrigated plants and dried with hot air, the percentage of OM compounds was more than 2.6-fold higher than that of compounds from the OS group (Table 9). When the drying process was carried out under natural conditions, the percentage of oxidized monoterpenes and sesquiterpenes (OM and OS) was similar.

Raw material drying method had a significantly greater effect on the amount of linalyl acetate (99%), linalool (82%), α-muurolol (94%), and β-caryophyllene (81%) than irrigation (Table 7 and Table 8). In oven-dried herb samples, more linalyl acetate and β-caryophyllene (by 95%), linalool (by 70%), and camphor (by 19%) were found, but less α-muurolol (by 67%), γ-cadinene (by 31%), borneol (by 25%), and caryophyllene oxide (by 13%) in comparison with drying under natural conditions. In 2016 and 2017, the percentage of borneol, γ-cardinene, and α-muurolol in nonirrigated and naturally dried plants was higher, while it was distinctly lower in crops grown with irrigation and dried in the oven (Table 5 and Table 6). It is difficult to determine a clear trend in the content of caryophyllene oxide in the EO. In 2016 the percentage of this compound in the EO extracted from irrigated and naturally dried plants was higher, whereas it was lower in irrigated and oven-dried plants. Conversely, in 2017 the percentage of caryophyllene oxide from nonirrigated and thermally dried plants was higher, whereas it was lower in the treatment with irrigation and natural drying.

In the EO from the herb harvested from irrigated and nonirrigated plants that were dried under natural conditions, the following compounds were found to be present, but did not occur in the case of thermal drying, whether WI or NI: β-phelandrene (0.65–0.15%), trans-linalool oxide (0.22–1.22%), 3-octanol acetate (0.13–0.07%), cis-p-menth-2-en-1-ol (0.26–0.16%), cis-p-mentha-2,8-dien-1-ol (0.20–0.10%), trans-sabinol (0.30–0.40%), cis-carvenol (0.62–0.92%), nerol (0.10–1.10%), isobornyl formate (0.40–1.40%), geraniol (0.18–0.58%), thymoquinone (0.06–0.06%), trans-verbenyl acetate (0.19–0.49%), α-santalene (3.16–1.16%), β-sesquiphellandrene (0.74–0.94%), and himachalol (0.61–0.11%) (Table 9). In the case of thermal drying of the herb, the following compounds were found in the EO but not identified under natural drying conditions, whether WI or NI: 3-octanone (0.24–0.18%), hexyl acetate (0.11–0.19%), cis-linalool oxide (0.17–1.17%), 1-octen-3-yl acetate (0.40–1.40%), piperitenone (0.31–0.21%), linalyl isobutanoate (1.90–1.50%), and 2-epi-sesquithujene (0.13–0.13%).

4. Discussion

In the present experiment, the obtained results and also the decreasing trends in fresh yield and in airdried herb and leaf yield in the treatments without irrigation confirm the strict dependence of plant production on the amount of water in the soil. Under water deficit conditions, the turgor pressure decreases, which is the reason for inhibition of the growth and development of cells in the aerial part of the plant, particularly stems and leaves [52]. In our study, the decrease in fresh yield and dry herb and leaf yield of L. angustifolia can be due to the differential growth in the root zone in the soil and the varying water content. García-Caparrós et al. [17] demonstrated that water stress caused a reduction in the fresh weight of L. latifolia, M. piperita, and T. capitatus when plants were subjected to moderate drought stress (70% Eto).

As reported by Khorasaninejad et al. [52], drought stress had a significant effect in terms of decreasing the dry weight of Lamiaceae species plants. Likewise, our study found a lower dry weight of the raw material from the plots without supplementary irrigation. It can be presumed that, under optimal moisture conditions during the growth of L. angustifolia, we can expect a high yield in the harvest year.

In the present research, in line with the expectations, the use of supplementary irrigation increased the water content in the fresh plant material. Di Cesare et al. [53] reported that a higher water content during thermal drying results in the preservation of a higher amount of EO, which was also confirmed in this study.

Drought-induced stress causes an excessive reduction in the synthesis of secondary metabolites, which may also affect the amount of essential oil accumulated by plants [54]. In our experiment, L. angustifolia plants grown with irrigation contained more EO. Similar results were obtained by García-Caparrós et al. [17] in the cultivation of L. latifolia, and by Karamzadeh [55] in the cultivation of L. angustifolia.

Changes in the content of phenolic compounds in raw plant materials are generally reflected in a change in antioxidant activity. In the present study, the content of TPA decreased in crops without supplementary irrigation, accompanied by a decline in the average AA based on the DPPH free radical scavenging activity in the plant material.

In the available literature, there is little information on the effect of irrigation on the L. angustifolia essential oil composition. In our research, regardless of the type of plant material (irrigated and nonirrigated plants), the percentage of oxidized compounds was higher (OM 46.0% and OS 26.1%), while that of hydrogenated compounds was lower (HS 13.7% and HM 13.2%). The L. angustifolia EO originating from Greece [56] is characterized by a much larger percentage of OM (87–89%). A comparison of these data proves the significant effect of genetic and environmental factors on the quantitative and qualitative composition of lavender EO. Nevertheless, most of the papers dealing with the qualitative composition of lavender EO show OM to be the dominant group of constituents [17,57].

Natural drying is a long-term process and does not allow one to control the drying parameters, which means that the quality of the obtained dried material is low. In practice, the best method of herb preservation is thermal drying, and the basic parameter of this process is temperature. Incorrect drying temperature may cause unwanted changes in the profile of EO and losses of thermolabile compounds, which results from the high sensitivity of these substances to the destructive effects of elevated temperature [31]. Therefore, a study was undertaken to determine to what degree the method used for drying determined the quality of the dried herb. A higher airdried herb and leaf yield was obtained in the case of drying under natural conditions than at a temperature of 30 °C, which is evidence of large variations in the herbal raw material. The plant material dried at 30 °C was characterized by a higher EO content than in the case of drying under natural conditions. Likewise, drying L. angustifolia flowers in the oven resulted in lower essential oil losses than drying under natural conditions [4]. The behavior of volatile substances is affected by drying method and temperature depending on the species from which the plant material is obtained. In most studies conducted, thermal drying provided greater EO retention, which was found with respect to the raw plant materials obtained from different species: O. basilicum [34], M. officinalis [39], Mentha sp. [58], and T. vulgaris [31]. As reported by Argyropoulos and Müller [39], an increase in secondary metabolites in the case of thermal drying can be related to the disintegration of cell structures and their easier release. Oil losses during drying under natural conditions are predominantly attributed to oxidation reactions [52]. It has also been noted that, during drying, EOs are unstable and easily degrade under the influence of oxygen, light, and increased temperature and pH [5]. Similarly, in our study, the herbal material contained fewer oxidized compounds and more hydrogenated ones.

In the present study, a higher amount of EO was determined in samples from plants irrigated using a drip line and dried in the oven. The large differences in the quantity of EO determined in the samples might be due to plant tissue hydration at the beginning of the drying process [59]. For instance, heating plant products in water causes heat to quickly penetrate into plant tissues, which, in turn, is the reason for the longer exposure of the entire volume of the product to this factor [60].

In this research, drying at a temperature of 30 °C produced triple the amount of TPA as compared to drying under natural conditions. The observed changes in the content of phenolic compounds in the dried material could have resulted from the combined effect of drying method and irrigation. In 2016 and 2017, the content of TPA was more than 50% greater in the raw material from plants grown with irrigation and dried at a temperature of 30 °C than its amount determined in the herbal material obtained from nonirrigated plants dried under natural conditions. It can be presumed that, at a temperature of 30 °C, the enzymes deactivated more quickly in the raw material than during drying under natural conditions, and hence the samples maintained their high antioxidant capacity and high total phenolic content in the extracts [61].

In this experiment, the oven-dried plant material, in which a larger amount of TPA and EO was determined, had greater AA. The effect of drying method on the content of TPA in airdried herbs was not significant after being calculated for the dry weight. On the other hand, a higher TPA content was associated with high AA and reducing activity in the DPPH test.

A clear decrease in AA activity can be seen in the raw material dried under natural conditions, mainly due to the lack of the following compounds: 3-octanone, hexyl acetate, cis-linalool oxide, 1-octen-3-yl acetate, piperitenone, linalyl isobutanoate, and 2-epi-sesquithujene In a previous study, Chrysargyris et al. [56] demonstrated a positive correlation between the quantity of EO in L. latifolia plant material and AA.

In the present study, the content of monoterpenoids in the oven-dried herb accounted for more than 65% of the oil composition, with borneol and linalyl acetate being the dominant oil constituents. Likewise, Ghasemi et al. [62] found that drying O. basilicum at a temperature of 40–45 °C contributed to higher retention of oxidized compounds OS and OH.

In this research, lavender dried in the oven with hot air retained linalool, camphor, linalyl acetate, and β-caryophyllene to the greatest degree. Drying under natural conditions led to high retention of limonene, 1,8-cineole, cryptone, γ-cadinene, caryophyllene oxide, and α-muurolol compared to thermal drying. Śledź and Witrowa-Rajcher [63] reported that, for many herbs, a lower temperature during the drying process results in the preservation of a significant number of aromatic compounds, which was also confirmed in the present study.

The water content in the herb at the beginning of the drying process caused differences in the oil chemical composition. The oil distilled from irrigated plants was characterized by a higher content of linalyl acetate, linalool, and borneol in the case of oven drying, whereas the herb dried under natural conditions contained more borneol, γ-cadinene, and α-muurolol. The EO from nonirrigated plants with a lower water content in plant tissues that was distilled from the oven-dried herb contained more borneol and α-muurolol. The content of linalyl acetate was also greater, but only in the case of thermal drying. The significant differences in the EO composition are evidence that these compounds are sensitive to temperature and have a greater affinity for the water fraction contained in plant tissues and therefore were lost together with water during the drying process. The conditions under which the technological process is carried out, among which drying temperature is important, are a major factor determining the essential oil composition [39].

Our results show that different drying methods can significantly (p < 0.05) change the chemical profiles of essential oils extracted from the lavender. The literature data cited above and the results obtained in this study indicate that it is advisable to conduct further research aimed at improving agronomic practices (irrigation, fertilization, and preservation methods) that allow the chemical composition of the raw material to be modified. It is even more important because the data regarding the lavender chemical composition, particularly of the essential oil fraction, are still the subject of many studies, while papers on the biologically active metabolites of L. angustifolia flowers continue to be rare.

5. Conclusions

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). The high AA of the lavender leaf extract was associated with the larger quantity of EO in the herbal material obtained from irrigated plants. The EO extracted from plants grown without irrigation contained more OM and less HM than irrigated plants. In crops with irrigation, linalool was predominant in the lavender EO from the monoterpene group, while β-caryophyllene was predominant among the sesquiterpenes. In nonirrigated crops, muurolol, linalyl acetate, borneol, γ-cadinene, camphor, and α-muurolol had a higher percentage in the EO.

The thermally dried lavender raw material contained 2.6-fold more OM than compounds from the OS group, while in the case of drying under natural conditions the percentage of OM was 1.2-fold higher than in the OS group. In samples of the oven-dried herb, more linalyl acetate, β-caryophyllene, linalool, and camphor was found in the EO, while less α-muurolol, γ-cadinene, borneol, and caryophyllene oxide was found in comparison to natural drying.

In the case of irrigation, oven drying provided a higher percentage of linalool, linalyl acetate, and β-caryophyllene in the EO than drying under natural conditions. As far as nonirrigated and naturally dried plants are concerned, the percentages of borneol, γ-cadinene, and α-muurolol were higher in the EO, but distinctly lower for irrigated and oven-dried plants.

It can be presumed that, in L. angustifolia cultivation, soil moisture content and drying temperature are factors that can influence the raw material yield and chemical composition. This can be achieved by providing optimal moisture conditions through supplementary irrigation, used to make up for rainfall deficits. However, it is necessary to conduct further research aimed at improving agronomic cropping practices and stabilizing the raw plant material in order to obtain more stable yields.