Henderson et al., 1970

© 2002 Taylor & Francis

© 2002 Taylor & Francis surface, which in total create a shield protecting from microorganism attack and UV-B radiation.

Structure One of the most frequently occurring in Lamiaceae kind of peltate glandular hairs is that composed of a 12-celled head (Table 2.7). This kind of hair is represented in the species of Origanum (Bosabalidis and Tsekos, 1982a, 1984; Werker etal, 1985b; Bosabalidis and Exarchou, 1995). At the developmental stage when cell divisions have been completed but secretion of the essential oil has not started yet (the oil-accumulating subcuticular space is not formed), the peltate glandular hairs appear in leaf cross section composed of a large vacuolated basal cell, a flattened stalk cell, and (most often) four plasma rich head cells arranged parallel to the epidermal surface (Figure 2.30). From the head cells, the two inner ones are smaller than the two lateral. In the fully-differentiated and secreting peltate hairs, the situation remains the same, as concerns the number of cells, and additionally a large subcuticular space filled with essential oil is formed (Figure 2.31). In leaf cross sections, thus, a mature peltate glandular hair usually appears composed of a four celled head, a unicellular stalk and also an unicellular base.

The anatomical picture of the Origanum peltate hair is quite different in leaf paradermal sections. In such sections, the head appears to consist of 12 cells, from which four are small and centrally located and eight are large and peripherally arranged (Figure 2.32). Serial paradermal sections cut toward the base of the peltate hair, disclose a progressive disappearance of the four central cells (the eight peripheral cells are still well distinguished), while simultaneously in the centre of the head the stalk region gradually emerges (Figures 2.33—2.35). This region becomes proved to be unicellular (Figure 2.36). At the most proximal level of section, the basal region appears also unicellular and radially surrounded by 12—18 elongated epidermal cells (Figure 2.37). Paradermal sections are thus necessary to elucidate the real number, as well as the manner of arrangement of the cells in all three hair regions. In conclusion, a typical peltate glandular hair in Origanum is composed of a 12-celled head, a unicellular stalk, a unicellular base and a peribasal crown of 12—18 radial epidermal cells.

Ontogeny The study of the ontogeny of the peltate glandular hairs is a key procedure elucidating obscure points related to the site of gland initiation, the sequence and manner of cell divisions, the final number and arrangement of the hair cells, the changes in the hair shape, the cytological alterations, the determination of the stage of secretion, the process of formation of the essential oil-accumulating chamber, the cell characteristics at the postsecretory and senescing phases, etc. Descriptions of peltate glandular hair ontogeny have been briefly or in detail reported for some Lamiaceae members, such as Origanum (Bosabalidis and Tsekos, 1982a, 1984; Bosabalidis and Exarchou, 1995), Thymus (Bruni and Modenesi, 1983), Pogostemon (Henderson etal, 1970), Salvia (Venkatachalam etal, 1984), Sideritis (Karousou et al., 1992), Leonotis (Ascensao et al., 1995), Mentha (Fischer and Hecht-Buchholz, 1985), Calamintha (Hanlidou et al., 1991), Monarda (Heinrich, 1973), etc.

In Origanum, a peltate glandular hair originates from a single protodermal cell dominating in volume (Figure 2.38). In this cell, the centrally located nucleus progressively migrates toward the apical region, while the bottom region becomes occupied by vacuoles. The thus established morphological polarization results in an asymmetric periclinal division from which two unequally sized and shaped cells are derived (Figure 2.39). The proximal daughter cell is peg-like and it does not divide any further constituting the future basal cell of the peltate hair. The distal cell is dome-like, much larger and it

Figure 2.30 Longitudinal section of a peltate glandular hair after conclusion of divisions (subcuticular space not yet formed). The hair appears in the section composed of one basal cell (bc), one stalk cell (sc) and four head cells (hc) (X750).

Figure 2.31 A fully-developed peltate glandular hair of Origanum dictamnus. The basal and stalk cells are highly vacuolated, whereas the head cells are plasma rich. The cuticle becomes greatly detached from the apical cell walls forming a large subcuticular space (ss) (X700).

redivides asymmetrically and periclinally to give rise to a lower flattened cell (future stalk cell of the hair) and an upper rounded cell (Figure 2.40). The latter corresponds to the mother cell of the head and it undergoes a series of anticlinal divisions to ultimately form the head of the peltate hair (Figures 2.41, 2.42).

Integration of cell divisions in the head of the peltate hair is followed by the stage of essential oil secretion. The secreted material becomes released into a space formed at the tip of the hair by detachment of the cuticle from the apical walls of the head cells. This

Figures 2.32—2.37 Leaf paradermal sections serially cut from the region of the head to the region of the base of a peltate glandular hair (X700). Figure 2.32 A fully-developed head composed of four small central cells and eight large peripheral cells (hc). Figure 2.33 A section at a lower level. In the centre of the head emerges the stalk region. Figure 2.34 Even lower section. From the four central cells, only small portions can be discerned, whereas the eight peripheral cells are still intact. Figure 2.35 The stalk region of the peltate hair surrounded by the eight peripheral cells in reduced volume (the four central cells become disappeared). Figure 2.36 The stalk region composed of one cell (sc). The nucleus is centrally located. Figure 2.37 The basal region, also composed of one cell (bc). This cell is radially surrounded by a number of epidermal cells (ec).

oil-containing subcuticular space initially is small (Figure 2.43) and later greatly increases in volume (Figure 2.44). After secretion is concluded, the peltate hair disintegrates (Figure 2.45). During this last ontogenetic stage, the cuticle does not remain any longer stretched and ballooned, but it sediments down to locally touch the head apical walls.

To follow the development of the head of the peltate glandular hair, leaf paradermal sections have been used (Bosabalidis and Tsekos, 1984). In such sections, the mother cell of the head (Figure 2.46) appears to divide twice symmetrically and anticlinally resulting in two (Figure 2.47) and then four (Figure 2.48) equally-sized daughter cells. Each of these cells redivides asymmetrically through a bent wall, to give a head with

Figures 2.38—2.45 Ontogeny of peltate glandular hairs in Origanum vulgare (X500). Figure 2.38 The initial cell of the peltate hair (ic). Figure 2.39 First periclinal division of the initial cell to give the future basal cell and an apical dome-like cell. Figure 2.40 Second periclinal division resulting in the basal cell, stalk cell and mother cell of the head. Figure 2.41 First anticlinal divisions in the head region. Figure 2.42 Integration of anticlinal divisions in the head region. Figure 2.43 Slight detachment of the cuticle from the apical walls of the head cells to form a small subcuticular space (ss). Figure 2.44 Advanced detachment of the cuticle creating a large subcuticular space (ss) in which the essential oil will be accumulated. Figure 2.45 Disorganized head cells in a senescent peltate hair.

four small lateral cells and four larger inner cells (Figure 2.49). The last developmental stage involves a morphological polarization and subsequent asymmetric division of the inner cells, so that finally the head is composed of four small central cells surrounded by eight large peripheral ones (Figure 2.50). In consideration of the above manner of head cell origination, the speculation could be expressed that the four central cells of the head produce different essential oil constituents from the eight peripheral cells and that the final mixture takes place within the subcuticular chamber.

The ontogeny of the peltate glandular hairs of Origanum was further studied at the submicroscopic level, in order to draw detailed information about the intracellular alterations taking place throughout hair lifetime (Bosabalidis and Tsekos, 1982a,b; Bosabalidis and Kofidis, unpublished). The ontogenetic course can be roughly distinguished into three main phases, i.e. the presecretory phase, the phase of active secretion and the phase of senescence.

Presecretory phase The initial cell of a peltate hair appears ultrastructurally largely occupied by a central nucleus (Figure 2.51). The cytoplasmic organelles are quite limited in number (Bosabalidis, 1990a). They are principally represented by inclusion-containing leucoplasts, mitochondria, rough endoplasmic reticulum elements and dictyosomes. The ground plasm contains densely arranged ribosomes and small vacuoles with a

Figures 2.46—2.50 Successive stages of head development in the peltate glandular hairs (leaf surface sections) (X700). Figure 2.46 The mother cell of the head. Figure 2.47 First symmetric anticlinal division of the mother cell. Figure 2.48 Second symmetric anticlinal division to give four equal cells. Figure 2.49 An eight-celled head arisen after an asymmetric division of the four cells. Figure 2.50 Final stage of head development. The head is composed of 12 cells, i.e. four small central cells and eight large peripheral cells.

Figures 2.46—2.50 Successive stages of head development in the peltate glandular hairs (leaf surface sections) (X700). Figure 2.46 The mother cell of the head. Figure 2.47 First symmetric anticlinal division of the mother cell. Figure 2.48 Second symmetric anticlinal division to give four equal cells. Figure 2.49 An eight-celled head arisen after an asymmetric division of the four cells. Figure 2.50 Final stage of head development. The head is composed of 12 cells, i.e. four small central cells and eight large peripheral cells.

Figure 2.51 Ultrastructure of the initial cell of a peltate glandular hair. The nucleus (nu) is large and centrally located. Among organelles, leucoplasts (le) and mitochondria (mi) proliferate. Vacuoles (va) are small and limited in number (X15 000).

translucent content. When the head of the peltate hair becomes 8-celled, the ground plasm is still dense and the nuclei centrally located (Figure 2.52). The leucoplasts appear scattered throughout the intercellular space and no inclusion bodies are any longer recognized in their stroma. Mitochondria have somehow increased in number and contain many sacculi.

At the stage when cell divisions have been completed and the head is composed of 12 cells, the nucleus becomes significantly smaller, whereas concomitantly vacuoles become larger and more numerous. In the leucoplast stroma, individual annular thy-lakoids and stacked electron dense lamellae are discerned (Figure 2.53). Leucoplasts have been often described to occur in the head cells of peltate glandular hairs with lipophilic secretions, as in the case of Viscaria (Tsekos and Schnepf, 1974), Comptonia (Bell and Curtis, 1985), Mentha (Fischer and Hecht-Buchholz, 1985), Monarda (Heinrich, 1973), Nepeta (Bourett etal., 1994), Artemisia (Duke and Paul, 1993) Calceolaria (Schnepf, 1969b), Newcastelia (Dell and McComb, 1975), Cannabis (Mahlberg et al, 1984), etc.

In Origanum, leucoplasts of peltate hair head cells do not ultrastructurally appear to directly participate in the biosynthesis of the essential oil, contrary to other types of secretory structures, like the oil cavities of Rutaceae (Heinrich, 1969; Bosabalidis and Tsekos, 1982c; Gleizes etal., 1983) and the oil ducts of Apiaceae (Bosabalidis, 1996), in which leucoplasts are considered to constitute the principal sites of essential oil biosynthesis (accumulations of secretory droplets within the plastid stroma). In some species [Inula viscosa (Werker and Fahn, 1982), Thymus vulgaris (Bruni and Modenesi, 1983), Nicotiana tabacum (Nielsen et al, 1991), Chrysanthemum morifolium (Vermeer and Peterson, 1979b), etc.], the head cells of the peltate glandular hairs have been observed to contain instead of leucoplasts, chloroplasts, which, however, do not have a well-developed grana system.

The stalk cell of the peltate hair of Origanum exhibits at early development significant ultrastructural similarities with the head cells. It, thus, contains a centrally located large nucleus, a reduced number of cytoplasmic organelles and a dense ground plasm

Figure 2.52 A cytoplasmic portion of a head cell at the 8-celled stage of head development. The leuco-plasts (le) are scattered throughout the cytoplasm and they do not contain any longer globular inclusions (X23 000).
Figure 2.53 Head cells at the stage of integration of cell divisions. The leucoplasts (le) exhibit in their stroma annular thylakoids and stacked electron dense lamellae. The mitochondria (mi) have significantly increased in number (X23 000).

(Figure 2.54). The periclinal walls facing the head cells and the basal cell, are thin, not cutinized and they are transversed by numerous plasmodesmata. The lateral (anticlinal) walls of the stalk cell are thicker than the periclinal ones and they remain uncutinized until the head is four-celled. Afterwards, they gradually become impregnated with cutin up to the stage the head obtains its final size (Figure 2.55). The process of progressive cutinization becomes evident by a centripetal increase of the electron density of the wall matrix, which is presumably due to a polymerization of the cutin precursors by the air oxygen (Frey-Wyssling and Muhlethaler, 1959).

Cutinization of the lateral walls of the peltate hair stalk cell has been also reported to occur in other Lamiaceae members (Bruni and Modenesi, 1983; Mc Caskill et al., 1992;

Figure 2.54 Early development of the peltate hair stalk cell (sc). The nucleus (nu) is flattened and centrally located. The cell walls between the stalk cell and head cells (hc), as well as between the stalk cell and the basal cell (bc) bear numerous plasmodesmata (X22 500).

Figure 2.55 Advanced cutinization of the lateral wall of the stalk cell (sc). ccw, cutinized cell wall (X15 000).

Ascensao et al, 1995, 1999; Corsi and Bottega, 1999) and it has been interpreted to prevent an uncontrolled by the protoplasm apoplastic movement of substances from the mesophyll cells to the hair head cells (Dell and McComb, 1977). At the same time, a similar movement of the essential oil to the mesophyll cells, where it might have a deleterious effect, is avoided (Wollenweber and Schnepf, 1970). According to Dolzmann (1964), such a cutinization contributes to a better support of the voluminous head which becomes heavy during accumulation of the essential oil within the subcuticular space. On this subject, Amelunxen (1965) claims that the cutinized lateral walls of the stalk cell constitute a cutin source for the raising cuticle, so that the latter does not rupture. In the next steps of stalk cell differentiation, the vacuole remarkably increases and the cytoplasm along with the nucleus become restricted to the parietal cell zone.

The basal cell of the Origanum peltate hairs is very large and highly vacuolated, even from early development (Figure 2.56). It contains a limited number of organelles (particularly mitochondria) compared to a secretory head cell. The plastids are quite similar to those of the typical epidermal cells and each of them contains a voluminous membrane-bordered inclusion with a fine-granular appearance (Figure 2.56). The upper periclinal wall facing the stalk cell, as well as the lower one facing the mesophyll parenchyma, are transversed by numerous plasmodesmata (Figure 2.54). This event, along with the large size and the great vacuole of the basal cell favour the suggestion that this cell serves as a collector of the mesophyll photosynthates, which through the apoplast and the symplast are led to the stalk cell and finally to the head cells to constitute precursors for essential oil biosynthesis. It should be recalled that the secretory

Figure 2.56 Section of a cytoplasmic strand transversing the large central vacuole (va) of the peltate hair basal cell. In the strand, the nucleus (nu) surrounded by several inclusion-containing leucoplasts (le), are embedded (X9500).

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