ORIGINAL ARTICLE

Insight into the structure and chemistry of glandular trichomesof Labiatae, with emphasis on subfamily Lamioideae

Claudia Giuliani Æ Laura Maleci Bini

Received: 14 March 2008 / Accepted: 12 August 2008 / Published online: 30 September 2008

� Springer-Verlag 2008

Abstract Glandular trichomes of Labiatae are among the

most investigated secretory structures. Most species stud-

ied belong to subfamily Nepetoidae, including plants with

aromatic properties, while so far a few species of subfamily

Lamioideae were examined. In this work, we studied the

micromorphology, ultrastructure, type and release of

secretion of the glandular trichomes present on leaves and

flowers of several species belonging to subfamily Lami-

oideae, (Stachys alopecuros (L.) Bentham subsp.

alopecuros, S. officinalis (L.) Trevisan subsp. officinalis,

S. germanica L. subsp. germanica, S. germanica L. subsp.

salviifolia (Ten.) Gams, S. sylvatica L., S. heraclea All.,

S. plumosa Griseb., S. annua L., Prasium majus L., Side-

ritis romana L.) and one to the sister group Scutellarioideae

(Scutellaria galericulata L.). Besides the well-known pel-

tate and small capitate trichomes, widely described in the

literature, other types of glandular trichomes were

encountered; stalked peltate hairs and large capitate hairs.

In particular, a new type of capitate trichome, exclusive of

calices and corollas, which presents a mode and release of

secretion never described before, is reported.

Keywords Labiatae � Lamioideae � Glandular trichomes �Histochemistry � Ultrastructure

Introduction

The Labiatae is one of the largest families among the

dicotyledons, being composed of more than 240 genera;

many species belonging to the family are highly aromatic,

due to the presence of external glandular structures that

produce essential oil.

The complete taxonomy of the family was carried out

firstly by Bentham (1832–1836) and later by Briquet

(1895–1897). Erdtman (1945), on the basis of the mor-

phology of the pollen grains, proposed a division of the

family into two subfamilies, Nepetoideae and Lamioideae.

This division was further investigated by Cantino and

Sanders (1986) who concluded that Nepetoideae are

monophyletic and pointed out that more research was

needed before the phyletic status of Lamioideae could be

determined. In an attempt to identify the monophyletic

taxa, Cantino (1990) studied some microcharacters (sto-

mata and glandular and non-glandular trichomes) and later,

taking into account a wider range of features (both

micromorphological and phytochemical), proposed a new

and more natural classification of the family, which he

divided into eight different subfamilies (Cantino 1992a, b;

Cantino et al. 1992): Symphorematoideae Briq., Viticoi-

deae Briq., Ajugoideae Kostel., Prostantheroideae Luerss.,

Scutellarioideae (Dumort.) Caruel, Lamioideae Harley,

Nepetoideae (Dumort.) Luerss., and Chloanthoideae.

The morphological differences between the two largest

subfamilies, Nepetoideae and Lamioideae, are not very

remarkable, the differing characters being pollen morphol-

ogy (Erdtman 1945), absence of endosperm (Wunderlich

1967) and presence of myxocarpy (Ryding 1992). Molec-

ular analyses (Olmstead et al. 1993; Wagstaff and Olmstead

1997; Wagstaff et al. 1998) confirmed the morphological

observations, showing that Nepetoideae were remotely

C. Giuliani � L. Maleci Bini (&)

Department of Vegetal Biology, University of Florence,

via La Pira, 4, 50121 Florence, Italy

e-mail: maleci@unifi.it

123

Plant Syst Evol (2008) 276:199–208

DOI 10.1007/s00606-008-0085-0

related with Lamioideae. More recently Harley et al. (2004)

reviewed Cantino’s classification recognizing only seven

different subfamilies, since the genera traditionally classi-

fied as Chloanthoideae were included in Prostantheroideae.

The micromorphological study of the species belonging

to different groups can evidence new similarities or dif-

ferences among the various taxa (Endress et al. 2000).

Within the micromorphological studies, an important tax-

onomic significance is attributed to the epidermis, in

particular to trichomes. Indeed, the type and distribution of

trichomes was one of the features used by Cantino (1990)

for differentiating the various subfamilies, as stated also by

Harley et al. (2004). Cantino’s study was mainly based on

the number of cells constituting the trichomes, but, at

present, the characterization of the different types of glands

has to take into account the shape of glands and also the

kind of secretion and its storage and mode of release

(Werker 2000).

Most works on the glandular trichomes of the Labiatae

concern species belonging to subfamily Nepetoideae, in

which two types of glandular trichomes are recognized:

peltate and capitate hairs (Hallahan 2000; Werker 2000).

Their morphology, ultrastructure, type and release of

secretion were widely studied (Amelunxen 1964, 1965;

Bosabalidis and Tsekos 1982, 1984; Bruni and Modenesi

1983; Modenesi et al. 1984; Bourett et al. 1994; Serrato-

Valenti et al. 1997; Bisio et al. 1999), probably due to the

economic importance of the essential oil. The peltate hairs

ultrastructure, which allows identification of typical leu-

coplasts, devoid of thylakoids and starch, and SER,

together with the histochemistry, show that these tric-

homes are the site of essential oil production and storage

(Fahn 2000, and literature therein). They are considered

long-term trichomes, because the accumulation of the

secreted material continues during the growth of the

organs that bear them (Werker 1993, 2000 and literature

therein).

Different types of capitate trichomes with stalk of var-

iable length have been described (Werker et al. 1985a,

1985b), but only short capitate hairs were studied in detail

(Modenesi et al. 1984; Serrato-Valenti et al. 1997; Bisio

et al. 1999; Werker 2000). Their ultrastructure, which

evidences Golgi stacks and RER, and the histochemistry

indicate the occurrence of a polysaccharidic secretion

(Fahn 2000, and literature therein). They are considered

short-term glandular trichomes because the entire process

of secretion is soon terminated and is particularly active in

the early stages of the organ’s development (Werker 1993).

Few studies were carried out on glandular trichomes of

species belonging to subfamily Lamioideae (Karousou

et al. 1992; Ascensao et al. 1995; Nik et al. 2004; Belhattab

et al. 2006). In particular, histochemistry and ultrastructure

were studied only on peltate and capitate trichomes of

Leonotis leonurus (Ascensao et al. 1997; Ascensao and

Pais 1998).

Since knowledge on the morphology and functioning of

Lamioideae trichomes is still limited, we carried out

observations on micromorphology, ultrastructure, type and

release of secretion of the glandular trichomes present on

leaves and flowers of several species belonging to sub-

family Lamioideae, with the aim of deepening these

aspects. In particular, numerous species of Stachys (S.

alopecuros (L.) Bentham subsp. alopecuros, S. officinalis

(L.) Trevisan subsp. officinalis, S. germanica L. subsp.

germanica, S. germanica L. subsp. salviifolia (Ten.) Gams,

S. sylvatica L., S. heraclea All., S. plumosa Griseb.,

S. annua L.), Prasium majus L., Sideritis romana L. and

Scutellaria galericulata L., belonging to subfamily Scu-

tellarioideae, considered a sister group of Lamioideae

(Wagstaff et al. 1998), were examined. In this work the

results of such observations are reported.

Materials and methods

Plant material

The aerial parts of the examined plants were collected

during the flowering period (April–July) from the follow-

ing sites:

Stachys alopecuros subsp. alopecuros, 20.07.2006 Fal-

cade (Belluno), Valle di Gares; Stachys officinalis subsp.

officinalis, 15.06.2005 Baratti (Livorno); 25.06.2006

Scarperia (Firenze); Stachys germanica subsp. germanica,

09.07.2005 Alpe della Luna (Arezzo); Stachys germanica

subsp. salviifolia, 02.07.2005 Monte Morello (Firenze);

Stachys sylvatica, 15.06.2005 Vaiano (Prato); Stachys

heraclea, 03.07.2005 Monte Morello (Firenze); Stachys

plumosa, 20.06.2005 Hortus Botanicus Benacensis di

Toscolano (Brescia) dell’Universita degli Studi di Milano;

Stachys annua, 15.07.2006 Scandicci Alto (Firenze);

Prasium majus, 03.05.2005 Baratti (Livorno); 12.05.2007

Isola del Giglio, Campese (Grosseto); Sideritis romana,

28.05.2005 Monte Morello (Firenze); Scutellaria galeri-

culata, 28.05.2005 Orto Botanico dell’Universita degli

Studi di Firenze.

Samples were determined following Flora Europaea

(Tutin et al. 1972), Flora d’Italia (Pignatti 1982) and An

Annotated Checklist of the Italian Vascular Flora (Conti

et al. 2005).

Laboratory methods

Micromorphological observations were carried out on fresh

materials (stems, leaves, bracts, calyces and corollas) of the

different plants by scanning electron microscopy (SEM),

200 C. Giuliani, L. Maleci Bini

123

light microscopy (LM) and transmission electron micros-

copy (TEM). TEM analysis allowed to observe the cellular

compartments involved in the secretion process.

SEM observations

Small pieces of plant material were fixed in 2.5% glutar-

aldehyde in 0.1 M phosphate buffer at pH 6.8, dehydrated

in ethanol in ascending grades up to absolute and then dried

using a Critical Point Dryer apparatus. The samples, coated

with gold, were observed with a Philips XL-20 SEM.

LM observations

Fresh material was frozen, sectioned and stained with

different techniques in order to evidence the different

components of the secretion. The stainings employed were

Fluoral Yellow-088 for total lipids (Brundrett et al. 1991),

Nile Red for neutral lipids (Greenspan et al. 1985), Nadi

reaction for terpenes (David and Carde 1964), Ruthenium

Red (Jensen 1962) and Alcian Blue (Beccari and Mazzi

1966) for acid polysaccharides, Ferric Trichloride for

polyphenols (Gahan1984) and Aluminum Trichloride for

flavonoids (Guerin et al. 1971). Observations were made

with a Leitz DM-RB Fluo optic microscope.

TEM observations

Small pieces of plant material were fixed in 2.5% glutar-

aldehyde in 0.1 M phosphate buffer at pH 6.8 and

postfixed in 2% OsO4, dehydrated in ethanol in ascending

grades up to absolute and embedded in Spurr’s resin.

Ultrathin sections were stained with uranile acetate and

lead citrate. Samples were examined with a Philips EM-

300 TEM.

Results

The glandular trichomes observed in the examined plants

can be grouped as follows: peltate hairs, type A; small

capitate hairs, type B; large capitate hairs, type C.

Type A is the typical peltate trichome and consists of a

basal epidermal cell, a short neck cell and a large head (50–

60 lm in size) with a variable number (4–16) of secreting

cells, depending on the species; the secreted material is

stored in a large subcuticular space, originated by the

detachment of the external part of the wall on the glandular

head, and released through the cuticle rupture. Its mor-

phology, ultrastructure, type and release of secretion were

widely described in the literature and will not be described

further in this paper. This type of trichome is present in

Stachys officinalis subsp. officinalis, S. alopecuros subsp.

alopecuros, S. annua and Scutellaria galericulata, partic-

ularly in the intervein areas of the adaxial and abaxial

surfaces of leaves, bracts and calices (Table 1). In S.

annua, the secretion is composed for the most part of

essential oil, but in the other species its composition is

more complex. Indeed, the secretion stains positive with

lipophilic stainings (Nile Red, Nadi reagent and Fluoral

Yellow 088) and with Ruthenium Red and AlCl3 (Table 2),

indicating the presence of essential oil and also of impor-

tant polysaccharides and flavonoids fractions (Fig. 1a, b).

The ultrastructure of mature secreting cells shows not only

leucoplasts and SER, typical organelles of a terpenoidic

secretion, but also Golgi stacks and RER (Fig. 1c, d),

indicating the presence of both lipophilic and hydrophilic

components that appear as electrondense droplets

immersed in an abundant granular matrix (Fig. 1e).

In Stachys germanica subsp. germanica, S. germanica

subsp. salviifolia, Prasium majus, Sideritis romana and

Scutellaria galericulata, a peculiar peltate trichome (type

A1) with a well-developed stalk (Fig. 1f) was observed,

both on leaves and inflorescences (Table 1), especially

along the margin of the calyx abaxial surfaces; the stalk is

constituted of the basal epidermal cell, which is particu-

larly elongated (about 40–60 lm in length) (Fig. 1g). The

neck cell, disposition of the secreting cells and the large

subcuticular space (Fig. 1f, g) are typical of peltate tric-

homes. The secretion proved positive to all the lipophilic

stainings (Table 2), particularly to the Nadi reagent

(Fig. 1h).

The ultrastructure evidences that the subcuticular space

is delimited by the cuticular layer and by the outer part of

the pectic-cellulosic wall of the secreting cells, typical

features of a peltate trichome (type A). In the young tric-

homes, the cuticular layer is characterized by a crowded

fibrillar network (Fig. 1i), originating from the underlying

pectic-cellulosic layer; this network decreases with the

aging of the trichomes. At the beginning of the secretion

process, the cytoplasm of secreting cells shows numerous

leucoplasts containing starch granules (Fig. 1j). With the

aging of the trichomes starch granules disappear, leucop-

lasts become ameboid in shape and are associated with the

ER cisternae (Fig. 1k); numerous autophagic vacuoles

containing myelin-like figures are also present (Fig. 1k).

Type B is a small capitate hair composed of one epi-

dermal cell, a neck cell functioning also as a short stalk,

and a head (20–25 lm in size) of 2–4 secreting cells. This

type of trichome is distributed both on the vegetative and

the reproductive organs of all the examined plants, espe-

cially along the veins of abaxial and adaxial surfaces of

leaves and of the abaxial side of calices (Table 1). On the

apex of the secreting cells a thin subcuticular space, in

which the secretion is accumulated, develops. The features

of the external layer delimiting the subcuticular space are

Glandular trichomes of Labiatae 201

123

similar to those described for type A and A1 hairs; indeed it

is composed by a thick cuticle layer, penetrated by a

fibrillar network that seems to be in continuity with the

cellulosic layer of the inner wall (Fig. 2a). No opening

structures were observed; the secretion, therefore, can cross

the external wall. In most species, these trichomes stain

positive with Ruthenium Red and Alcian Blue (Table 2),

indicating a hydrophilic secretion of polysaccharides.

Nevertheless, in Stachys officinalis subsp. officinalis, S.

alopecuros subsp. alopecuros and S. annua the secretion

stains also with Nadi reagent and FeCl3 (Table 2), indi-

cating a more complex composition with both essential oil

and polyphenolic fractions (Fig. 2b, c). The ultrastructure

of mature secreting cells in most cases evidences abundant

Golgi stacks, RER and electrondense leucoplasts with

scarce tubular membrane elements and large globular in-

traplastidial bodies (Fig. 2a). In Stachys officinalis subsp.

officinalis also, abundant dilated cisternae of SER are

present (Fig. 2d).

Trichomes of type C are large capitate hairs. Two dif-

ferent types, type C1 and type C2, may be distinguished

from the morphological point of view.

Type C1, observed on the intervein areas of the adaxial

and abaxial surfaces of leaves, bracts, calices and corollas

in S. sylvatica and S. plumosa (Table 1), is composed of an

Table 1 Distribution of the different types of glandular trichomes

present on the examined taxa

Leaf Bract Calyx Corolla

Adax Abax Adax Abax Adax Abax Adax Abax

Stachys alopecuros subsp. alopecuros

A ± ?? ? ?? - ? - ?

A1 - - - - - - - -

B ? ? ? ? ? ? - ?

C1 - - - - - - - -

C2 - - - - - - - -

Stachys officinalis subsp. officinalis

A ? ?? ? ?? ± ?? ? ??

A1 - - - - - - - -

B ?? ? ?? ? ? ? - ?

C1 - - - - - - - -

C2 - - - - - - - -

Stachys germanica subsp. germanica

A - - - - - - - -

A1 - ? ± ? ? ?? - ?

B ?? ? ?? ? ? ? ? ?

C1 - - - - - - - -

C2 - - - - ± ? - -

Stachys germanica subsp. salviifolia

A - - - - - - - -

A1 ? ± ? ? ? ? - ?

B ?? ? ?? ? ? ?? ? ?

C1 - - - - - - - -

C2 - - - - ± ?? - -

Stachys sylvatica

A - - - - - - - -

A1 - - - - - - - -

B ? ?? ?? ?? ± ?? ± ?

C1 - ? ? ? - ?? ± ?

C2 - - - - - ?? - ??

Stachys heraclea

A - - - - - - - -

A1 - - - - - - - -

B ? ? ± ± ? ?? ± ?

C1 - - - - - - - -

C2 - - - - ± ?? - -

Stachys plumosa

A - - - - - - - -

A1 - - - - - - - -

B ? ? ? ? ? ?? - ?

C1 ?? ?? ? ? ? ?? - ?

C2 - - - - - ? - ?

Stachys annua

A - ? - ? - ? - ?

A1 - - - - - - - -

B ? ?? ? ? ? ? ? ??

Table 1 continued

Leaf Bract Calyx Corolla

Adax Abax Adax Abax Adax Abax Adax Abax

C1 - - - - - - - -

C2 - - - - ? ? - -

Prasium majus

A - - - - - - - -

A1 - - - - ± ? - -

B ± ± ± ± - ? - ±

C1 - - - - - - - -

C2 - - - - ± ?? - ??

Sideritis romana

A - - - - - - - -

A1 ± ? ± ? - ? - -

B ? ?? ? ?? ? ?? ? ?

C1 - - - - - - - -

C2 - - - - ? ? ? -

Scutellaria galericulata

A ± ?? ? ?? - ? - ?

A1 - - - - ± ? - -

B ?? ?? ?? ?? ? ?? - ?

C1 - - - - - - - -

C2 - - - - ± ? - -

(-) Absent, (±) scarce, (?) present, (??) abundant

202 C. Giuliani, L. Maleci Bini

123

elongated epidermal cell which forms a long stalk, a neck

cell and a glandular head (30–40 lm in size) of 2–4

secreting cells surrounded by a large subcuticular space

(Fig. 2e–g). The secreting material is a typical essential oil

secretion (Table 2), well stained with the Nadi reagent

(Fig. 2h). The ultrastructure shows that the subcuticular

Table 2 Histochemical tests of the different types of glandular trichomes

Staining procedure Target compounds Observed color Type A Type A1 Type B Type C1 Type C2

Nile Red Neutral lipids Golden–yellow ? ? - ? ?

Fluoral Yellow-088 Total lipids Yellow to orange ?? ?? - ?? ?

NADi reagent Terpenes Violet–blue ?? ?? ±a ?? ??

AlCl3 Flavonoids Blue–green ? - ?a - ??

FeCl3 Polyphenols Emerald–green ? - ?a - ?

Ruthenium Red Acid polysaccharides Pinkish to red ? - ? - ??

Alcian Blue Acid polysaccharides Pale–blue ? - ? - ??

Results: (-) absent; (±) scarce; (?) intense; (??) very intensea In Stachys officinalis subsp. officinalis, S. alopecuros subsp. alopecuros and S. annua

Fig. 1 a–e Type A peltate

trichome: secretion stained

a with Ruthenium Red and

b with AlCl3 in S. alopecurossubsp. alopecuros. c Secreting

cell ultrastructure of a mature

trichome in S. officinalis subsp.

officinalis. d Rough

endoplasmic reticulum in

S. annua. e Secreting material

stored in the subcuticular space

in S. officinalis subsp.

officinalis. f–k Type A1 peltate

trichome. f, g An overview in

S. germanica subsp. salviifolia.

h Secretion stained with Nadi

reagent in Prasium majus.

i External cell wall with the

crowded fibrillar network in

Prasium majus. j Secreting cell

ultrastructure of a young

trichome in Prasium majus.

k Secreting cell ultrastructure of

a mature trichome in Sideritisromana. av Autophagic

vacuoles, c chloroplasts, clcuticular layer, g Golgi stacks,

ld lipidic droplets, lpleucoplasts, m mithocondria, nnucleus, Nc Neck cell, pcpectic-cellulosic layer, rer RER,

s starch, Sc stalk cell, ser SER,

Ss Subcuticular space, vvacuoles. a, b, g Bars 25 lm;

f, h bars 20 lm; c, k bars 2 lm;

d, e, i bars 0.5 lm; J bar 1 lm

Glandular trichomes of Labiatae 203

123

space is delimited by a thin electrondense layer, probably

constituted only of cutin (Fig. 2g, i). At the beginning of

the secretion phase, the secreting cells cytoplasm shows

numerous electrondense leucoplasts with starch granules

(Fig. 2j); in full-active trichomes, leucoplasts lack starch

granules and appear surrounded by dilated SER cisternae

(Fig. 2k). It is likely that the secretion crosses the thin

external wall, but in few cases cuticle rupture was

observed.

Type C2 is present in all the examined plants, except

Stachys officinalis subsp. officinalis and S. alopecuros

subsp. alopecuros (Table 1). It is typical of the only

inflorescences and, in particular is localized on the whole

abaxial surface of calices and corollas. It is a large capitate

trichome either with a stalk composed of three or more

cells on calices (Fig. 3a) or with a shorter stalk on corollas.

The neck cell bears a head composed of a variable number

of secreting cells (4 or more, up to 16, 40–80 lm in size,

depending on the species) (Fig. 3b); on the apex of each

cell, a small subcuticular space develops from the raising

of the thin external wall (Fig. 3c). Occasionally some

prearranged openings for the release of the secretion were

observed (Fig. 3c), but most part of the secreting material

seems to be extruded through the whole external wall,

raised or not raised, and flows along the stalk (Fig. 3d–h).

The secretion is characterized by a complex composition,

positive to Alcian Blue, Ruthenium Red, Fluoral Yellow

088, Nadi reagent, FeCl3 and AlCl3 (Table 2). Therefore,

secretion is constituted of polysaccharides, essential oil and

polyphenols, particularly flavonoids (Fig. 3d–h).

Ultrastructure observations allowed recognizing differ-

ent stages in the secretion process. At the beginning of the

Fig. 2 a–d Type B small

capitate trichome. a Secreting

cell ultrastructure in

S. germanica subsp. salviifolia.

Secretion stained b with Nadi

reagent in S. plumosa and c with

FeCl3 in S. heraclea.

d Secreting cell ultrastructure in

S. officinalis subsp. officinalis.

e–k Type C1 capitate trichome.

e, f An overview in S. plumosa.

g Ultrastructure of the glandular

head in S. plumosa. h Secretion

stained with Nadi reagent in

S. sylvatica. i Particular of the

thin cuticular layer delimiting

the subcuticular space in

S. plumosa; j secreting cell

ultrastructure of an young

trichome in S. plumosa.

k Secreting cell ultrastructure of

a mature trichome in

S. plumosa, note the dilated

SER cisternae (arrows)

surrounding leucoplasts.

a Bar 2 lm; b, c bars 25 lm;

d bar 1 lm; e, f, h bars 20 lm;

g bar 10 lm; i, j bars 1 lm;

k bar 0.5 lm. Symbols as in

Fig. 1

204 C. Giuliani, L. Maleci Bini

123

secretion phase, the cytoplasm of the glandular cells is

characterized by large vacuoles, mithocondria, multi-

shaped leucoplasts rich in starch granules, Golgi stacks and

RER cisternae (Fig. 4a). The thick inner layer of the

secreting cells tangential wall is overlapped by abundant

flattened vesicles, externally delimited by a thin electron-

dense layer, probably constituted of only cutin (Fig. 4b).

On the apex of each cell a small portion of the vesicle layer

raises, originating a small space in which electrondense

fibrillar or granular material is stored (Fig. 4c). This

material probably is released through the apical pore

observed during SEM investigation.

The stalk cells and the neck cell show large vacuoles

and numerous chloroplasts with well developed thylakoids,

and probably take part in the secretion process, as evi-

denced by the numerous plasmodesmata present on the

tangential walls of the trichome (Fig. 4d).

Mature trichomes present a dense cytoplasm, numerous

small vacuoles, electrondense leucoplasts devoid of starch,

abundant dilated SER cisternae surrounding plastids, and

lipidic droplets (Fig. 4e). Golgi stacks and RER elements

occur occasionally. In the neck cell and in the stalk cells,

lipidic droplets and small crystals are present; the internal

membranes system of chloroplasts is reduced, and plasto-

globules appear (Fig. 4f).

The vesicle layer develops in very young trichomes and

is maintained in mature trichomes; it probably constitutes

the secreting material, which abundantly flows out and can

be observed along the stalk and on the epidermal surfaces

(Fig. 3d–h).

Discussion and conclusions

Most research reported in the literature (Hallahan 2000 and

the literature therein) was carried out on plants belonging

to subfamily Nepetoideae. Peltate hairs and small capitate

hairs, referred to as type A and type B in this paper, were

recorded; accordingly, our observations described these

two types of trichomes.

Type A1 trichomes, which we observed in some species,

were described also in Salvia officinalis (Corsi and Bottega

1999). They can be considered a variant of type A,

although their external morphology (SEM) is similar to a

large capitate trichome (type C). However, the morphology

of the secreting cells, and, in particular, the large subcu-

ticular space and the cell wall delimiting it are typical of

the peltate trichomes. The crowded fibrillar network

described by Ascensao et al. (1997) in young peltate hairs

of Leonotis leonurus, was observed also in the external

wall delimiting the subcuticular space of these trichomes,

further confirming that they are peltate hairs.

According to the current literature (Bruni and Modenesi

1983; Werker 1993; Ascensao et al. 1997), the secretion of

type A trichomes is constituted of essential oil; this is true

for most plants that we studied, but in Stachys officinalis

Fig. 3 Type C2 capitate

trichome. a Calyx abaxial

surface of Scutellariagalericulata. b Semithin section

in S. germanica subsp.

germanica. c Glandular head

with prearranged openings

(arrow) in S. germanica subsp.

salviifolia. Secretion stained

d with Alcian Blue in S.germanica subsp. salviifolia,

e with Fluoral Yellow 088 in

Prasium majus, f with Nadi

reagent in S. sylvatica, g with

FeCl3 in Prasium majus and

h with AlCl3 in S. germanicasubsp. salviifolia. a Bar100 lm; b, c bars 20 lm;

d, e, f, g, h bars 25 lm. Symbolsas in Fig. 1

Glandular trichomes of Labiatae 205

123

subsp. officinalis and S. alopecuros subsp. alopecuros,

peltate trichomes present a complex mixture of essential

oil, polyphenols and polysaccharides as already observed

in Salvia aurea (Serrato-Valenti et al. 1997) and Salvia

blepharophylla (Bisio et al. 1999).

Concerning type B trichomes, the literature reports a

secretion composed mainly of polysaccharides (Modenesi

et al. 1984; Werker et al. 1985a), as we observed in

the most species studied. Instead, in S. officinalis subsp.

officinalis the secretion has a more complex composition of

polysaccharides, polyphenols and essential oil, as already

described in Leonotis leonurus (Ascensao and Pais 1998)

and Stachys recta (Giuliani et al. 2008). Considering

the tcurrent literature data, and our observations, type B

trichomes seem to be the only type common to all the

species examined of both subfamilies Nepetoideae and

Lamioideae.

Large capitate trichomes were already described in

several Nepetoideae species (Werker et al. 1985a; Serrato-

Valenti et al. 1997; Ascensao et al. 1999; Bisio et al. 1999),

studying their external morphology and histochemistry.

However, in the reported works the stalk was constituted of

a variable number of cells and the glandular head of one

large single cell. Instead, type C capitate trichomes (C1,

C2), described here, present a multicellular glandular head

and a stalk of variable length.

Type C1 hairs are typical essential oil secreting hairs,

present on species lacking peltate trichomes. For their

dimensions, number of secreting cells and kind of secre-

tion, these trichomes could be confused with type A1

Fig. 4 Ultrastructure of type C2

capitate trichome. a Secreting

cell cytoplasm of a young

trichome in S. heraclea.

b Vesicle layer (arrow) on the

not-raised wall. c Secreting

material stored in the

subcuticular space delimited by

the vesicle layer (arrow) in

S. heraclea. d Cytoplasm of the

neck and stalk cells of a young

trichome in S. heraclea, note the

numerous plasmodesmata

(arrow). e Secreting cell

cytoplasm of a mature trichome

in S. germanica subsp.

salviifolia. f Cytoplasm of the

neck and stalk cells of a mature

trichome in S. germanica subsp.

salviifolia. a, c, d, e Bars 1 lm;

b bar 0.5 lm; f bar 5 lm.

Symbols as in Fig. 1

206 C. Giuliani, L. Maleci Bini

123

trichomes. However, the ultrastructure evidences a thin

external layer, strongly electrondense, delimiting the sub-

cuticular space, which clearly distinguishes this type of

trichomes from peltate A1 hairs.

Type C2 trichomes present a particular mode of secretion

storage and release, never described before, with vesicles

surrounding the whole external wall. The ultrastructure,

evidencing different organelles according to the secreting

stages, suggests a different type of secretion corresponding

to various stages: firstly hydrophile-polysaccharidic,

secondly lipophile-terpenoidic. These trichomes are char-

acteristic only of the reproductive organs of the plants

(calyces and corollas).

We observed these trichomes in most of the species

studied, belonging to the genera Stachys, Sideritis, Prasium,

Scutellaria; probably they are present on other species of

the subfamilies Lamioideae and Scutellarioideae. The

capitate trichomes described by Karousou et al. (1992) in

Sideritis syriaca subsp. syriaca could belong to this type.

Concerning the secretion composition, each type of tri-

chome usually is characterized by one type of secretion;

nevertheless some ‘‘exceptions’’ were recorded, e.g., pel-

tate trichomes are typical essential oil trichomes, but in

Stachys officinalis subsp. officinalis and S. alopecuros

subsp. alopecuros abundant polyphenols and polysaccha-

rides are also present. The secreting cells are probably able

to produce all the different kinds of secretion, but usually

only one type is prevailing. It is also possible that small

quantities of substances, which the histochemical methods

do not evidence, are currently produced.

Our study was carried out on several species of Labiatae

belonging to the subfamily Lamioideae, and allowed evi-

dencing a variety of types of glandular trichomes, some of

them not yet reported in the current literature. The char-

acterization of the different types of trichomes concerned

not only the morphology, but also the functioning (kind of

secretion, storage and mode of its release). Thus, some

differences are emerging, which the morphology alone

could not evidence.

In conclusion, although glandular trichomes of Labiatae

are among the more studied glands, there is still much to be

learned about the structure, ultrastructure and secretory

substances of the glandular trichomes. Current knowledge

does not comprehend a sufficient number of species to

utilize trichomes as distinguishing characters of large taxa

like subfamilies or genus. Nevertheless, in some cases

glandular trichomes were used as characters for better

distinguishing plants at subgeneric or subspecific level, as

evidenced, for instance, in Stachys germanica group (Fal-

ciani 1997), in Thymus striatus Vahl (Maleci Bini et al.

1999), in Teucrium L. (Navarro and El Qualidi 2000) and

in Stachys recta (Giuliani et al. 2008).

Acknowledgments We greatly acknowledge the two anonymous

reviewers for their useful suggestions which contributed to the final

form of this paper, and Mr. Gabriele Tani for technical assistance.

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