Biology Reference
In-Depth Information
Ascorbic acid (vitamin C) is well known as an antioxidant in floral nectar
(Baker & Baker, 1975). It was identified early on in nectar at moderately
high concentrations (>2 mg/ml) (Griebel & Hess, 1940). Ascorbate has sub-
sequently been identified in the nectars of many plant species (Bukatsch &
Wildner, 1956; Baker & Baker, 1975; Carter & Thornburg, 2004a; Naef et al.,
2004; Horner et al., 2007); it is, however, not known how widespread ascorbate
accumulation is in nectar. Most of the ascorbate is lost when honeybees
convert nectar into honey. Ascorbate is an essential nutrient for many insects
and also an antioxidant (see “Nectar redox cycle” on page 241) that minimizes
the negative effects of phenols in ingested plant tissue (Barbehenn et al.,
2001)—this might help pollinators to cope with phenolic compounds in nectar.
7.3
Phenolics
Phenolic substances are quite widespread in nectars (Radzevenchuk et al.,
1976; Baker & Baker, 1982a; Ferreres et al., 1996). Their accumulation may
render the nectar toxic, so that it then becomes repellent to some visitors
(Frankie et al., 1982; Hagler & Buchmann, 1993). Recently, Johnson et al.
(2006b) examined the responses of potential pollinators to the dark nectar of
Aloe vryheidensis
, which contains phenolics—honeybees and sunbirds were
deterred by the bitter taste, but larger passerine birds that are likely to be
more effective pollinators were not.
Phenolic substances are also relatively common scent products of flowers
(Knudsen et al., 1993; Sroka et al., 2001; del Bano et al., 2003; Deachathai
et al., 2006). As well as attracting pollinators or repelling nectar thieves,
these scent compounds may have a defensive function, either due to anti-
microbial activity or because they serve as signalling molecules to predators
and parasitoids (Pichersky & Gershenzon, 2002). Because of their solubility
in aqueous solutions and their production in the vicinity of the floral nectar
these phenolic compounds may also dissolve in nectar (Raguso, 2004).
Phenolic compounds in honey can serve as markers for its botanical ori-
gin (Bogdanov et al., 2004). Ferreres et al. (1996) have shown that
Portuguese heather nectar (
Erica
sp.) collected from the crop of bees con-
tained at least four phenolic compounds. The phenolic aglycones were
identified as quercetin, kaempferol, myricetin, and isorhamnetin. Gil et al.
(1995) identified a phenolic profile in rosemary nectar that showed 15 dif-
ferent flavonoids, the most prominent being kaemferol-3-sophoroside and
quercetin-3-sophoroside. In some cases, nectar phenolics appear to be meta-
bolized by honeybees during the manufacture of honey (Liu et al., 2005).
