Acta bot. bras. 24(4): 909-915. 2010.
Characterization and ontogeny of the glandular
trichomes of Ocimum selloi Benth. (Lamiaceae)
Letícia de Almeida Gonçalves1,3, Aristéa Alves Azevedo2 and Wagner Campos Otoni2
Recebido em 2/09/2009. Aceito em 19/08/2010
RESUMO – (Caracterização e ontogenia dos tricomas glandulares de Ocimum selloi Benth. - Lamiaceae). Ocimum selloi Benth. (Lamiaceae) é uma espécie
nativa da América do Sul e na medicina popular tem sido usada devido suas propriedades analgésica, anti-inflamatória e antiespasmódica. O objetivo do
presente trabalho foi identificar os tipos de tricomas glandulares que ocorrem nos órgãos vegetativos e reprodutivos de O. selloi e determinar a ontogenia
desses tricomas. Ramos laterais em início de formação, folhas totalmente expandidas, flores em diferentes estádios de diferenciação, amostras de caule e
do eixo das inflorescências foram analisados em microscopias de luz e eletrônica de varredura. Tricomas glandulares do tipo peltado e capitado subséssil
foram observados no caule, nas folhas, no eixo da inflorescência e na superfície adaxial das sépalas. Nas sépalas foi encontrado, além dos tricomas secretores peltados e capitados subsésseis, o tricoma glandular capitado pedunculado. A ontogenia inicia-se com a expansão de uma célula protodérmica que,
de acordo com a seqüência de divisões periclinais e anticlinais (ora simétricas, ora assimétricas), dá origem aos tricomas. A diferenciação dos tricomas
glandulares peltados e capitados não é sincrônica e ocorre muito cedo no desenvolvimento da folha, do caule e do eixo floral.
Palavras chave: alfavaca, elixir paregórico, estruturas secretoras, plantas medicinais
ABSTRACT – (Characterization and ontogeny of the glandular trichomes of Ocimum selloi Benth. - Lamiaceae). Ocimum selloi Benth. (Lamiaceae)
is native to South America and in traditional medicine has been used due to its analgesic, anti-inflammatory, and antispasmodic properties. The aim of
this study was to identify the types of glandular trichomes that occur on the vegetative and reproductive organs of O. selloi and to determine trichome
ontogeny. Lateral branches at the initial formation phase, fully opened leaves, flowers at different differentiation stages, and stem and inflorescence axes
were analyzed under light and scanning electron microscopy. Glandular trichomes of the peltate and subsessile capitate types were observed on the stem,
leaves, inflorescence axis and the adaxial surface of the sepals. On the sepals, in addition to the peltate and subsessile capitate secretory trichomes, the
stalked capitate glandular trichome type was detected. Ontogeny began with the expansion of a protodermic cell, which according to the sequence of
periclinal and anticlinal division (either symmetric or asymmetric) gave rise to the glandular trichomes. Differentiation of the peltate and capitate glandular
trichomes was not synchronized and occurred very early in the leaf, stem and floral axis.
Key words: basil, paregoric elixir, secretory structures, medicinal plants
Introduction
Lamiaceae is rich in species that produce essential oils
whose compounds are largely used in food (as flavorings),
cosmetics (fragrances and aftershaves), and medicine
(Burt 2004). The essential oils are lipophilic complexes
synthesized and stored in different secretory structures,
including the glandular trichomes in the Lamiaceae (Simões
& Spitzer 2002).
Ocimum selloi Benth. is a perennial shrub, native to South
America where it is known as “paregoric elixir”, “basil” or
“atroveran” (Lorenzi & Matos 2002). In traditional medicine
O. selloi has been used as analgesic, anti-inflammatory,
and antispasmodic. The essential oil extracted from leaves,
stem and flowers contain anethole and methyl chavicol as
their major components (Moraes et al. 2002; Gonçalves et
al. 2003; Paula et al. 2003). The O. selloi essential oil is an
efficient repellent of the Anopheles braziliensis mosquito
(Paula et al. 2003) and it has modest antimicrobial activity
against strains of Escherichia coli and Staphylococcus
aureus (Farago et al. 2004).
Even though some of the glandular trichomes of O.
selloi have been described by Martins (1996) and Costa et
al. (2007a) information is still scarce.
Glandular trichomes, frequently called glands, are
epidermal appendices formed by the head of a single cell or
1
2
3
pluricellular secretory cells and a non-glandular stalk (Fahn
1990). The number of secretory cells, number and length of
the stalk cell(s), and the density, location and arrangement of
these glandular trichomes in the epidermis can vary. Their
structure and ultrastructure in vegetative and reproductive
organs have been widely described in the literature (Werker
et al. 1985; Maffei et al. 1989; Werker et al. 1993; Ascensão
et al. 1995; Serrato-Valentin et al. 1997; Milaneze-Gutierre
et al. 2007; Martins et al. 2009).
Increased interest in the therapeutic use of O. selloi has
motivated researchers to investigate not only its biology
regarding reproduction (Facanali et al. 2009) but also optimal
growth conditions, e.g. cultivation (Costa et al. 2007b,
2010), harvest time, and temperature for dryness, that will
lead to improved content and composition of the essential
oil (David et al. 2006; Costa et al. 2007c). Studies on the
occurrence, location and characterization of the glandular
trichomes can provide important insights regarding harvest,
post-harvest and quality control of essential oil producing
plants. Furthermore, the adaptive and taxonomic value of
the glandular trichomes, and consequently the essential oils,
is of great importance in botanical studies (Abu-Asab &
Cantino 1987; Martins et al. 1997). The present study was
undertaken to identify the types of glandular trichomes that
occur on the vegetative and reproductive organs of Ocimum
selloi Benth. and to characterize trichome ontogeny.
Universidade Federal de Goiás, Instituto de Ciências Biológicas, Departamento de Biologia Geral, Goiânia, GO, Brasil
Universidade Federal de Viçosa, Departamento de Biologia Vegetal, MG, Brasil
Author for correspondence: leticia.icb.ufg@gmail.com
910
Gonçalves et al.: Characterization and ontogeny of the glandular trichomes of Ocimum selloi Benth. (Lamiaceae)
Materials and methods
Seed-derived plants of Ocimum selloi Benth. (Lamiaceae) were collected
in the “Grupo Entre Folhas Medicinal Plants”, located at Universidade
Federal de Viçosa (UFV), Viçosa (20°45´20” S; 42°52´40” W), Minas
Gerais state, Brazil. The climate of the region is the “Tropical Altitude”
type (Cwb), with warm summers and temperatures in the hottest month
below 22°C (Köppen 1948).
Seedlings (average 3 cm tall) were collected and placed in plastic bags
containing soil, sand and manure (1:1:1), and were initially kept partially
shaded (50% light reduction) and watered on a daily basis. After 41 days,
seedlings with 4 nodes (approximately 7 cm tall) were transplanted to sixliter polyethylene containers, filled with soil, manure and sand (3:1.5:1
ratio). After transplant, six plants were kept under greenhouse conditions.
The substrate was kept at 80% of field capacity. Maximum and minimum
temperatures were recorded daily.
Voucher materials were prepared according to standard herbarium
protocol and placed in the VIC Herbarium at the Plant Biology Department,
Viçosa, MG (# 23,642).
Lateral branches at the initial formation stage were collected and
embedded in paraffin. Fully opened leaves were used for clarification,
embedding in paraffin, and for scanning electron microscopy. The samples
were collected from the mid-region of leaves located at the second node, from
tip to base, of the main axis of the plants. Stem and inflorescence axis samples
were prepared for embedding in paraffin and scanning electron microscopy.
Buds and open flowers were embedded in paraffin and sepals from open
flowers were prepared for scanning electron microscopy. In addition, freshly
prepared sections were made from stem and fully expanded leaves.
The samples were fixed in FAA50 for 24 hours, dehydrated in
progressive ethylic-butyric series and embedded in paraffin according to
the usual techniques (Johansen 1940). Transverse sections were made in the
inflorescence axis, leaves (petiole and blade) and in the stem. Transverse and
longitudinal sections were made in the buds, opened flowers, and branches
at the initial formation phase. Thick sections (12 µm) from all samples were
obtained in a rotating microtome. The inflorescence axis, leaves and stem
were stained with astra blue 1% (for 2 minutes) and ferric hematoxylin
(for 30 minutes), while the branches at initial formation phase and flowers
were stained with ferric hematoxylin (for 120 minutes). The sections were
mounted between a slide and slide cover in Canadian balsam.
For clarification, some samples were placed in 50% ethanol, stored in
70% ethanol and further processed according to the procedure adapted from
Johansen (1940): 10% NaOH solution, for 3h, 3% sodium hypochlorite
solution until bleached, and 10% hydrated chloral (CCl3CHO.H2O) until
the material was completely translucent. The material was stained with
safranin 1% (for approximately 24 hours), and after dehydration was
mounted between slide and slide cover in Canadian balsam. The remaining
samples were placed directly in hydrogen peroxide until bleached, stained
with safranin and hematoxylin for a few seconds, and mounted between
slide and slide cover in hydrated glycerin (3:1).
Photographic documentation was done by an Olympus microscope
model AX-70 with the U-PHOTO system.
The samples for scanning microscopy were fixed in 2% glutaraldehyde
and 1% osmium tetroxide and after using 1% thiosemicarbazide and postfixed with 1% osmium tetroxide (Silveira 1989). The material was dehydrated
in a progressive ethanol series, dried to the critical point with liquid CO2,
fixed on a support with silver glue and covered with metallic gold (10 nm).
Photographic documentation was done by a JEOL-T 200 scanning
microscope at 15 Kv.
The types of glandular trichome were classified according to Werker et
al. (1985) and Werker et al. (1993).
Results
Peltate and capitate glandular trichomes were detected
in O. selloi. The O. selloi capitate type was further divided
into subtypes designated here as subsessile capitate and
stalked capitate.
Peltate and subsessile capitate glandular trichomes
(Figure 1) were detected in the stem, leaves (petiole and
blade), inflorescence axis and sepals.
The peltate type consists of four cells in the secretory
head, one cell in the stalk (low and necklace-shaped) and
a base cell. This type of glandular trichome was frequently
observed in depressions in the epidermis (Figure 1A-B). The
subsessile capitate glandular trichome consists of two cells
in the secretory head, one cell in the stalk and one basal cell
(Figure 1C-D).
The stalked capitate type glandular trichome is formed
by two cells in the stalk and one secretory cell (forming the
head) (Figure 2A-B), where small droplets were observed
(Figure 2B). The stalked capitate type was observed on the
adaxial surface of the sepals (Figure 2C-D), however it was
not found on the petals, stamens and gynoecia.
Several events were defined in peltate and capitate
glandular trichome ontogeny (Figure 3). Firstly, a
protodermic cell expands, differentiating itself from the other
cells and the nucleus migrates to the distal pole of the cell.
The equal distribution of cytoplasm was visible at this point,
with the vacuole located almost completely on the opposite
side of the nucleus (Figure 3A). Secondly, the cell divides
periclinally giving rise to a more vacuolated basal cell and
an apical cell with fairly dense cytoplasm and voluminous
nucleus (Figure 3B-C). Thirdly, the apical cell divides
periclinally originating the cell from which the secretory
head and the stalk cell will derive (Figure 3D-E); there
was an anticlinal symmetric division with the subsequent
formation of the two secretory cells (Figure 3F-G). These
two apical cells and the stalk cell expand without further
division and the final product is a capitate glandular trichome
with two head cells (Figure 3H). Two apical cells, displaying
typical voluminous nucleus and dense cytoplasm, divide
again anticlinally, and form four secretory head cells, thus
creating the stalked capitate type glandular trichome (Figure
3I). It was observed whether the division of the two cells
into four occurred simultaneously. Cells in anaphase and
telophase were observed in several of the events (Figure 3).
The O. selloi peltate and subsessile capitate glandular
trichome differentiation occurred very early in leaf, stem
and flower development. These glandular trichomes were
observed at various differentiation stages even before the
differentiation of stomata mother cells. Protodermic cells
were observed in expansion in very young primordial
leaves and in the internodes farthest from the stem apex,
and occasionally, glandular trichomes at more advanced
differentiation stages. Glandular trichomes at different
differentiation stages were also observed in the same
region of these primordial leaves or the stem, showing that
development was not synchronized.
The initial events of the stalked capitate glandular
trichome ontogeny are probably similar to those of peltate
and subsessile capitate ones. However, stalked capitate
glandular trichomes were observed only in the end stage of
Acta bot. bras. 24(4): 909-915. 2010.
911
Figure 1. Glandular trichomes present in the Ocimum selloi Benth. stem, leaves and flowers. A and C, transverse sections in light microscopy. B and D, scanning
electron microscopy. A and B, peltate type. C and D, subsessile capitate type. Bars = 20 μm.
differentiation, with short stalk cells (Figure 3J), and it was
not possible to define the precise sequence of development.
The maximum and minimum average temperatures
throughout the experiment were 33.5°C and 15.5°C,
respectively.
Discussion
Although the number of species examined in detail in the
Lamiaceae family is considered small, peltate and capitate
glandular trichomes are common to the species already
studied in the family and have been described in detail in
the literature.
The occurrence of capitate and peltate glandular trichomes
in leaves of Ocimum selloi has been documented by Martins
(1996) and Costa et al. (2007a; 2010). In the present study,
besides confirming previous findings, further work was
accomplished in characterizing the types of trichomes that
occur on the vegetative and reproductive organs of O. selloi.
According to Werker et al. (1985) the capitate type
glandular trichomes are less variable than the peltate
glandular trichome regarding the length of the stalk cells
and the shape of the secretory head. The secretory head of
the peltate trichomes consists of four cells in the middle and
variable number of cells surrounding them. Others consist
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Gonçalves et al.: Characterization and ontogeny of the glandular trichomes of Ocimum selloi Benth. (Lamiaceae)
of four secretory cells only. The capitate trichomes are
generally formed by one or two cells in the stalk and by
one or two secretory cells. Werker et al. (1985) classified
the capitate glandular trichomes in the Lamiaceae family
in three subtypes. The subsessile and pedunculated capitate
glandular trichomes observed in Ocimum selloi are similar
to subtypes I and III described by these authors, respectively.
Type I consists of 1-2 cells in the stalk and 1-2 secretory
cells, whereas type III consists of 2-5 cells in the stalk and
one cell in the secretory head.
Figure 2. Glandular trichomes present in the Ocimum selloi Benth. flowers. A, transverse section in light microscopy. B-D, scanning electron microscopy. A and B,
pedunculated capitate trichome, with one secretory cell. C and D, overall view of the adaxial and abaxial sepal surfaces, respectively. Bars = 20μm.
Acta bot. bras. 24(4): 909-915. 2010.
913
Figure 3. Differentiation pattern of the Ocimum selloi Benth. glandular trichomes. A, Initial diferenciation of the glandular trichome. B, C, D e E, periclinal divisions. F,
anticlinal division. G and H, expansion of the two apical cells or, I, formation of the capitate trichome. J, stalked capitate with the stalk cells in expansion. Bars = 20μm.
914
Gonçalves et al.: Characterization and ontogeny of the glandular trichomes of Ocimum selloi Benth. (Lamiaceae)
Small droplets were observed in the stalked capitate
glandular trichome, indicating that the secretion could be
released without rupturing the cuticle. This type of secretion
was reported in Salvia aurea L. and S. blepharophylla
Brendegee ex Epling capitate glandular trichomes (SerratoValentin et al. 1997; Bisio et al. 1999).
The clarification process and paraffin embedding
considerably damaged the peltate glandular trichome
structure, which presented deformed secretory cells and
subcuticle space, characteristic of this type of ruptured
glandular trichome. However, the subcuticular space was
observed in fresh sections of the stem and leaves, indicating
one of the possible ways of storing and eliminating the
material synthesized in this type of glandular trichome.
According to Werker et al. (1985), in several Lamiaceae
species a continuous layer, formed by the cuticle and by
part of the external wall of the secretory head cells, detaches
itself from the rest of the walls forming a space where the
secretions are accumulated. External factors, such as high
temperatures, low air humidity or aggression by animals,
may break this layer leading to the release of the oil
(Ascensão et al. 1995).
Glandular trichomes were not observed in the stamens
and gynoecia of O. selloi. However, although few species
have been examined, several authors have reported
the occurrence of glandular trichomes in reproductive
Lamiaceae organs. Glandular trichomes of the peltate and
capitate types were detected on reproductive organs of
Leonotis leonurus (L.) R. Br. Those of the capitate type
were observed in the calyx, corolla, stamens and gynoecia,
but the capitate type glandular trichomes occurred rarely
and were numerous only on the abaxial surface of the calyx
(Ascensão et al. 1995). It is possible that the number of
glandular trichomes on the stamens and gynoecia of O.
selloi is indeed small, or that the embedding process in
paraffin may have contributed to damaging the structures,
though impairing observation.
Capitate and peltate glandular trichomes also occur in
other species of the genus Ocimum. In O. basilicum the
capitate trichomes are composed of one basal cell, one stalk
cell and the head of either one elongated, oval cell or two
cells broad (Werker et al. 1993). In O. gratissimum (Martins
et al. 2009), the capitate and peltate glandular trichomes
on the leaves are similar to those found on the leaves of O.
selloi, and cannot be used to distinguish the two species.
It is noteworthy that the oil produced on the leaves of
O. selloi has repellent (Paula et al. 2003) and antimicrobial
(Farago et al. 2004) activities, indicating the possible role
of glandular trichomes and their essential oil in protecting
the plant against herbivores and pathogens.
The ontogeny of glandular trichomes of Ocimum selloi
follows the pattern described for other species of the
Lamiaceae family such as Origanum sp. (Bosabalidis &
Tsekos 1984), Leonotis leonurus (Ascensão et al. 1995),
and Mentha x piperita (Turner et al. 2000b).
Bearing in mind the differentiation of the glandular
trichomes in Ocimum selloi, it can be presumed that
the number of glandular trichomes in this species is
fixed at the initial development stages of the leaf and
stem, similar to O. basilicum L. (Werker et al. 1993),
Leonotis leonurus (Ascensão et al. 1995) and Origanum
sp. (Bosabalidis & Tsekos 1984). In some species this
fact has not been observed. It was shown that during
Mentha x piperita L. leaf development the total number
of peltate type glandular trichomes continued to form until
leaf expansion ceased (Maffei et al. 1989; Turner et al.
2000a), indicating sustained gland production during leaf
growth. Similar results were reported for Salvia officinalis
L. (Croteau et al. 1981).
The ontogeny of the stalked capitate type glandular
trichomes is probably similar to that of peltate and subsessile
capitates ones. The hypothesis is that cell expansion occurs
only after the formation of the secretory cell is completed.
The stalked capitate glandular trichome probably forms
later in the flower differentiation process. The peltate and
subsessile capitate glandular trichomes are found on the
sepals of incompletely differentiated buds to fully opened
flowers. Conversely, the stalked capitate glandular trichomes
were only observed in flowers just before anthesis.
It should be pointed out that in the flowers, the peltate
and subsessile capitate glandular trichomes seemed to
differentiate first in the most basal region of the abaxial
surface of the sepals. The maximum number of these
glandular trichomes was also observed in this region when
the flowers were open. This fact can be explained considering
the influence of light on glandular trichome differentiation.
According to Maffei et al. (1989), for example, in Mentha
x piperita the abaxial and adaxial surfaces of the epidermis
generally present different development rates for glandular
trichomes. The abaxial surface showed little change in the
number of glandular trichomes during leaf development
and the density quickly decreased with leaf opening,
while the number of glandular trichomes increased on
the adaxial surface. This difference between the numbers
of glandular trichomes on the two epidermis during leaf
development is caused by the influence of solar radiation on
the differentiation of these structures. The abaxial surface
of the leaf is directly exposed to solar radiation during the
first stages of development; in the subsequent stages the
situation is reversed. Consequently, glandular trichome
development is inhibited on the abaxial epidermis and
induced on the adaxial epidermis. The abaxial surface of
Ocimum selloi sepals, unlike the adaxial surface, is exposed
to light throughout the flower differentiation stages that
probably induces the formation of the glandular trichomes
first on the surface.
Acknowledgements
CNPq (National Council for Scientific and Technological Development)
is acknowledged for the financial support.
915
Acta bot. bras. 24(4): 909-915. 2010.
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