The Structure of Elaiophores in Oncidium Cheirophorum Rchb.F
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ACTA AGROBOTANICA Vol. 60 (2): 9–14 2007 THE STRUCTURE OF ELAIOPHORES IN ONCIDIUM CHEIROPHORUM RCHB.F. AND ORNITHOCEPHALUS KRUEGERI RCHB.F. (ORCHIDACEAE) Agata Pacek, Małgorzata Stpiczyńska Department of Botany, Agricultural University in Lublin, Akademicka 15, 20-950 Lublin, Poland e-mail: [email protected] Received: 20.11.2007 Summary The fl oral oils are secreted by structures called The shining appearance of the fl owers of Oncidium che- elaiophores. Vogel (1974) recognized two different irophorum and Ornithocephalus kruegeri results from the presen- types of elaiophores: epithelial and trichomal ones. In ce of lipids on the fl ower surface. The lipids are produced by ela- Orchidaceae, both of them occur. Epithelial elaiopho- iophores – secretory structures situated symmetrically at the base res are composed of one layer of glandular epidermis of the labellum or upon the callus. and subgladular parenchyma. Usually, they occur on the In O. cheirophorum, the elaiophores are epithelial labellar callus (S inger and Cocucci, 1999; Sin- type. They consist of one layer of cuboidal secretory cells and ger et al. 2006). The fl oral oil is secreted directly onto subsecretory parenchyma. The thick cuticle covering the outer, the epidermis surface, or accumulates under the cuticle tangential wall of epithelial cells becomes distended and wrin- (Buchmann, 1974; S inger et al. 2006). Trichomal kled as secreted oil accumulates beneath its surface. Oil secre- elaiophores consist of numerous glandular hairs. Se- tion begins at the bud stage and lasts till the end of anthesis, cretion from trichomal elaiophores is easily accessible that is 22 days, on average. Pollination does not infl uence oil production. to a wide spectrum of visitors, but oil production is al- In O. kruegeri, trichomatous elaiophores are situated on ways lower than in fl owers with epithelial elaiophores the central part of the callus. Unicellular trichomes project from (Vogel, 1974; Buchmann, 1987). the epidermis cells. Their outer walls are covered by a thin cutic- The majority of oil-producing species rely exclu- le. In the dense cytoplasm of the trichomes, small plastids with sively on oil-collecting bees for pollination (Silvera, few starch grains occur, whereas subsecretory parenchyma cells 2002). Bees that harvest and use fl oral oils are found pri- contain amyloplasts with large starch grains and raphides. The marily in the families Apidae and Melittidae (Buch- oil is already produced at the bud stage, about one week before mann, 1987; M ichener, 2000). Floral oil is collec- fl ower opening, and lasts till the end of anthesis. ted usually by female bees, which use it mixed with pol- len as provision for larvae, as nest construction lining Key words: elaiophore, structure, oil secretion, Oncidium cheiro- their brood cells in the nest to waterproof them or as an phorum, Ornithocephalus kruegeri, Orchidaceae adult food resource (B uchmann, 1987; Endress, 1994; Rasmussen and Olesen, 2000). Contrary to the nectar, bees do not collect and transport the fl oral INTRODUCTION oil with their mouth parts but with their legs or abdomen Plants attract fl ower visitors by a range of pro- and these body parts are covered with special branched ducts such as nectar and pollen, more rarely perfumes, hairs (Vogel, 1974; B u chmann, 1987). resins, waxes or fl oral oils (Buchmann, 1987; E n - The lipoid non-volatile material secre- dress, 1994). Eight angiosperm families (Orchidace- ted by oil-fl owers is usually colourless and con- ae, Iridaceae, Krameriaceae, Malpighiaceae, Primula- sists predominantly of diacyl- and triacylglyce- ceae, Scrophulariaceae, Solanaceae, Cucurbitaceae) are rols, saturated fatty acids, paraffi ns, and esters. known to secrete oils as the reward for their pollinators. Other components related with elaiophore exudates Three additional families: Caesalpiniaceae, Melastoma- are diglycerides, amino acids, glucose, caroteno- taceae and Gesneriaceae were previously included in ids, phenolics, glycosides, nonvolatile isoprenoi- this list but it is not still clear if their oil is a reward des, and saponins (Vogel, 1974; B uchmann, (Vogel, 1974; B u chmann, 1987). 1987; Reis et al. 2000, 2006; Silvera, 2002). The 10 Agata Pacek, Małgorzata Stpiczyńska quantity of oil can vary among members of the same RESULTS genus. The difference may also relate to the quality of The fl owers of O. cheirophorum are yellow, sub- oil and, for example, oil from Sigmatostalix picturatissi- tly fragrant and shining (Fig. 1). The size of each fl ower ma is more similar to the oil of Ornithocephalus sp. than is about 1.5 x 1.2 cm. The average anthesis period is 22 of other Sigmatostalix species (Silvera, 2002). days. The lip is three lobed and it has white callus in the The structure of elaiophores is relatively obscu- central part from which the lobes branch out (Fig. 1). re, and only epithelial elaiophores in four species have The elaiophores have a roundish structure with a crin- been studied in any degree of detail (Singer and Cocuc- kled surface, and their size is 2.0 x 1.5 mm (Fig. 2). ci, 1999; Stpiczyń ska and Davies, 2007; Stpi- They are situated symmetrically at the base of each late- czyń ska et al. 2007). Therefore, data on the struc- ral lobes, adjacent to the callus (Fig. 1). ture of elaiophores in Orchidaceae is still insuffi cient. The elaiophores are epithelial type and consist of The aim of this study is to describe the elaiophore single-layered cuboidal epithelium cells that measure structure of morphologically different members of 37 x 20 μm, on average (Figs 3-5). Under the epithelial Orchidaceae: Oncidium cheirophorum Rchb.f. and layer, there are three layers of small subepithelial pa- Ornithocephalus kruegeri Rchb.f. syn. Ornithocep- renchyma cells and ground parenchyma with vascular halus ciliatus Lindl. bundles (Fig. 4). The epithelial cells contain dense cy- toplasm and a large, centrally positioned nucleus, 9 μm MATERIALS AND METHODS in diameter, on average (Figs 6, 7). Treatment with IKI Fresh fl owers of Oncidium cheirophorum Rchb. does not indicate the presence of starch in the plastids. f. and Ornithocephalus kruegeri Rchb.f. were ob- In the thick outer cell wall, small numerous cavities oc- tained from the Botanical Garden of the Maria Curie- cur (Fig. 7). Cavities do not occur in the thin, radial cell Skłodowska University (UMCS) in Lublin. walls. The epithelial cell wall is covered by a thick cutic- The position of the elaiophores in complete fl ow- le of mean thickness of 1.4 μm (Figs 5, 6). The oil, both ers of Oncidium cheirophorum Rchb.f. and Ornithoceph- on the surface and within the cytoplasm of the epithelial alus kruegeri Rchb.f. was determined using an Olympus cells stains intensely with Sudan III (Fig. 3). The cuticle SZX12 stereo-microscope. Hand-cut sections through covering the outer, tangential wall distends as secreted the living elaiophore were tested for lipids and starch oil accumulates beneath its surface (Figs 5, 6 and 8, 9). using saturated alcoholic Sudan III solution and Lugol Oil secretion begins at the bud stage, about 10 days be- solution, respectively. Stained sections were viewed fore fl ower opening, and lasts till the end of anthesis. with a Nikon H-III Rower light microscope (LM). During the course of anthesis, one fl ower secretes 0.27 The elaiophores were fi xed in 2.5% glutaraldehyde – 0.48 mg of oil. The highest secretory activity was no- and 5% sucrose in phosphate buffer (pH 7.0; 0.1M) for ted on the eighth day of anthesis. The pollination does 2h at 20°C, after that in phosphate buffer for 24h at 4°C, not infl uence oil production. post-fi xed in 1% osmium tetroxide at 4°C for 15 min. and The fl owers of O. kruegeri are very small, mea- washed in distilled water two times for 5 min. The fi xed suring 5.0 x 4.0 mm, on average (Fig. 10). The period material was then dehydrated in ethanol, infi ltrated and of anthesis is about 15 days. Tepals are covered by mul- embedded in LR White resin, according to detailed pro- ticellular trichomes with an unicellular, roundish head ducer protocol (London Resin Company Limited, UK). at the tip. The labellum is whitish-green and elonga- Semi-thin sections (about 0.7 μm thick) were cut ted. The callus is also whitish-green, and semi-roun- with a Reichert Ultracut S microtome and stained for ded in shape with two prominent outgrowths (Figs 10, general histology using 1% methylene blue and 1% az- 11). At anthesis, the whole surface of callus is covered ure II (1:1) for 5-7 min. Micrometry and photomicrogra- by oily secretion (Fig. 11). Oil is produced by unicellu- phy of the elaiophores were accomplished using a Jena- lar trichomes situated on the central part of the callus val or Nikon Eclipse 600 microscope. (Figs 12-15). The trichomes are quite small during the For scanning electron microscopy (SEM), the fi rst day of anthesis and they grow successively up to elaiophores fi xed as above, were dehydrated in acetone 78 μm in length. The trichomes project from the epider- mis cells (Figs 14, 16, 17). The oil, both inside the cells and critical-point dried using liquid CO2. They were then sputter-coated with gold and examined by means and on the surface of the trichomes, stains intensely of TESLA BS-300. with Sudan III (Figs 12, 13). Conversely to the previo- To estimate the weight of secreted oils, secretion us species, cavities do not occur in the outer cell wall. accumulated at the surface of the fl ower was absorbed The cell wall is covered by the cuticle but, unlike that of into fi lter paper strips, previously extracted with acetone O.