Biology Reference
In-Depth Information
Table 2.
Sugar ratios in nectar. (From Baker & Baker, 1982a.)
Class
S/G + F
% sucrose
Sucrose-dominant
>1.0
51-100
Sucrose-rich
0.5-1.0
34-50
Hexose-rich
0.1-0.5
10-33
Hexose-dominant
<0.1
0-9
on the sucrose content of nectar. The transition from “hexose-rich” to “su-
crose-rich” occurs at 33% sucrose, when it should occur at 50% sucrose. The
overemphasis on sucrose may have arisen from considering a “balanced”
nectar as one with equal weights of the three sugars and thus a sugar ratio of
0.33/0.67 = 0.5 (Baker & Baker, 1983b), when in fact a sugar ratio of 1.0 is
more appropriate to describe a balanced nectar (containing equal weights of
sucrose and hexoses). The statement, firmly entrenched in the literature, that
hummingbird flowers are prevailingly sucrose-rich or sucrose-dominant
(Baker & Baker, 1983b) merely indicates that most of their nectars have a
sugar ratio above 0.5, i.e., sucrose concentrations greater than 33% of total
sugar. However, the idea of sucrose dominance is supported by the mean
value of 64.4% sucrose in the nectars of 278 plant species pollinated by
hummingbirds (Nicolson & Fleming, 2003).
A further reason to avoid the use of sugar ratios is that percentages of the
three sugars are not independent; acknowledging this, Baker and colleagues
omitted ratios in a later review of nectar and fruit sugars (Baker et al., 1998).
Sugar ratios are still in widespread use but fortunately many authors present
the percentage sugar composition as well (e.g., Perret et al., 2001; Torres &
Galetto, 2002; Galetto & Bernardello, 2003; Petanidou, 2005; Wolff, 2006).
The use of sugar ratios will be avoided in the following discussion, to-
gether with the terms “sucrose-dominant”, etc. The alternative is to express
the proportions of sucrose, glucose, and fructose as percentages of total
sugar; usually the percentage of sucrose is sufficient information (unless
there is particular interest in the relative proportions of glucose and fruc-
tose). Data presented as sugar ratios can be converted as follows: where the
ratio R = S/(F + G), the percentage of sucrose is % sucrose = 100R/(1 + R)
(Freeman et al., 1984).
