Biology Reference
In-Depth Information
3
SUGARS
Nectar chemistry is dominated by three simple sugars: the disaccharide su-
crose and its component monosaccharides—fructose and glucose. All are
derived from sucrose translocated in phloem sap or synthesized in the nec-
tary. The relative amounts of each are determined by nectary invertase which
hydrolyses sucrose to glucose and fructose, before or during nectar secretion
(Pate et al., 1985). The sharp dichotomy between sucrose and hexose nectars
seen within genera such as
Erica
and
Leucospermum
(Barnes et al., 1995;
Nicolson & van Wyk, 1998), with nectars at both extremes of the sucrose-
hexose continuum, is indicative of the absence or presence of invertase
activity. Partial hydrolysis is apparently responsible for the mixed sugar
composition seen in the majority of nectars (Baker & Baker, 1983b).
Other minor sugars are present in trace amounts in nectar (Baker & Baker,
1982a, 1983b). These may be monosaccharides, (e.g., mannose, arabinose,
xylose), disaccharides (maltose, melibiose) or, more rarely, oligosaccharides
(raffinose, melezitose, stachyose). Sorbitol is also a frequent constituent of
Mediterranean nectars (Petanidou, 2005). Tests of a pollinator's preference
among nectar sugars are often restricted to sucrose, glucose, and fructose
(Chapter 7); where the less abundant nectar sugars have been included, they
are generally less attractive and less useful nutritionally (Barker & Lehner,
1974). Trisaccharides are more common in honeydew, another carbohydrate-
rich fluid that is produced by homopteran insects, its sugar composition
depending on both the insect and the host plant (Völkl et al., 1999).
An exception to the dominance of the three main sugars is seen in two
sister genera of the Proteaceae,
Protea
and
Faurea
, in which the pentose
sugar xylose comprises up to 39% of total nectar sugars (Nicolson & van
Wyk, 1998). Xylose is, however, absent from the nectar of other genera of
Proteaceae in South Africa and Australia. Studies of sugar preference and
sugar absorption efficiencies among various pollinators of Proteaceae are
reviewed by Jackson and Nicolson (2002)—insect and bird pollinators are
strongly averse to xylose and show very poor intestinal absorption of this
sugar. In contrast, the Namaqua rock mouse,
Aethomys namaquensis
, feeds
on
Protea
nectar with a relatively high xylose concentration compared to
that of bird-pollinated species (Nicolson & van Wyk, 1998) and is able to
utilize xylose via microbial fermentation in the hindgut (Johnson et al.,
2006a). Xylose has also been reported in the gelatinous nectar of a rodent-
pollinated African lily,
Massonia depressa
(Johnson et al., 2001).
