Trichomes

The aerial parts of many plants are covered with trichomes, which may be glandular or nonglandular. The members of each category exhibit a broad spectrum of variability, as concerns their shape, size, anatomy, ultrastructure, function, origin, pattern of development, etc. More than a century ago, microscopists tried to investigate some of the above trichome characteristics (particularly the hair form) and used them as a criterion for plant classification (Weiss, 1867; de Bary, 1877). The importance of the various types of trichomes in systematics has been greatly acknowledged and many works on this subject have appeared since that time (Gupta and Bhambie, 1978; Metcalfe and Chalk, 1979; Cantino and Sanders, 1986; Karousou etal., 1992). Modern techniques, such as sophisticated microscopy (scanning and transmission electron microscopy), analytical chemistry, biochemistry, genetics, molecular biology, biotechnology, etc. have been use as fundamental means for the study and classification of plant trichomes (Wollenweber, 1984; Harborne etal, 1986; Walt van der and Demarne, 1988; Bini-Maleci and Servettaz, 1991; Berta et al, 1993). As to the functional and ecological roles of the plant pubescence, the nonglandular hairs were long time ago considered to have a significant contribution to the obstruction of the free movement of the water vapours from the stomata (transpiration), as well as to the reduction of leaf overheating (Gradmann, 1923; Staudermann, 1924). They have been also considered to create difficulties in the movement, feeding and oviposition of mites and aphids on the leaf surface (Walters et al, 1989; Goertzen and Small, 1993).

The glandular hairs seem to operate in different ways than the nonglandular ones. Their secretions (monoterpenes, sesquiterpenes, phenolics, sucrose esters, etc.) have been observed to have a repellent character on harmful insects (Levin, 1973; Werker, 1993), to be toxic when eaten (Klingauf et al, 1983; Bestmann et al, 1987), to inhibit egg hatching (Sharaby, 1988; Konstantopoulou etal., 1992) and to function as sticky traps (Kowalski etal., 1988). Diterpenes and triterpenes have been further found to be deterrent, toxic and severe skin irritants to herbivorous mammals (Rosenthal and Berenbaum, 1991). Of significance are, in addition, the antimicrobial and allelopathic properties of the terpenoid components of the essential oils secreted by the glandular hairs (Vokou and Margaris, 1986; Sivropoulou et al., 1996). Recently, an important role has been attributed to the glandular hairs and their secretions (phenolic compounds of the essential oils), as concerns their implication in the protection of the mesophyll cells from UV-B radiation (Fahn and Shimony, 1996; Bosabalidis and Skoula, 1998b; Manetas, 1999). Glandular hair products may not only have a repellent character for insects, but also an attractive one, as in the case of nectaries, where they play a decisive role in the process of pollination (Fahn, 1979; Sawidis et al, 1989).

Nonglandular hairs

In Origanum taxa, nonglandular hairs have been observed to occur on the vegetative and reproductive organs as well (Werker et al., 1985b). In the leaves, they are present on both surfaces, but their number is higher on the veins of the lower leaf surface (Figures 2.1, 2.2). Their density and length increase toward the base of the blade.

Structure

The nonglandular hairs of Origanum are usually falcate filiform, uniseriate, composed of 4—7 cells (branched configurations may also exist, as in the case of O. dictamnus). The most distal cell of each hair ordinarily points to the leaf tip (Werker et al, 1985 b; Bosabalidis and Exarchou, 1995).

Apart from Origanum, this type of nonglandular hairs has been also observed in other genera of Lamiaceae, such as Ocimum (Werker etal, 1993), Teucrium (Bini-Maleci and Servettaz, 1991), Nepeta (Bourett etal, 1994), Monarda (Heinrich, 1973), Thymus (Economou-Amilli et al, 1982), Satureja (Bosabalidis, 1990b), Salvia (Serrato-Valenti et al, 1997), Mentha (Gavalas et al, 1998), Plectranthus (Ascensao et al, 1998), Leonotis (Ascensao et al, 1995), Coridothymus, Majorana, Micromeria, Melissa (Werker et al, 1985a), etc.

Though the falcate form of nonglandular hairs seems to be the dominant in Lamiaceae, other types, like flask-shaped in Teucrium (Bini-Maleci and Servettaz, 1991), conical (unicellular or bicellular) in Salvia, Monarda and Thymus (Heinrich, 1973; Economou-Amilli etal, 1982; Bisio etal, 1999) and capitate (uniseriate and multi-seriate) in Ocimum (Gupta and Bhambie, 1978) may also exist.

Ontogeny

The nonglandular hairs of Origanum initiate and develop very early before the mesophyll becomes differentiated into palisade and spongy parenchymas. They originate from a single protodermal cell which is much voluminous than its neighbouring cells (Figure 2.15). This initial cell contains a centrally located nucleus and it divides periclinally and asymmetrically to give two daughter cells i.e. a large vacuolated basal cell and a second dome-shaped cell seated above the former (Figure 2.16). The basal cell may further divide twice to produce four cells, or it may divide successively resulting in the formation of a pedestal. The apical daughter cell actually constitutes the mother cell of the hair and it undergoes a series of periclinal divisions to derive two (Figure 2.17), three (Figures 2.18—2.20), four, five (Figure 2.21), and so on, up to seven cells. The derivatives are initially short (Figures 2.17, 2.18), but they progressively increase in length (Figures 2.19, 2.20). The free end of the most distal cell of the hair becomes acute and in the progress of development it bends toward the leaf tip (Figure 2.20). Branching of nonglandular hairs (Figure 2.22) may be frequent (O. dictamnus) or rare (O. vulgare, O. onites, Origanum X intercedens, etc.). The cytological characteristics of the cells composing a nonglandular hair comprise a large central vacuole with no dark deposits, as well as a thin peripheral

Unicellular Hairs Leaves

Figures 2.15—2.22 Ontogeny of nonglandular hairs of Origanum vulgare (X 450). Figure 2.15 The initial cell of the hair (ic). Figure 2.16 The 2-celled stage (one periclinal division). Figure 2.17 The 3-celled stage (two periclinal divisions). Figure 2.18 A young hair composed of one basal cell and three piled vacuolated cells above it. The most distal cell is pointed.

protoplasmic layer in which the nucleus and other organelles are embedded. The plastids are represented by leucoplasts. The cell walls (anticlinal and periclinal) are thin with primary texture and no deposition of cutin or suberin.

Glandular hairs

The glandular hairs are epidermal appendices, as the nonglandular hairs, but they are more complicated from the anatomical, ultrastructural and functional points of view. They biosynthesize and secrete a broad spectrum of substances, like essential oils, resins, gums, slimes, nectar, salty solutions, etc. In the family of Lamiaceae, glandular hairs principally produce essential oils and our report will be limited to only this sort of hairs.

Glandular hairs in members of Lamiaceae develop on all aerial plant organs. The majority of the relevant studies have been conducted on the leaf, but there is a remarkable number of observations made on the calyx, the corolla, the stamens and the carpels of the flower (Modenesi etal., 1984; Werker etal., 1985b; Servettaz etal., 1994; Ascensao et al., 1995; Corsi and Bottega, 1999). The glandular hairs initiate very early, when leaves are still at the primary stage. This situation seems to hold true not only for Lamiaceae (Bosabalidis and Tsekos, 1982a; Werker etal, 1985b; Maffei etal, 1986; Danilova and Kashina, 1989; Bourett etal, 1994), but also for other families with trichomes secreting essential oils, such as the Compositae (Vermeer and Peterson, 1979a; Werker and Fahn, 1982; Spring and Bienert, 1987), Geraniaceae (Oosthuizen and Coetzee, 1984), Simarubaceae (Bory and Clair-Maczulajtys, 1981), Valerianaceae (Corsi and Pagni, 1990), Dioscoreaceae (Bruni etal., 1987), etc. In the primary leaves, all stages of gland development can be found and consequently these leaves are the most appropriate for ontogenetic studies.

Microscopic investigations on aromatic plants have revealed that young leaves bear both developing and mature glandular hairs (Werker etal., 1993; Duke and Paul, 1993; Ascensao etal., 1995; Bosabalidis and Skoula, 1998b), whereas fully expanded leaves only mature glandular hairs (Amelunxen, 1965; Bory and Clair-Maczulajtys, 1981; Werker et al, 1993; Ascensao et al, 1999). Cases are reported, however, in which grown leaves possess both mature and young oil glands (Dell and McComb, 1975; Venkatachalam etal, 1984; Oosthuizen and Coetzee, 1984; Bell and Curtis, 1985; Fahn and Shimony, 1998). The fact that in the same leaf (regardless of whether it is primary or grown) initiating, developing and senescing glandular hairs may simultaneously occur, reveals that the process of essential oil secretion does not proceed synchronously. The formation and pattern of development of the glandular hairs, as well as the biosynthesis of the terpenoids and the mechanism of their secretion seem to be governed by genes (Nielsen etal, 1982; Rosenthal and Berenbaum, 1991). Environmental factors (light, water, nutrients, etc.) may though affect these parameters. An interesting point of consideration is the density of the glandular hairs on the leaf surface. In some aromatic plants [Thymus (Economou-Amilli et al, 1982; Letchamo and Gosselin, 1996), Rosmarinus (Werker et al, 1985a), Mentha (Maffei et al, 1986; Gavalas

Figure 2.19 Advanced elongation of the hair cells. Figure 2.20 A fully-developed hair. The basal region is composed of more than one cells, while the piled cells have undergone maximal elongation. The most distal cell is bent. Figure 2.21 A non-glandular hair composed of the basal region and five piled cells. Figure 2.22 A branched nonglandular hair.

et al, 1998), Salvia (Venkatachalam et al, 1984; Corsi and Bottega, 1999) Plectranthus (Ascensao etal, 1999), Leonotis (Ascensao etal, 1995), Nepeta (Bourett etal, 1994), Rosmarinus (Maffei et al, 1993), Teucrium (Bini-Maleci and Servettaz, 1991), etc.], glandular hairs are more numerous on the lower leaf side, whereas in some others [Origanum (Werker et al, 1985b; Bosabalidis and Kokkini, 1997), Artemisia (Corsi and Nencioni, 1995), Tamus (Bruni et al, 1987), Ailanthus (Bory and Clair-Maczulajtys, 1981), etc.] on the upper leaf side. The density of glandular hairs on the leaf surface is functionally associated with transpiration, leaf overheating, insect attack, UV-B radiation, etc.

During leaf development and expansion, the number of glandular hairs may remain more or less stable (Henderson et al, 1970; Werker et al, 1993; Ascensao et al, 1995), or it may change (Maffei et al, 1986). In several articles the view has been expressed that during leaf growth there is a continuous formation of new glandular hairs, so that old leaves are richer in glandular pubescence than young ones (Dell and McComb, 1975; Oothuizen and Coetzee, 1984; Bruni etal, 1987; Mahlberg etal, 1984). Goertzen and Small (1993) consider, however, that the density of glandular hairs decreases with leaf maturity, indicating that the young leaves have denser hair coverings. This could be an adaptive feature of the plant, wherein the young, most tender and appetizing to herbivores leaves, are given the highest level of protection (many secretions of glandular hairs were found to be deterrent or toxic to insects). A denser glandular hair covering in the young leaves has been also observed in Inula (Werker and Fahn, 1982), Mentha (Maffei etal, 1986), Origanum (Bosabalidis and Skoula, 1998b), Pelargonium (Walters et al, 1989), Valeriana (Corsi and Pagni, 1990), Nicotiana (Nielsen etal, 1991), etc.

The number of glandular hairs on the leaves of aromatic plants is linearly associated with the essential oil yield. Thus, the greater the number of glandular hairs on the leaves is, the higher the amount of essential oil derived from them by distillation (Bosabalidis and Kokkini, 1997; Gavalas et al, 1998). This is due to the fact that the glandular hairs are the exclusive leaf sites of essential oil biosynthesis, as biochemical studies with isolated glandular hairs have evidenced (Gershenzon etal., 1989; Mc Caskill and Croteau, 1995). No other leaf compartments, epidermal or mesophyl-lic, are able to metabolize terpenoids into essential oils. An indirect participation of the photosynthesizing leaf tissue in the provision of the secretory cells of the glandular trichomes with precursors is, however, acknowledged.

The glandular hairs on the upper and lower leaf surfaces may not only differ in number and density, but also in the qualitative and quantitative constitutions of the secretions they produce. Comparative studies of essential oil analyses separately for the hairs of the upper and the lower surfaces of mature leaves have shown that oil composition may either slightly fluctuate, as in the case of Ocimum basilicum (Werker et al, 1993) and Origanum X intercedens (Bosabalidis and Skoula, 1998b), or it may exhibit remarkable alterations, as in the case of Mentha piperita (Maffei etal, 1989). In the young leaves, the differences between the upper and lower leaf sides, as concerns the content and composition of the essential oil produced by the glandular hairs, are quite sharper (Werker et al, 1993).

The shape, density, size and position of the glandular hairs, as well as the oily property, the chemical constitution and the fragrant character of the essential oils, are parameters not accidentally created by nature on aromatic plants and much more investigation is needed to increase data on the structural, functional and ecological points of the glandular hairs and their secretions.

Types of glandular hairs

From the morphological studies conducted so far on members of Lamiaceae, it follows that on the aerial plant organs and particularly on the leaves, two types of glandular hairs mainly exist, the peltate and the capitate. These types co-occur to create a mixed population. An integrated glandular hair (peltate or capitate) has been described to be composed of a basal region (unicellular or multicellular), a stalk region (unicellular or multicellular) and a head region (unicellular or multicellular). In our opinion, however, a structurally and functionally integrated glandular hair, additionally to the above three regions, also contains the epidermal cells which radially surround the basal region. Such cells have been observed in the peltate hairs of Origanum (Bosabalidis and Tsekos, 1984; Bosabalidis and Exarchou, 1995; Bosabalidis and Kokkini, 1997), Calamintha (Hanlidou etal., 1991), Ocimum (Werker etal., 1993), Satureja (Bosabalidis, 1990b), Mentha (Gavalas etal, 1998), Monarda (Heinrich, 1973), etc. The beribasal cells do not operate like the typical epidermal cells, but they become induced by the glandular hair [presumably in a way analogous to that between the guard and subsidiary cells of stomatal complexes (Stebbins and Shah, 1960)] to constitute an accessory of the hair and serve the process of essential oil secretion. Thus, the large size of the beribasal cells, their shape, arrangement, vacuolation, density of plasmodesmata at the periclinal walls, etc, most probably contribute to the collection of the photosynthates from the chlorenchymatic mesophyll and to their further centripetal transport to the basal cell of the glandular hair. The latter cell in this way becomes a central pool in which the photosynthates are temporarily deposited, which is why it is very voluminous (significantly larger than the typical epidermal cells) and greatly vacuolated. The photosynthetic products become then moved to the stalk region of the glandular hair and finally to the head region, in which their elaboration into essential oil takes place through the enzymatic machinery exclusively possessed by the head secretory cells.

Capitate glandular hairs

Structure The capitate glandular hairs are much smaller than the peltate ones, occur in thicker populations and exhibit a greater morphological variability. Werker etal, (1985a) distinguish three main types of capitate hairs in Lamiaceae, i.e. Type I (short) with one basal cell, 1—2 stalk cells and 1—2 head cells (rounded, ovoid or pear-like), Type II (medium) with one basal cell, 1—2 stalk cells and one head cell (finger-or pestle-like) and Type III (long) with one basal cell, 2—5 stalk cells and one head cell (rounded). In Table 2.6, some morphological data from Lamiaceae members are summarized, as concerns the above-mentioned types of capitate glandular hairs occurring on the leaf surface. From these data it follows that the most frequent type of capitate hairs met in all species studied, is Type I. This type may be exclusive (Satureja, Mentha, Thymus, Coridothymus, Rosmarinus, Nepeta) or it may be accompanied either by Type II capitate trichomes (Majorana, Micromeria, Melissa, Origanum, Ocimum, Calamintha) or by Type III capitate trichomes (Salvia, Plectranthus, Teucrium).

In the leaves of Sideritis syriaca subsp. syriaca, another type of capitate glandular hair has been observed composed of four symmetric basal cells, three elongated stalk cells, a single plasma rich neck cell and four symmetric head cells (Karousou etal., 1992). Peculiar is also the anatomy of a sort of capitate glandular hair occurring on the leaves of Leonotis leonurus (Ascensao etal., 1995). This hair is constructed of an 8—32 celled pedestal base, a 2—3 celled stalk, an 1—2 celled neck and a four celled head. Both of

Table 2.6 Types of capitate glandular hairs in members of Lamiaceae

Taxon

Type I

Type II

Type III

References

Satureja thymbra

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