Biology Reference
In-Depth Information
with a 1.56 times increase in volume (Nicolson, 2002). Sucrose hydrolysis in
the nectary also serves to maintain a favourable concentration gradient for
sucrose transport across the nectary tissue: thus total sugar production may
be significantly greater in flowers producing hexose nectars. This may ex-
plain differences in composition and concentration between the nectars of
flowers visited by hummingbirds and passerine birds (Nicolson & Fleming,
2003).
In addition to sugars, many nectars contain other solutes in sufficient
amounts to mask the true concentration of sugar in nectar. Corbet et al.
(1979) plotted the refractive index of various nectars against either ambient
relative humidity or the nectar osmolality: microclimatic and chemical ef-
fects, respectively, were shown by deviations from the corresponding curves
for pure sugar solutions. The effects of non-sugar solutes will depend on
their refractive index (which is highly correlated with molecular size) and
their concentration (Inouye et al., 1980). The latter authors calculated poten-
tial refractive index contributions from various non-sugar solutes, obtaining
values ranging from 1.9% to 3.6% as sucrose w/w. Such solutes may account
for a significant proportion of the apparent sugar content estimated by refrac-
tometry, resulting in overestimates of energy content (Inouye et al., 1980).
Regardless of chemical effects on concentration, microclimatic effects
tend to predominate, and the usual post-secretory change in nectar is an in-
crease in concentration due to evaporation, especially in open flowers. This
results in the commonly observed inverse relationship between volume and
concentration (Corbet et al., 1979; Plowright, 1981; Nicolson, 2002). To illus-
trate the power of evaporation, a 20% sucrose solution will lose water to air
at all relative humidities below 98% (Corbet et al., 1979). Fortunately for many
nectar consumers, humidity gradients inside flowers are modified by long
corollas that slow the exchange of water between nectar and air (Plowright,
1987), and large volumes of nectar evaporate more slowly because of the
reduced surface/volume ratio. Evaporation can be rapid when small drops
of sugar solution are placed inside the corolla but not in contact with the nec-
taries (Castellanos et al., 2002). Other features of floral morphology may
reduce evaporation, such as the massed stamens in brush-type flowers, e.g.,
in
Eucalyptus
. A dense barrier of stamen filaments in the red flowers of the
cactus
Echinocereus coccineus
protects the abundant nectar for humming-
bird visitors (Scobell & Scott, 2002). Microclimatic effects must also be
taken into consideration when flowers are protected from visitors in order to
compare nectar production rates with standing crops (Wyatt et al., 1992;
Corbet, 2003).
