pollinators. Highly concentrated nectars are related to insect-pollinated flowers,
whereas flowers pollinated by birds and bats generally produce more dilute nectars [57–59, 65]. Thus, in most cases, morphological floral constraints play a greater
role in limiting the access of a flower to interested visitors (potential pollinators,
nectar robbers, etc.), rather than the sugar composition of its nectar.
Amino acids are the second-most common components of nectar [57, 66, 67].
The amino acid concentration and composition may vary significantly within a
population and even within a single plant [68–70]. However, the overall amino
acid composition is generally more highly conserved than the individual amino acid
concentrations [71]. Amino acids contribute to the taste of nectar and are important
in regard to the types of pollinators that visit or avoid the flowers [71–75]. Some
proteins (nectarins) also occur in nectar; these have only been studied in a few genera
(e.g., Nicotiana, Allium) [76–78].
Small amounts of other substances also occur in nectar, such as lipids, organic
acids, phenolic compounds, alkaloids, and terpenoids [57]. In some species, these
substances, especially lipids, may even comprise a large proportion of the solutes
of nectar, which is then referred to as mixed secretions [10, 21, 29, 41, 57, 59, 79,
80]. Lipids often provide a particular flavor and odor that can be essential for certain
pollinators; in addition, they are twice as energy-rich as sugars and an important food
resource [81–83]. The major lipids in nectar are fatty acids of different chain length,
but recently volatile oils, such as monoterpenes, have also been identified, produced
either by the nectary itself or by microorganisms that inhabit the gland (Fig. 6)
[84–87]. While lipids are linked to the attraction of pollinators, such compounds as
alkaloids, phenolics, coumarins, and saponins may be toxic and/or repellent to
some groups of floral visitors [57, 67, 88–90]. The main alkaloids thus far identified
are nicotine, anabasine, caffeine, and amygdalin, which have been detected in
nectar of different families of angiosperms (e.g., Rosaceae, Rutaceae, Solanaceae)
(Fig. 6) [91]. Regarding phenolics, most of them are flavonoids, such as quercetin,
kaempferol, myricetin, and isorhamnetin, but aurones may be also found in some
species with colored nectar (Fig. 6) [92, 93]. Therefore, the interactions between
the substances present in nectar and the pollinators are complex and not only related
to alimentary reward but also to modulating insect behavior [94, 95].
2.3
Evolution of Floral Nectaries and Nectar
Floral nectaries are enormously diverse and have evolved independently several
times within angiosperms; in all cases, nectar is related to interactions with pollinators [14, 95]. The shape, structure, and location of nectaries vary greatly among
plants [10, 14, 56, 96–100], each lineage with unique peculiarities. Variations within
the same family or genus are common, depending on phylogenetic and/or ecological
constraints [14, 82, 101].
Beetles seem to have been the chief insects to pollinate early angiosperms,
and it has been claimed that pollen was the only attractant and reward [102–105].
However, recent evidence has shown that the first reward to pollinating insects in
718
E. D. Tölke et al.
whereas flowers pollinated by birds and bats generally produce more dilute nectars [57–59, 65]. Thus, in most cases, morphological floral constraints play a greater
role in limiting the access of a flower to interested visitors (potential pollinators,
nectar robbers, etc.), rather than the sugar composition of its nectar.
Amino acids are the second-most common components of nectar [57, 66, 67].
The amino acid concentration and composition may vary significantly within a
population and even within a single plant [68–70]. However, the overall amino
acid composition is generally more highly conserved than the individual amino acid
concentrations [71]. Amino acids contribute to the taste of nectar and are important
in regard to the types of pollinators that visit or avoid the flowers [71–75]. Some
proteins (nectarins) also occur in nectar; these have only been studied in a few genera
(e.g., Nicotiana, Allium) [76–78].
Small amounts of other substances also occur in nectar, such as lipids, organic
acids, phenolic compounds, alkaloids, and terpenoids [57]. In some species, these
substances, especially lipids, may even comprise a large proportion of the solutes
of nectar, which is then referred to as mixed secretions [10, 21, 29, 41, 57, 59, 79,
80]. Lipids often provide a particular flavor and odor that can be essential for certain
pollinators; in addition, they are twice as energy-rich as sugars and an important food
resource [81–83]. The major lipids in nectar are fatty acids of different chain length,
but recently volatile oils, such as monoterpenes, have also been identified, produced
either by the nectary itself or by microorganisms that inhabit the gland (Fig. 6)
[84–87]. While lipids are linked to the attraction of pollinators, such compounds as
alkaloids, phenolics, coumarins, and saponins may be toxic and/or repellent to
some groups of floral visitors [57, 67, 88–90]. The main alkaloids thus far identified
are nicotine, anabasine, caffeine, and amygdalin, which have been detected in
nectar of different families of angiosperms (e.g., Rosaceae, Rutaceae, Solanaceae)
(Fig. 6) [91]. Regarding phenolics, most of them are flavonoids, such as quercetin,
kaempferol, myricetin, and isorhamnetin, but aurones may be also found in some
species with colored nectar (Fig. 6) [92, 93]. Therefore, the interactions between
the substances present in nectar and the pollinators are complex and not only related
to alimentary reward but also to modulating insect behavior [94, 95].
2.3
Evolution of Floral Nectaries and Nectar
Floral nectaries are enormously diverse and have evolved independently several
times within angiosperms; in all cases, nectar is related to interactions with pollinators [14, 95]. The shape, structure, and location of nectaries vary greatly among
plants [10, 14, 56, 96–100], each lineage with unique peculiarities. Variations within
the same family or genus are common, depending on phylogenetic and/or ecological
constraints [14, 82, 101].
Beetles seem to have been the chief insects to pollinate early angiosperms,
and it has been claimed that pollen was the only attractant and reward [102–105].
However, recent evidence has shown that the first reward to pollinating insects in
718
E. D. Tölke et al.