How Biological and Environmental Factors Affect the Quality of Lavender Essential Oils

Abstract

Background/Objectives The plants of the Lavandula genus are widely investigated because of their significance for pharmaceuticals and food. The composition of lavender essential oil is determined by genotype and can be induced by environmental, ontogenetic factors, and morphogenetic features. Linalool and linalyl acetate are the most abundant compounds, performing essential ecological functions and participating in lavender’s therapeutic properties. This review reports on the biosynthesis of lavender oil compounds and summarises the environmental, developmental, and molecular factors contributing to essential oil composition in lavender flowers. Results Floral developmental stage and ontogeny are fundamental for optimal harvest time. The harvesting period for high-quality lavender essential oil is affected by environmental and developmental factors that influence the gene expression of monoterpene biosynthesis. Conclusions These findings indicate the appropriate features for high-quality lavender essential oil and contribute to information that may allow for the manipulation of monoterpenes biosynthesis in lavender breeding efforts.

1. Introduction

The Lavandula genus consists of a diverse cluster of small perennial herbaceous plants or shrubs that vary in morphological, habitat, and chemical composition. It is well known for its aromatic leaves and flowers [1]. These plants include 39 highly hybridised species and approximately 400 registered cultivars [2,3]. Lavender is grown worldwide, indicating its ability to adapt to different climates [4]. Lavender essential oil (EO) primarily comprises mono- and sesquiterpenoids, predominantly stored in the peltate glandular trichome (PGT) [5,6]. Lavandula angustifolia and the hybrid between L. angustifolia and Lavandula latifolia, known as Lavandula x intermedia (commercially reported as lavandin), are cultivated principally for the distinctive characteristics of their essential oils, used mainly in the perfume and cosmetic industry and aromatherapy [7,8,9].

Furthermore, the essential oils of Lavandula species are valuable and have tremendous commercial value in the pharmaceutical, food and flavour industries [10]. The pleasant aroma of lavender depends on the various monoterpenes synthesised, and these accumulate primarily in flowers [11]. The most valuable lavender oil characteristic in the perfume and cosmetic industries include the increased amounts of linalyl acetate and linalool. Meanwhile, lavender oil, more rich in camphor amounts, is exclusively used in aromatherapy [12].

Lavender EO demonstrates sedative, anxiolytic, analgesic, anticonvulsant, and anaesthetic activity in the central nervous system. It also exhibits antioxidant, antimicrobial, anti-inflammatory, spasmolytic, and carminative properties [13]. Spike lavender EO, by tradition, is used for its antibacterial, carminative, antifungal, anti-depressive, and anti-inflammatory activities. It is also effective for burn injuries and wounding. Moreover, it is also utilised as a component in perfumes and fragrances, as well as being used as a flavouring agent [14].

Lavandula essential oil is produced annually in an amount of about 1500 tons [15]. Bulgaria and France are the leading lavender production countries, followed by countries in Europe, Asia, Australia, Africa, and the USA [16].

This manuscript examines the biological and environmental factors affecting the production and quality of lavender EO, particularly highlighting studies from the last twenty years. Specifically, this review describes how environmental, geographical, diurnal, plant characteristics and developmental factors contribute to the production and quality of lavender EO. Furthermore, factors affecting the biosynthesis of lavender terpenes at transcriptional levels are analysed.

2. Lavender Essential Oil Production

2.1. Lavender Essential Oil Composition

The International Organization for Standardization (ISO) strictly regulates the properties of essential lavender oils, and their economic value is determined. The L. angustifolia essential oil composition is described in the ISO 3515:2002/Cor 1:2004, and its description concerns its different origins [17]. The essential oil composition of the variety ‘Grosso’ (LI) is defined by ISO 8902:2009 as follows: linalool (24–37%), linalyl acetate (25–38%), camphor (6–8.5%) and 1.8-cineole (4–8%) [18]. Moreover, Pharmacopoeia regulates the pharmaceutical quality of true lavender EO [19]].

Table 1. Lavender essential oil standards.

Compound	ISO 3515:2002 L. angustifolia [17]	ISO 8902:2009 Lavandin Grosso [18]	L. angustifolia var. etherio [13]	European Pharmacopoeia L. angustifolia [20]
Linalool	25–38	24–35	28–35	20–45
Linalyl acetate	25–45	28–38	39–47	25–46
1,8-Cineole	1–2	4–7	0.4–2	<2.5
Camphor	0.5–1	6–8	2.4–3.5	<1.2
Limonene	<1	0.5–1.5	0.3–0.6	<1
b-Ocimene	2.5–6	0.5–1.5	-	–
Terpinen-4-ol	2–6	1.5–5	tr	0.1–0.6
Lavandulyl acetate	3.4–6.2	1.5–3	1.7–2.4	>0.2
Lavandulol	>0.1	0.2–0.8	-	>0.1
a-Terpineol	<2	–	0.6–0.9	<2
Borneol	–	1.5–3	2.1–3	–
3-Octanone	–	–	0.1–0.6	0.1–2.5

presents the main compounds in lavender’s composition, according to two ISOs [17,18], Etherio from Greek native L. angustifolia [13] and European Pharmacopoeia [20].

Many factors alter lavender EO quality, including genotype, plant organs, plant growth, flower development, harvesting time, environmental conditions, and drying and extraction methods [19,21].

2.2. Biosynthesis of Lavender Essential Oil Compounds

Lavender EOs are primarily monoterpenes and sesquiterpenes obtained from the reduction of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) [22] (Figure 1). Prenyltransferases, including geranyl diphosphate synthase (GPPS), reduce the IPP and DMAPP to synthesise geranyl diphosphate (GPP) [23]. Moreover, the action of a cis-prenyl transferase, lavandulyl diphosphate synthase (LPPS), results in the synthesis of lavandulol [24]. Terpene synthases (TPSs) catalyse GPP into cyclic and non-cyclic monoterpenes within the lavender glandular trichomes’ secretory cells [25]. Five of the seven TPS gene subfamilies are present in lavender, accounting for 100 genes [6]. The expression of the TPS-b gene subfamily that encodes for monoterpenes synthases is high in Lavender [26]. The first two monoterpene synthases characterised by lavender plants were (R)-linalool synthase (LaLINS) and (R)-limonene synthase (LaLIMS) [27], responsible for producing linalool and limonene, respectively. The enzymatic activity of 1,8-cineole synthase (LaCINS) converts GPP into 1,8-cineole [28].

BPP synthase (BPPS) catalysis produces bornyl diphosphate (BPP) from GPP and initiates the synthesis of Borneol and camphor [29]. Borneol oxidation by borneol dehydrogenase (BDH) results in the formation of camphor [30]. Another secondary modification of the monoterpenes is acetylation; for example, the acetylation of lavandulol by lavandulyl acetyltransferase produces Lavandulyl acetate [31]. Furthermore, linalyl acetate, the primary compound of lavender EO, is catalysed from linalool by linalool acetyltransferase [32].

Linalool and linalyl acetate are the major compounds of lavender EO, followed by camphor and 1,8-cineole. The latter compounds weaken the EO quality used in perfume and aromatherapy [33]. Plant developmental stages and biotic factors regulate the biosynthesis of monoterpenes and display important physiological and ecological roles in Lavandula plants [6,34,35].

3. Biological and Environmental Factors Affecting EO Production

Essential oil composition verifies lavender oil’s superiority, which is subsequently influenced by genotype, environmental and developmental factors.

3.1. Environmental Factors

EO quality is variable based on geographical location and climatic fluctuations [36]. Among the environmental factors, temperature fluctuations and precipitation can contribute to differences in quantity and quality in aromatic plants [33]. Specifically, rain precipitation affected the ratio between linalool and linalyl acetate in lavender [37]. Differences observed in spike lavender EO yield have clear correspondence with the variation in rainfall between years [38]; as a result, higher EO yields may arise from an increased number of glandular trichomes or from the ability of trichomes to store more EO. The wettest years result in a higher proportion of linalool, 34%, and a significantly lower proportion of camphor.

In contrast, the 1,8-cineole percentage increases dramatically in dryer years. Furthermore, drought stress results in the accumulation of 1,8-cineole [39]. On the contrary, drought before harvest results in an increased amount of linalool [40]. Temperature drops followed by rainfall decrease the linalool content in flowers; however, linalyl acetate was found to remain unaffected [41]. In contrast, rainfall and temperature drops lead to an accumulation of terpinen-4-ol [41]. For Lavandula latifolia, the Thermo-Mediterranean climate favours essential oil more than the Meso- and Supra-Mediterranean climates [14].

Altitude and slope orientation influence lavender essential oil production and compound contribution. L. angustifolia var. etherio grew from 330 to 710 m, resulting in variable amounts of essential oil. Cultivars with a southern orientation, characterised by lower rainfalls and higher temperatures, stimulate essential oil production, reaching 1.89%, compared to that of north orientation cultivars, which accumulate at 0.81% in dry weight [13]. In the same work, a southern orientation and low altitude have led to much higher linalool and linalyl acetate contents, albeit the lower 1.8 cineole. Generally, high production of lavender essential oil can be achieved in the Mediterranean climate and at medium altitudes [42].

Populations grown at lower latitudes yield higher EO with an increased ratio of 1,8-cineole to linalool in the EO composition. Meanwhile, populations grown at higher latitudes produce lower EO, and the ratio of 1,8-cineole to linalool content is also low [43]. On the contrary, lavender plants growing at higher altitudes have double the content of essential oil [44].

Furthermore, optimal essential oil production correlates positively with the presence of pollinators [45]. Specifically, linalool and linalyl acetate attract pollinators [17].

3.2. Developmental Factors and Plant Age

EO quality is variable based on the plant organs used for extraction [14] and depends on plant developmental stages. Remarkably, the amount and constituents of essential oil also vary during inflorescence and floral development [46]. Usually, higher amounts of EO are observed in younger plants, while richer compositions are found in 8-year-old plants [47]. Almost 25% of lavender cultivars grown in Hungary display remarkable differences in their essential oil compounds with plant age [37]. From one year to another, there are variations in the same genotype of lavender.

The highest-quality EO composition is obtained from flowers, as shown by [48]; therefore, stems and leaves should be avoided during harvesting. Lavender flowers are 4–5 times richer than leaves in L. angustifolia, L. latifolia and L. x intermedia; as a result, essential oil from lavender flowers has the finest fragrance [14,49]. Furthermore, the inflorescence stem accumulates less essential oil than the leaves [50]. The composition of EO is also variable within floral organs; for instance, sepals and gynoecium have 50 times higher EO than petals and stamens [50]. Therefore, the flower developmental stage and ontogeny are fundamental for optimal harvest time [33,50]. EO accumulation in flowers of Grosso lavandin increases and reaches a maximum at 30% of anthesis, followed by a gradual drop towards senescence [51]. The harvest time considerably affects lavender’s production and EO composition parameters [13,35,41]. The optimum harvesting stage to achieve the maximum lavender EO content is at 60% blooming [41,52,53]. However, in L. x intermedia plants, the ideal harvesting period is at the early stage of flowering [54].

Ontogenesis is a crucial factor affecting the production of linalool and linalyl acetate in lavender. The linalool amount correlates highly with lavender plant age and accumulates within the first and fourth years; this contrasts the trend observed for linalyl acetate [54]. Age was also positively correlated with 3-octanone and á-terpineol content [54]. During the lavender anthesis period, linalool content increases slightly towards the end of flowering, reaching 43.5%. In contrast, linalyl acetate content displays a reverse trend, reaching 18.7% at the end of flowering [26,31,37,55]. Linalyl acetate and several sesquiterpenes attract pollinators during the flowering period [56]. The highest amounts of the major EO components, such as linalool, 1,8-cineole, terpinene-4-ol, limonene and ocimene, accumulate in the flower buds and at the end of the flowering period [26,55,57]. In contrast, in L. x intermedia plants, the linalool content is at a maximum of 50% during the blooming period. On the other hand, the linalyl acetate content is at its optimum at the end of flowering [58].

When the first lavender flowers open, the lavandulyl acetate content is at its maximum [26]. Borneol and bornyl acetate accumulate in immature flowers and leaves [41]. The terpinen-4-ol amount starts to accumulate in the first lavender flowers, reaching the maximum level at the end of the flowering period when the first seeds are mature. Additionally, the amounts of terpinen-4-ol accumulate in mature inflorescences of L. latifolia [59].

Interestingly, lavender EO production is regulated diurnally [13], suggesting that the circadian rhythms affect the biosynthesis of monoterpenes.

Table 2. Factors affecting the specific compounds of the EO produced by lavender flowers. (− means decrease and + means increased amount).

	Environmental Factors			Plant Development		Genotype
	Rain	Temperature	Diurnal	Ontogeny Inflorescence Development	Plant Age
Linalool	− [39,41] + [43]	+ [41]	ZT4 [13]	+ Middle [58] + Late [26,37,52]	+ up to 4y [46]	[37,45]
Linalyl acetate	− [41]	+ [41]	ZT12 [13]	+ Late [26,58]		[38,45]
Camphor	+ [41] − [43]	+ [41]	Max at 10 h [13]	+ [41]		[45]
1–8 Cineole	+ [41]	+ [41] − [38]	Max at 10 h [13]	+ [26,41] − Late [41]
Borneol			Max at 10 h [13]	+ [41]
Limonene			Max at six h [13]	+ Late [26]
3-Octanone			Max at 10 h [13]		+ up to 4y [46]
α-Terpineol			Max at 10 h [13]	+ Late [26,41,52]	+ up to 4y [46]
Terpinen-4-ol	+ [41]	+ (temperature drop) [41]		+ [26] + Late [41,52]
Lavandulyl acetate			Max at 18 h [13]	+ [26]
Ocimene				+ Late [26]
trans-β-Ocimene			Max at six h [13]	+ Late [26,41,52]
Lavandulol				+ Late [26,41,52]

summarises the significant factors affecting the specific compounds of the EO produced by lavender flowers.

3.3. Genetic Factors

EO production is usually highly variable between populations and within the same plant population [14]. Genotype predominantly affects the amounts of lavender oil compounds (Table 2) [37,38,44,45]. In twelve varieties grown in Canada, linalool was the primary compound, ranging from 17% to 43% [44]. On the other hand, linalyl acetate ranged from 0 to 14.5% [44]. Moreover, the amounts of camphor were reasonably abundant, ranging from 2% to 13%, among the investigated varieties [43]. Similarly, cultivated genotypes in Hungary affect linalool and linalyl acetate percentages [37].

Factors Affecting Monoterpene Biosynthesis Gene Expression in Lavender Flowers

The synthesis of volatile organic compounds (VOCs), including monoterpenes, is a dynamic process that varies throughout plant development. This variation arises from patterns of differentially expressed genes from distinct cell types [60]. In lavender, monoterpene biosynthesis is highly controlled at the transcription level [61]. Table 3 presents the effect of several environmental and developmental factors on the gene expression of monoterpene biosynthesis in lavender.

Most genes that encode enzymes involved in the accumulation of linalool, linalyl acetate and lavandulyl acetate are expressed in flowers and glandular trichomes [6]. The genotype affects the gene expression involved in lavender monoterpene biosynthesis, like GGPPS, GPPS, LPPS and (R)-linalool synthase. Furthermore, several reports suggest that monoterpene biosynthetic gene expression is regulated by flower developmental stage [6,34,62,63]. In early flower development, the content of limonene synthases and one transcript of alcohol acetyltransferase expression are high, and subsequently, their expression decreases in the later developmental stages [6,34]. During inflorescence development, genes that encode enzymes involved in the accumulation of linalool, linalyl acetate and lavandulyl acetate displayed increased expression, with the flowers being picked at 50% of blossoming [6,34,62,63]. Moreover, lavender EO production is regulated diurnally [13]. Likewise, linalool synthase, limonene synthase and terpene synthase expression are regulated during the day, with picking being carried out at mid-day until 15:00, suggesting that circadian rhythms affect the transcription of monoterpene synthases in lavender flowers [64].

Linalool and linalyl acetate attract pollinators [17], a process mediated by jasmonates [65]. Methyl jasmonate (MeJa) affects the diurnal emission of lavender volatiles in a quantitative and chronologically dependent manner [66]. The expression of LaGPPS and LaCINS was considerably increased by MeJA [66]; moreover, TPS genes (La05G1453/ La05G1454) are highly expressed in MeJa-treated flowers [6]. Interestingly, the expression levels of five BAHD acyltransferase genes (LaBAHD57, 63, 104, 105, and 119) also increased significantly after MeJA treatment, especially in sepals [67]. These genes, which are thought to be involved in the biosynthesis of linalyl acetate and lavandulyl acetate, and their expression are also induced by abiotic stress, including drought and salt treatments [67].

Table 3. Effect of several environmental and developmental factors on gene expression of monoterpene biosynthesis in lavender.

Genes	Factors	Reference
(R)-linalool synthase (LaLinS)	Ιnflorescence development (flower stage 50%)	[62,63]
	Genotype	[62]
	Increased at mid-day	[64]
lavandulyl pyrophosphate synthesis (LPPS)	Genotype, ιnflorescence development (picked at bud II)	[62]
geranylgeranyl pyrophosphate synthase (GGPPS)	Genotype	[62]
GPP synthase (GPPS)	Genotype, ιnflorescence development (picked at flower stage 50%)	[62]
GPP synthase (GPPS)	Up-regulated by methyl jasmonate	[66]
LIMS	Down-regulated during flower development	[6]
	Increased at mid-day	[64]
LaCINS	Up-regulated by methyl jasmonate	[66]
alcohol acetyltransferase (AAT2)	Up-regulated before blossom	[6]
LaAAT	Increased early flower development, decreased late flower development	[34]
La05G1453/La05G1454 (TPS)	Up-regulated with methyl jasmonate	[34]
La02G01528/La02G01529/La02G01630 (TPS)	Up-regulated with methyl jasmonate	[34]
TPS-l	Increased at mid-day	[64]

Table 3. Effect of several environmental and developmental factors on gene expression of monoterpene biosynthesis in lavender.

Genes	Factors	Reference
(R)-linalool synthase (LaLinS)	Ιnflorescence development (flower stage 50%)	[62,63]
	Genotype	[62]
	Increased at mid-day	[64]
lavandulyl pyrophosphate synthesis (LPPS)	Genotype, ιnflorescence development (picked at bud II)	[62]
geranylgeranyl pyrophosphate synthase (GGPPS)	Genotype	[62]
GPP synthase (GPPS)	Genotype, ιnflorescence development (picked at flower stage 50%)	[62]
GPP synthase (GPPS)	Up-regulated by methyl jasmonate	[66]
LIMS	Down-regulated during flower development	[6]
	Increased at mid-day	[64]
LaCINS	Up-regulated by methyl jasmonate	[66]
alcohol acetyltransferase (AAT2)	Up-regulated before blossom	[6]
LaAAT	Increased early flower development, decreased late flower development	[34]
La05G1453/La05G1454 (TPS)	Up-regulated with methyl jasmonate	[34]
La02G01528/La02G01529/La02G01630 (TPS)	Up-regulated with methyl jasmonate	[34]
TPS-l	Increased at mid-day	[64]

4. Conclusions

Lavender essential oil is extensively used, not only in the aromatherapy, perfume, and the cosmetic industries but also in medicine and therapeutic applications, due to its antiviral and antibacterial activities. Furthermore, lavender monoterpenes perform essential physiological and ecological functions. This review summarises the environmental, developmental, and molecular factors contributing to EO composition in lavender flowers. These features predict the optimum harvesting period for high-quality lavender EO and highlight the environmental factors, such as rainfall and temperature during harvesting, and the developmental factors, like the circadian rhythm, that regulate linalool and linalyl acetate biosynthesis. Therefore, the highest quality of lavender EO is obtained when the flowers are harvested in the afternoon for the highest content of linalool and linalool acetate at the end of the flowering period.

The regulation of the pathways involved in monoterpene biosynthesis in lavender is limited, which constrains the manipulation of VOC emission. In conjunction with new technologies, modern approaches will generate transgenic plants and improved genotypes with more precise, fine-tuned VOC emissions and high-quality lavender EO for industrial and medicinal purposes.