generally unprotected, although in some cases lipids may accumulate in a subcuticular space [229, 230]. Trichomatous elaiophores occur in five orders
and nine families of angiosperms and quite variable regarding their location
[221, 222]. In monocots the elaiophores are unicellular and occur only in
two families of Asparagales: Iridaceae and Orchidaceae. In these families,
trichomatous elaiophores are distributed on tepals (Fig. 13b and c) or on the
staminal column as in Sisyrinchium (Iridaceae) (Fig. 13d) and Grobya and
Ornithocephalus (Orchidaceae) [216, 221, 222, 226, 228, 230–235]. In eudicots,
trichomatous elaiophores are multicellular and located mainly on sepals or
petals (e.g., Calceolariaceae and Cucurbitaceae), inside the tube of
the gamopetalous corolla (e.g., Stilbaceae and Solanaceae), or on spurs
(Colpias and Diascia) and on the staminal column (Lysimachia) [128, 216, 217,
219, 236–242].
4.2
Floral Oil: Production and Chemistry
Aside from being either epidermal and trichomatous, the ultrastructural characteristics of secretory cells of elaiophores are very similar [226, 227, 235, 243–245].
These cells with their machinery for lipid production are quite abundant and
characterized by the presence of plasmodesmata connecting all glandular cells,
dense cytoplasm, extensive endoplasmic reticulum, abundant mitochondria, numerous plastids with lipid inclusions, lipid droplets in the cytoplasm, elaioplasts, and
conspicuous nuclei [226–228, 234, 237, 243].
The floral oils released by elaiophores are usually colorless or yellow and
odorless [216, 218]. They are composed of complex mixtures of mostly nonvolatile
lipids but also small amounts of aldehydes, amino acids, carbohydrates, phenolic
compounds, hydrocarbons, and ketones [216–218, 246]. Studies on the chemical
composition of floral lipids are scarce, but generally these nonvolatile lipids consist
of fatty acids and/or glycerides, and their composition varies within plant families
and among genera and species [216, 217, 224]. All chemical analyses available have
shown that glycerides generally occur in the form of monoglycerides or diglycerides
and rarely triglycerides. The most common free fatty acids found in floral oils are
myristic, palmitoleic, palmitic, oleic, stearic, and eicosenoic acids (Fig. 15) [216,
242, 246–252].
The synthesis of lipids is complex and occurs through different routes involving
fatty acids and acylglycerols (we here present a general scheme of synthesis;
see Fig. 16). In general, the synthesis of lipids depends on the available carbohydrate sources (sucrose and/or glucose). The carbohydrate may be transported
directly from the phloem to the secretory cells via symplast or apoplast and/or
may become synthesized in the parenchyma of the elaiophore. Additionally, the
carbohydrates may be temporarily stored in vacuoles and/or plastids (Fig. 16).
Lipid synthesis occurs first through the glycolytic pathway, where the sucrose is
first degraded into pyruvate molecules [253, 254]. Long-chain fatty acids with 16
or 18 carbons in length are synthesized in plastids and then transported to the
28 Diversity of Floral Glands and Their Secretions in Pollinator Attraction
731
and nine families of angiosperms and quite variable regarding their location
[221, 222]. In monocots the elaiophores are unicellular and occur only in
two families of Asparagales: Iridaceae and Orchidaceae. In these families,
trichomatous elaiophores are distributed on tepals (Fig. 13b and c) or on the
staminal column as in Sisyrinchium (Iridaceae) (Fig. 13d) and Grobya and
Ornithocephalus (Orchidaceae) [216, 221, 222, 226, 228, 230–235]. In eudicots,
trichomatous elaiophores are multicellular and located mainly on sepals or
petals (e.g., Calceolariaceae and Cucurbitaceae), inside the tube of
the gamopetalous corolla (e.g., Stilbaceae and Solanaceae), or on spurs
(Colpias and Diascia) and on the staminal column (Lysimachia) [128, 216, 217,
219, 236–242].
4.2
Floral Oil: Production and Chemistry
Aside from being either epidermal and trichomatous, the ultrastructural characteristics of secretory cells of elaiophores are very similar [226, 227, 235, 243–245].
These cells with their machinery for lipid production are quite abundant and
characterized by the presence of plasmodesmata connecting all glandular cells,
dense cytoplasm, extensive endoplasmic reticulum, abundant mitochondria, numerous plastids with lipid inclusions, lipid droplets in the cytoplasm, elaioplasts, and
conspicuous nuclei [226–228, 234, 237, 243].
The floral oils released by elaiophores are usually colorless or yellow and
odorless [216, 218]. They are composed of complex mixtures of mostly nonvolatile
lipids but also small amounts of aldehydes, amino acids, carbohydrates, phenolic
compounds, hydrocarbons, and ketones [216–218, 246]. Studies on the chemical
composition of floral lipids are scarce, but generally these nonvolatile lipids consist
of fatty acids and/or glycerides, and their composition varies within plant families
and among genera and species [216, 217, 224]. All chemical analyses available have
shown that glycerides generally occur in the form of monoglycerides or diglycerides
and rarely triglycerides. The most common free fatty acids found in floral oils are
myristic, palmitoleic, palmitic, oleic, stearic, and eicosenoic acids (Fig. 15) [216,
242, 246–252].
The synthesis of lipids is complex and occurs through different routes involving
fatty acids and acylglycerols (we here present a general scheme of synthesis;
see Fig. 16). In general, the synthesis of lipids depends on the available carbohydrate sources (sucrose and/or glucose). The carbohydrate may be transported
directly from the phloem to the secretory cells via symplast or apoplast and/or
may become synthesized in the parenchyma of the elaiophore. Additionally, the
carbohydrates may be temporarily stored in vacuoles and/or plastids (Fig. 16).
Lipid synthesis occurs first through the glycolytic pathway, where the sucrose is
first degraded into pyruvate molecules [253, 254]. Long-chain fatty acids with 16
or 18 carbons in length are synthesized in plastids and then transported to the
28 Diversity of Floral Glands and Their Secretions in Pollinator Attraction
731