Biology Reference
In-Depth Information
longiflora
(Polemoniaceae) without changing the concentration (Villarreal &
Freeman, 1990). Conversely, supplemental watering increases nectar pro-
duction in
Delphinium nelsonii
(Ranunculaceae) (Zimmerman, 1983) and
Echium vulgare
(Boraginaceae), particularly in
E. vulgare
plants that have
been selected as low nectar producers (Leiss & Klinkhamer, 2005). After
nectar removal, plants appear to be more conservative in replacing the sugar
component of nectar than the water, although a meta-analysis of removal
effects showed considerable variability (Ordano & Ornelas, 2004).
2.1
Nectar concentration
Corbet (2003) gives a useful account of techniques for field measurements
of nectar volume and concentration, necessary for collecting data on both
standing crop and rate of production. Concentrations are usually measured
using refractometers, although alternative procedures are needed for flowers
that produce very small nectar volumes (Kearns & Inouye, 1993; Dafni
et al., 2005). The use of filter paper wicks to remove small nectar volumes was
recommended by McKenna and Thomson (1988), and is especially valuable
in the study of butterfly nectar resources, although the method yields total
sugar rather than volume and concentration (Holl, 1995). Refractometers
give the sugar concentration on a % weight/weight basis (sucrose equiva-
lents; g sucrose per 100 g solution), so these units are commonly used in the
literature concerning nectar and nectar consumers (and are used throughout
this chapter). There are, however, several sources of error that could be
avoided by expressing sugar concentrations on a molar basis instead:
•
Some authors report concentrations without distinguishing between % w/w
and % w/v.
•
Others calculate energy values without first converting sugar concentra-
tions to units of % w/v or molarity, and this error is greater at higher
concentrations (Bolten et al., 1979).
•
Sugars mixed on a % w/w basis have been assumed to be equicaloric, but
hexose solutions prepared in this way have only 95% of the energy value
of the corresponding sucrose solutions (Fleming et al., 2004).
•
An additional source of error comes from ignoring non-sugar solutes such
as amino acids and inorganic ions (see discussion on page 219).
Nectar sugar concentrations vary significantly, both within and between
species. Extreme concentrations are found in the nectars of
Aloe castanea
(Asphodelaceae), in which sugar concentration is less than 10% (0.3 M)
(Nicolson & Nepi, 2005), and caraway
Carum carvi
(Apiaceae), in which
the average sugar concentration is 66.5% (2.5 M) (Langenberger & Davis,
