by active transport [51]. According to this model, there is no combination of
the apoplast and symplast mechanisms. Current evidence suggests, however, that
pre-nectar is mainly transported through symplast via plasmodesmata into vesicles
where its composition changes to then be released by exocytosis (Fig. 4) [11, 54, 55].
Indeed, plasmodesmata (Fig. 3e) have been shown to occur in great quantity, this
being evidence for symplast transport of sugars [10, 11, 16, 22]. Nectar secretion by
nectariferous trichomes excludes an apoplastic transport of nectar due to the presence of barriers in the external cell walls in the stalk and intermediate cells of
the trichomes [55].
Nectariferous cells remain intact after the release of nectar, the latter taking
place via two different mechanisms – granulocrine or eccrine secretion [10, 11, 15,
16, 23, 24, 56]. In granulocrine secretion, molecules are grouped and transported
in ER- or dictyosome-derived vesicles that fuse with the plasmalemma and release
the molecules to the outside of the protoplast, while eccrine secretion involves
transport of individual molecules across the plasmalemma (Fig. 4) [10, 11, 16, 23,
24, 56]. Parenchyma cells rich in endoplasmic reticulum cisternae, dictyosomes, and
vesicles are evidence of granulocrine secretion, while cells poor in endoplasmic
reticulum and dictyosomes have more likely an eccrine mechanism (Fig. 4) [10, 11,
16, 23, 24, 56].
Finally, the source of nectar carbohydrates may be immediate photosynthesis
by the nectary itself, mainly by the subnectary parenchyma, or by any other part
of the plant, or may result from starch storage in plastids present in the parenchyma
cells (Fig. 4) [16, 39, 51]. While both mechanisms may occur, the absence of starch
grains confirms that the sugars of the nectar result from photosynthesis, supplied
in most instances by the phloem that reaches the nectary parenchyma [16, 51].
2.2
The Chemical Constituents of Nectar
Nectar chiefly consists of sugars, especially of the disaccharide sucrose and the
monosaccharides fructose and glucose (Fig. 5) [57]. Also, there are minor amounts
of other monosaccharides (mannose, arabinose, xylose) and disaccharides (maltose,
melibiose) as well as oligosaccharides and sugar alcohols (Fig. 5) [57–59].
Possible correlations between the type and composition of nectar sugars and
the kind of associated pollinators have long been discussed. Flowers with highsucrose nectars are more likely to be visited by bees, butterflies, moths, and
hummingbirds, while flowers with high-hexose nectars are usually pollinated
by small, unspecialized insects, passerine birds, or Neotropical bats [57, 59]. There
are certain phylogenetically restricted trends in the sugar composition of the nectar
within each of the families Asteraceae, Gesneriaceae, and Scrophulariaceae,
yet their flowers are visited by a diversity of generalist pollinators [60–64]. In
Anacardiaceae, also with a more generalist pollination system, there seems to
be no clear pattern of nectar sugar composition, here apparently affected by
various environmental factors [21, 41]. On the other hand, plants with similar,
more defined concentrations of nectar tend to have the same kind of more specialized
28 Diversity of Floral Glands and Their Secretions in Pollinator Attraction
715
the apoplast and symplast mechanisms. Current evidence suggests, however, that
pre-nectar is mainly transported through symplast via plasmodesmata into vesicles
where its composition changes to then be released by exocytosis (Fig. 4) [11, 54, 55].
Indeed, plasmodesmata (Fig. 3e) have been shown to occur in great quantity, this
being evidence for symplast transport of sugars [10, 11, 16, 22]. Nectar secretion by
nectariferous trichomes excludes an apoplastic transport of nectar due to the presence of barriers in the external cell walls in the stalk and intermediate cells of
the trichomes [55].
Nectariferous cells remain intact after the release of nectar, the latter taking
place via two different mechanisms – granulocrine or eccrine secretion [10, 11, 15,
16, 23, 24, 56]. In granulocrine secretion, molecules are grouped and transported
in ER- or dictyosome-derived vesicles that fuse with the plasmalemma and release
the molecules to the outside of the protoplast, while eccrine secretion involves
transport of individual molecules across the plasmalemma (Fig. 4) [10, 11, 16, 23,
24, 56]. Parenchyma cells rich in endoplasmic reticulum cisternae, dictyosomes, and
vesicles are evidence of granulocrine secretion, while cells poor in endoplasmic
reticulum and dictyosomes have more likely an eccrine mechanism (Fig. 4) [10, 11,
16, 23, 24, 56].
Finally, the source of nectar carbohydrates may be immediate photosynthesis
by the nectary itself, mainly by the subnectary parenchyma, or by any other part
of the plant, or may result from starch storage in plastids present in the parenchyma
cells (Fig. 4) [16, 39, 51]. While both mechanisms may occur, the absence of starch
grains confirms that the sugars of the nectar result from photosynthesis, supplied
in most instances by the phloem that reaches the nectary parenchyma [16, 51].
2.2
The Chemical Constituents of Nectar
Nectar chiefly consists of sugars, especially of the disaccharide sucrose and the
monosaccharides fructose and glucose (Fig. 5) [57]. Also, there are minor amounts
of other monosaccharides (mannose, arabinose, xylose) and disaccharides (maltose,
melibiose) as well as oligosaccharides and sugar alcohols (Fig. 5) [57–59].
Possible correlations between the type and composition of nectar sugars and
the kind of associated pollinators have long been discussed. Flowers with highsucrose nectars are more likely to be visited by bees, butterflies, moths, and
hummingbirds, while flowers with high-hexose nectars are usually pollinated
by small, unspecialized insects, passerine birds, or Neotropical bats [57, 59]. There
are certain phylogenetically restricted trends in the sugar composition of the nectar
within each of the families Asteraceae, Gesneriaceae, and Scrophulariaceae,
yet their flowers are visited by a diversity of generalist pollinators [60–64]. In
Anacardiaceae, also with a more generalist pollination system, there seems to
be no clear pattern of nectar sugar composition, here apparently affected by
various environmental factors [21, 41]. On the other hand, plants with similar,
more defined concentrations of nectar tend to have the same kind of more specialized
28 Diversity of Floral Glands and Their Secretions in Pollinator Attraction
715