Elaiophore structure and oil secretion in flowers of Oncidium trulliferum Lindl. and Ornithophora radicans (Rchb.f.) Garay & Pabst (Oncidiinae: Orchidaceae)

Abstract

Background and aims: Many orchid flowers have glands called elaiophores and these reward pollinating insects with oil. In contrast to other reward-producing structures such as nectaries, the anatomy of the elaiophore and the process of oil secretion have not been extensively studied. In this paper, elaiophore structure is described for two members of Oncidiinae, Oncidium trulliferum Lindl. and Ornithophora radicans (Rchb.f.) Garay & Pabst.

Methods: Elaiophores of both species were examined using light microscopy, scanning electron microscopy and transmission electron microscopy.

Key results and conclusions: In flowers of Oncidium trulliferum and Ornithophora radicans, oil is secreted by morphologically distinct elaiophores associated with the labellar callus. However, in O. trulliferum, elaiophores also occur on the lateral lobes of the labellum. In both these species, the epithelial elaiophores are composed of a single layer of palisade-like epidermal cells and a distinct subepithelial layer. Secretory elaiophore cells may contain numerous, starchless plastids, mitochondria and smooth endoplasmic reticulum profiles. In O. trulliferum, the cytoplasm contains myelin-like figures but these are absent from O. radicans. In the former species, cavities occur in the cell wall and these presumably facilitate the passage of oil onto the elaiophore surface. In O. radicans, the accumulation of oil between the outer tangential wall and the cuticle causes the latter to become distended. Since it is probable that the full discharge of oil from the elaiophores of O. radicans occurs only when the cuticle is ruptured by a visiting insect, this may contribute towards pollinator specificity. The structure of the elaiophore in these species resembles both that found in previously investigated species of Oncidiinae and that of certain members of the Malpighiaceae.

Fig. 1.

Flower and elaiophore surface (SEM) of Oncidium trulliferum. (A) Flower with surface of elaiophore on lateral lobe of labellum covered with oil (arrow). Scale bar = 1·6 mm. (B) Part of callus (Ca) and lateral lobe of labellum (L) – the sites of floral oil secretion in this species. Scale bar = 1·0 mm. (C) Folds of cuticle covering epithelial cells of lateral lobe elaiophore. Notice the absence of pores or breaks in the cuticle. Scale bar = 10 µm. (D) Labellar callus showing position of stomata (arrows). Scale bar = 200 µm.

Fig. 2.

Sections through elaiophores (A–C, lateral lobe elaiophore; D, callus elaiophore) of Oncidium trulliferum (LM). (A) Fresh, hand-sectioned elaiophore tissue showing palisade-like secretory cells containing lipid droplets (arrows) stained with Sudan III. Scale bar = 20 µm. (B) Section of elaiophore tissue showing secretory epithelium and subepithelial layer (Se). Note the position of cavities in the cell wall (arrow heads) and the vascular bundle (Vb). Scale bar = 50 µm. (C) Detail of secretory epithelium and subepithelial cells (Se) of elaiophore tissue showing position of myelin-like figures (arrows). The position of the cavity in the cell wall is indicated by an arrowhead. Scale bar = 20 µm. (D) Detail of secretory epithelium showing myelin-like figures (arrows) and cell wall cavities (arrow heads). Scale bar = 20 µm.

Fig. 3.

Elaiophore cells (A, D–F, lateral lobe elaiophore; B and C, callus elaiophore) of Oncidium trulliferum (TEM). (A) Outer tangential cell wall (CW) with reticulum of cellulose microfibrils and associated cavity. Note also the lamellate cuticle (C) on cell wall surface. Scale bar = 1 µm. (B) Epithelial cell with nucleus (N) and perinuclear plastids (P) containing darkly stained material (arrow). Scale bar = 1 µm. (C) Nucleus (N) surrounded by numerous plastids (P) containing irregularly distributed, darkly stained material. Scale bar = 1 µm. (D) Epithelial cell showing plastids (P) and myelin-like figures (Mf). Scale bar = 1 µm. (E) Detail of myelin-like figures (Mf) containing osmiophilic globules. Scale bar = 0·5 µm. (F) Subepithelial cell containing lipid droplets (L) and myelin-like figures (Mf). Scale bar = 2·5 µm.

Fig. 4.

Flower and elaiophore surface (SEM) of Ornithophora radicans. (A) Flower with yellow callus, the site of the labellar elaiophore. Scale bar = 1·7 mm. (B) Labellum showing labellar crest and callus outgrowths bearing elaiophore tissue. Scale bar = 900 µm. (C) Lobed, labellar elaiophore showing wrinkled surface. Scale bar = 500 µm. (D) Detail of elaiophore surface showing distended cuticle (arrow). Scale bar = 50 µm

Fig. 5.

Sections of elaiophore tissue of Ornithophora radicans (LM). (A) Section of callus outgrowths showing position of elaiophore epithelium and distended cuticle (arrows). Scale bar = 195 µm. (B) Section through callus showing palisade-like, secretory epithelium (E), subepithelial cells (Se) and vascular bundle (Vb) embedded in ground parenchyma. Scale bar = 10 µm. (C) Epithelial cells in palisade arrangement with intercellular spaces (arrows) and centrally placed nuclei surrounded by darkly stained plastids. Beneath this secretory tissue occur subepithelial cells with amyloplasts. Scale bar = 15 µm.

Fig. 6.

Ultrastructure of elaiophore epithelial cells of Ornithophora radicans (TEM). (A) Outer cell wall (CW) with dark deposits covered with a thick layer of reticulate cuticle (C). Scale bar = 0·7 µm. (B) Intercellular spaces with osmiophilic content (arrows). Note that the cytoplasm mainly contains smooth endoplasmic reticulum (SER), plastids (P) and mitochondria (m). Scale bar = 0·7 µm. (C) Large intercellular spaces (Is) with dark inclusions (arrow) and cytoplasm with numerous smooth endoplasmic reticulum (SER) profiles. Scale bar = 1 µm. (D) Secretory cytoplasm with small, starchless plastids (P) and mitochondria (m) associated with the nucleus (N). Note the cell wall (CW) with dark inclusion (arrow). Scale bar = 1 µm. (E) Nucleus (N) with associated plastids (P) and mitochondria (m). Note the large intercellular spaces and the presence of dark inclusions in the cell wall (arrow). Scale bar = 3 µm. (F) Mitochondria (m) and typical, starchless plastids (P) with few plastoglobuli. Scale bar = 0·5 µm.