between the flowers and the pollinators is mostly mutualistic, where the flowers offer
the resource and the pollinators guarantee the transfer of pollen grains to the stigmas.
Thus, the substances produced by these secretory structures may play a central role
in pollination. Flowers that do not offer any kind of resource are not dependent on
animals for their reproduction and, in most cases, are wind or water pollinated. Such
abiotic pollination occurs in only 30% of flowering plants [5]. Therefore, knowledge
about floral secretory structures is fundamental for understanding how the relationships between pollinators and flowers have evolved.
The main resource offered by flowers to their pollinators is pollen, but additional
resources, such as nectar, oils, perfumes, and resins, are often present, which
may also act as attractants [4, 6, 7]. The secretory structures responsible for the
production of these substances include nectaries, osmophores, elaiophores, and resin
glands [8–13]. The purpose of this chapter is to describe these floral glands that serve
in support of pollination and the diversity of compounds that they produce and to
discuss some of the involved functional and evolutionary aspects.
2
Floral Nectaries
2.1
Nectary Structure and Nectar Production
Floral nectaries are glands responsible for the production of nectar, which is the
main food resource offered to potential pollinators [10, 14]. They may vary in
location and morphology but are similar regarding their anatomical structure [10,
11, 15]. Floral nectaries generally comprise an epidermis, a nectary parenchyma, and
a subnectary parenchyma (Fig. 1a–c).
The epidermis may be composed of elongated secretory cells in palisades or
of small nonsecretory cells. When cells are nonsecretory, stomata are often present,
which is a very common path for the release of nectar [10, 11, 14–16] (Fig. 1d).
Stomata involved in nectar release have been called nectarostomata and are characterized by having lost the capacity to regulate their aperture [17–20]. Their position
on the nectary may be homogeneous, occupying the entire gland, or the stomata
may also be restricted to some special locations (e.g., on the top surface of the
nectariferous disk in Spondias, Anacardiaceae) [21]. In cases where the epidermis
is secretory (or nectariferous) or does not display stomata, the nectar is released
through the cell wall and cuticle [22–24]. However, the release of nectar through
stomata and cuticle may take place simultaneously [10, 11, 22].
Different families of plants, such as Anacardiaceae, Bignoniaceae,
Caprifoliaceae, Fabaceae, Malvaceae, and Orchidaceae, have characteristic
receptacular nectaries, but in some species they are completely absent or replaced
by secretory trichomes (e.g., Anacardium of Anacardiaceae, Adenocalymma
of Bignoniaceae) (Fig. 2a–c) [10, 21, 25–29]. A unique characteristic of
nectariferous trichomes is the presence of transfer cells, characterized by a
thicker anticlinal wall with irregular ingrowths, which are important to the facilitation of transmembrane flow of solutes [28, 30].
28 Diversity of Floral Glands and Their Secretions in Pollinator Attraction
711
the resource and the pollinators guarantee the transfer of pollen grains to the stigmas.
Thus, the substances produced by these secretory structures may play a central role
in pollination. Flowers that do not offer any kind of resource are not dependent on
animals for their reproduction and, in most cases, are wind or water pollinated. Such
abiotic pollination occurs in only 30% of flowering plants [5]. Therefore, knowledge
about floral secretory structures is fundamental for understanding how the relationships between pollinators and flowers have evolved.
The main resource offered by flowers to their pollinators is pollen, but additional
resources, such as nectar, oils, perfumes, and resins, are often present, which
may also act as attractants [4, 6, 7]. The secretory structures responsible for the
production of these substances include nectaries, osmophores, elaiophores, and resin
glands [8–13]. The purpose of this chapter is to describe these floral glands that serve
in support of pollination and the diversity of compounds that they produce and to
discuss some of the involved functional and evolutionary aspects.
2
Floral Nectaries
2.1
Nectary Structure and Nectar Production
Floral nectaries are glands responsible for the production of nectar, which is the
main food resource offered to potential pollinators [10, 14]. They may vary in
location and morphology but are similar regarding their anatomical structure [10,
11, 15]. Floral nectaries generally comprise an epidermis, a nectary parenchyma, and
a subnectary parenchyma (Fig. 1a–c).
The epidermis may be composed of elongated secretory cells in palisades or
of small nonsecretory cells. When cells are nonsecretory, stomata are often present,
which is a very common path for the release of nectar [10, 11, 14–16] (Fig. 1d).
Stomata involved in nectar release have been called nectarostomata and are characterized by having lost the capacity to regulate their aperture [17–20]. Their position
on the nectary may be homogeneous, occupying the entire gland, or the stomata
may also be restricted to some special locations (e.g., on the top surface of the
nectariferous disk in Spondias, Anacardiaceae) [21]. In cases where the epidermis
is secretory (or nectariferous) or does not display stomata, the nectar is released
through the cell wall and cuticle [22–24]. However, the release of nectar through
stomata and cuticle may take place simultaneously [10, 11, 22].
Different families of plants, such as Anacardiaceae, Bignoniaceae,
Caprifoliaceae, Fabaceae, Malvaceae, and Orchidaceae, have characteristic
receptacular nectaries, but in some species they are completely absent or replaced
by secretory trichomes (e.g., Anacardium of Anacardiaceae, Adenocalymma
of Bignoniaceae) (Fig. 2a–c) [10, 21, 25–29]. A unique characteristic of
nectariferous trichomes is the presence of transfer cells, characterized by a
thicker anticlinal wall with irregular ingrowths, which are important to the facilitation of transmembrane flow of solutes [28, 30].
28 Diversity of Floral Glands and Their Secretions in Pollinator Attraction
711