Biology Reference
In-Depth Information
Phenolics have fluorescent properties, and it was suggested that fluores-
cent compounds accumulated in nectar might serve as a guide for polli-
nators, especially honeybees, which can see in the UV, although this has
been disputed (Thorp et al., 1975; Kevan, 1976). Two bright blue fluorescent
compounds in the nectar of
Fremontia
sp. were identified as 5,7-dimethoxy-
genistein-4′glucoside and its aglycone, 5,7-dimethoxygenestein (Scogin,
1979). The chemical causes of coloured nectar in some plant genera such as
Melianthus
,
Aloe
, and
Schiedea
are not well known (Hansen et al., 2007),
although aurones (flavonoid pigments) were identified in the red nectar of
Nesocodon mauritanicus
(Campanulaceae) (Olesen et al., 1998).
7.4
Alkaloids
Alkaloids have been detected in the nectar of a large number of plants
(Hazslinsky, 1956; Baker & Baker, 1975; Galetto & Bernardello, 1992; Adler
& Wink, 2001). Alkaloids are generally thought to have a detrimental effect
on pollinator visitation (Baker & Baker, 1982a; Adler, 2000); however, a
direct test showed that the pyrrolizidine alkaloid, monocrotaline, had no sig-
nificant effect on insect feeding (Landolt & Lenczewski, 1993). Recently,
Singaravelan et al. (2005) tested the responses of honeybees to four secondary
compounds found naturally in floral nectar: nicotine, anabasine, caffeine, and
amygdalin. Except for anabasine, naturally occurring concentrations did not
have a deterrent effect, and low concentrations of nicotine and caffeine elicited
a significant feeding preference in the bees. This is interesting in view of the
common observation of bees drinking from Coca-Cola cans!
The relationship between nectar alkaloids and plant fitness has been ex-
amined for gelsemine, the main alkaloid of
Gelsemium sempervirens
(Loganiaceae). When Adler and Irwin (2005) increased the gelsamine con-
tent in nectar of this plant, both nectar robbers and pollinators visited fewer
flowers and for shorter times, and pollen transfer decreased. The presence of
this alkaloid in nectar does not appear to benefit the plant, and may be a
pleiotropic consequence of its production in other plant tissues.
7.5
Terpenoids
Diverse and odiferous terpenoid compounds are produced by almost all
flowers. They are primary constituents of essential oils, and have been iden-
tified as accumulating in nectars of a number of plants (Juergens, 2004; Naef
et al., 2004; Raguso, 2004). Terpenoids are generally thought to be insect
attractants (Plepys et al., 2002; Andersson, 2003; Tholl et al., 2004). However,
many terpenoids have antifeedant activity, reflected in their interactions with
