Biology Reference
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will naturally obscure the fine-scale variation between nectaries and flowers.
On a broader scale, Herrera et al. (2006) point out that much higher nectar
sucrose levels have been recorded in populations of
H. foetidus
from other
parts of Europe.
In six species of Brassicaceae, including
Arabidopsis thaliana
, the glu-
cose/fructose ratio differs between lateral and median nectaries in the same
flowers (Davis et al., 1998), but pooling between flowers was necessary to
show this because of the very small nectar volumes. Unbalanced glu-
cose/fructose ratios are an indication that more than simple hydrolysis in the
nectary is involved in the determination of nectar sugar composition. Low
glucose in relation to fructose was apparent in late season nectar samples of
H. foetidus
(Herrera et al., 2006), and seems to be characteristic of the Ges-
neriaceae (Stiles & Freeman, 1993; Baker et al., 1998; Perret et al., 2001,
2003). Higher glucose than fructose (10 times higher for some species) is
characteristic of the genus
Haworthia
(Asphodelaceae) (van Wyk et al.,
1993; Smith et al., 2001), and has also been recorded in nectar of
Lycium
species (Solanaceae) (Galetto et al., 1998). Glucose also dominates the
hexose fraction in
Combretum fruticosum
(Combretaceae), the difference
increasing with flower age (Bernardello et al., 1994). However, other popu-
lations of this widely distributed species were shown to have more balanced
hexose sugars (Gryj et al., 1990).
3.2
The use of sugar ratios can be misleading
Abundant data on nectar sugar composition (and the assumption that nectar
chemistry of a species is constant) have led to the suggestion that there are
co-evolutionary relationships between the sugar proportions in nectar and the
types of floral visitors. Based on extensive but largely unpublished analyses of
nectar sugar composition, Baker and Baker (1982a, 1983b) grouped unrelated
plant species according to pollinator type and demonstrated convergence in
sugar composition between plants with the same visitors. To summarize their
findings briefly, high sucrose in nectar was broadly correlated with pollination
by moths and butterflies, long-tongued bees, and hummingbirds. In contrast,
high proportions of glucose and fructose were characteristic of species polli-
nated by flies, short-tongued bees, passerine birds, and Neotropical bats.
This categorization was based on nectar/sugar ratios, defined as the ratio
by weight of sucrose to the combined hexose sugars, S/(G + F) (Baker &
Baker, 1982a, 1983b). Four classes of nectar were recognized (Table 2). This
terminology, although widely adopted by later authors, places undue emphasis
