Myrtus communis L. subsp. communis

Myrtus communis L. subsp. communis: History

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Myrtus communis subsp. communis is an evergreen shrub or a small tree, growing spontaneously throughout the Mediterranean basin. The stem is branched from the basal portion and the bark is brownish or reddish in color. The leaves are simple, opposite, sessile or sub-sessile, glossy, and dark green in color, lanceolate or ovoid-elliptical in shape with entire or slightly revolute margins and acute apices; they are very aromatic due to the presence of numerous secretory cavities. The flowers, white in color with yellowish streaks, are solitary or coupled at the leaf axil. The fruits are ellipsoidal or subspherical berries, red-violet or blackish in color at maturity, with persistent calyx residues.

myrtle
botanic gardens
secretory cavities
essential oils
α-pinene
1
8-cineole and linalool chemotype
light microscopy
GC-MS
Open science

1. Introduction

Myrtle has a consolidated ethnobotanical tradition and is used in different parts of the world, against colds and coughs ^[1], for self-medication of digestive problems, for skin disorders ^[2], for the treatment of obesity, hypercholesterolemia, and diabetes ^[3]^[4]. The essential oils (EOs), obtained from shoots, leaves and sometimes flowers and berries, are used eminently in perfumery. The berries are also employed in the production of bitters and famous liquors. Due to its broad use in folk medicine, myrtle has been widely investigated, especially with regards to the EO composition ^[5]^[6]^[7]^[8]^[9]^[10]^[11]. In Italy, previous studies were focused on plants from Sardinia, Sicily, Campania, and Liguria regions ^[8]^[9]^[12]^[13]^[14]^[15]. Concerning literature data on the micromorphology, previous investigations were focused on the structure and ontogeny of the secretory cavities of leaves and flowers by means of both light and electronic microscopy ^[16]^[17]^[18]^[19].

2. Micromorphological Investigation

The micromorphological survey showed the occurrence of secretory cavities both in the leaves and in the shoots. In the leaves these structures were variously distributed: in the palisade parenchyma, where they are located immediately below the adaxial epidermis; in the spongy parenchyma, and especially in the transition region with the palisade mesophyll or adjacent to the abaxial epidermis (Figure 1a,b). In the shoots, the cavities had a reduced diameter and generally occurred in the cortical parenchyma.

Figure 1. (a,b) Cross section of Myrtus communis subsp. communis leaf colorless (a) and stained with Toluidine blue dye, (b); (c) Detail of a leaf secretory cavity, colorless; (d) Detail of a shoot secretory cavity; (e–g) Histochemistry of secretory cavities: Nadi reagent (e), Fluoral Yellow-088 (f), AlCl3 (g). Scale bars = 100 µm (a,b); 25 µm (c,d,g); 50 µm (e,f).

The secretory cavities, regardless of the distribution pattern, were globose-spheroidal in shape and had a diameter ranging from 10 to 50 µm (Figure 1c). The cavities displayed a 1-layered epithelium consisting of secreting cells that released the secreted material inside the cavity (Figure 1d). The cavities generally appeared empty, but sometimes the accumulation of colorless or pale-yellow material was observed; the secreted material consisted of small, densely packed droplets or of large clusters which filled the entire volume of the cavity. These results are in accordance with those reported in the literature ^[16]^[19].

The histochemical tests revealed consistent results between the cavities of leaves and shoots (Figure 1e–g). All the dyes specific for lipophilic substances gave positive responses, with special reference to the NADI reagent indicating the presence of terpenes (Figure 1e,f). Furthermore, the production of flavonoids was for the first time highlighted in the cavities of M. communis subsp. communis following the treatment with AlCl3 (Figure 1g), while the tests specific for polysaccharides and proteins gave negative results.

3. Phytochemical Investigation

In 2018 the leaves were treated according to four different preparation methods and separately hydro-distilled: as fresh material (FL), after freezing at −20 °C (20FL), after freezing at −80 °C (80FL), and after air-drying at room temperature (DL). The profiles of the leaf EOs are reported in Table 1.

Table 1. GC-MS profiles of the essential oils obtained from the leaves of Myrtus communis subsp. communis collected in July 2018 following different preservation procedures: hydro-distillation as fresh material (FL), freezing at −20 °C (20FL), freezing at −80 °C (80FL), drying at room temperature (DL).

	LRI	Class	Compound	Relative Abundance (%)
	LRI	Class	Compound	FL	80FL	20FL	DL
1	769	OTHER	hexanal	0.1	tr	0.1	tr
2	844	OTHER	2-hexenal	0.6	0.3	1.2	0.6
3	912	OTHER	isobutyric acid, isobutyl ester	1.0	0.4	0.9	1.1
4	927	MH	α-thujene	0.1	0.2	0.4	0.6
5	935	MH	α-pinene	38.6	36.2	41.2	41.6
6	961	MH	camphene	0.2	0.1	0.1	0.1
7	983	MH	β-pinene	0.3	0.1	0.2	0.3
8	990	MH	myrcene	0.6	0.2	0.3	0.3
9	1005	MH	α-phellandrene	0.2	0.2	0.1	tr
10	1008	MH	3-carene	0.8	0.2	0.3	0.5
11	1019	MH	α-terpinene	0.1	0.1	0.1	0.1
12	1028	AH	o-cymene	0.1	-	0.3	0.5
13	1030	MH	limonene	-	-	2.9	3.3
14	1036	MH	cis-sabinene hydrate	0.7	-	-	-
15	1049	OM	1,8-cineole	31.0	25.5	28.2	26.1
16	1052	MH	α-ocimene	0.4	0.3	0.4	0.3
17	1064	MH	γ-terpinene	0.5	0.3	0.5	0.4
18	1074	OM	cis-linalool oxide	0.1	tr	0.2	0.3
19	1088	MH	α-terpinolene	0.5	0.5	0.7	0.7
20	1100	OM	linalool	10.7	28.6	18.7	18.2
21	1125	OM	fenchol	0.1	tr	tr	-
22	1140	OM	pinocarveol	0.2	tr	0.2	-
23	1143	OM	nerol oxide	0.1	-	-	-
24	1154	OM	pinocarvone	0.1	-	-	-
25	1159	OM	δ-terpineol	0.1	tr	-	-
26	1162	OM	borneol	0.1	tr	-	-
27	1168	OM	terpinen-4-ol	0.5	0.2	0.2	0.3
28	1202	OM	α-terpineol	1.8	2.1	1.4	2.0
29	1248	OM	linalyl acetate	0.9	0.6	0.4	0.8
30	1255	OM	trans-geraniol	1.3	0.2	-	-
31	1296	OM	trans-pinocarvyl acetate	0.1	tr	-	-
32	1318	OM	methyl geranate	0.1	tr	-	-
33	1347	OM	α-terpinyl acetate	0.7	0.3	-	-
34	1356	OM	nerol acetate	0.5	0.2	-	-
35	1376	OM	geranyl acetate	0.8	0.8	-	-
36	1402	OM	methyleugenol	0.9	0.3	-	-
37	1423	SH	β-caryophyllene	1.0	0.4	0.5	0.6
38	1440	SH	aromadendrene	0.1	-	-	-
39	1461	SH	humulene	1.0	0.5	0.4	0.6
40	1492	OTHER	2-tridecanone	0.1	-	-	-
41	1518	OTHER	durohydroquinone	0.8	0.7	-	0.7
42	1562	OS	trans-nerolidol	0.4	-	-	-
43	1588	OS	caryophyllene oxide	0.4	tr	-	-
44	1603	OS	trans-bisabolene oxide	0.1	tr	-	-
45	1616	OS	humulene oxide II	0.3	tr	-	-
46	1660	OTHER	5,8,11-heptadecatrien-1-ol	0.1	-	-	-
47	1705	OTHER	methyl ketostearate	tr	0.1	-	-
			Oil yields	0.36%	0.46%	0.49%	1.08%
			Total identified	98.9	99.9	99.7	100.0
			Monoterpene hydrocarbons (MH)	42.9	38.5	47.2	48.2
			Oxygenated monoterpenes (OM)	50.0	59.0	49.2	47.7
			Sesquiterpene hydrocarbons (SH)	2.1	0.9	0.9	1.3
			Oxygenated sesquiterpenes (OS)	1.1	0.1	tr	tr
			Aromatic hydrocarbons (AH)	0.1	tr	0.3	0.5
			Other compounds (OTHER)	2.7	1.5	2.1	2.4

The main common compounds are highlighted in grey color. LRI = Linear Retention Index, experimentally obtained on a HP-5MS column using a C7–C30 mixture of n-alkanes.

The obtained oil yields ranged from 0.36% in the fresh samples up to 1.08% in the dried ones. The samples stored at −20 °C and at −80 °C showed similar values (0.49% and 0.46%).

The more complex profile, due to the presence of the highest number of total compounds, was that obtained from FL (46), followed by 80FL (38). The other two leaf samples displayed 26 (20FL) and 25 (DL) total compounds. However, the additional compounds in FL occurred in amounts ranging from 0.1% up to 0.9%.

This entry is adapted from the peer-reviewed paper 10.3390/plants11060754

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