Biology Reference
In-Depth Information
2002). These two species show little variance in their nectar sugar concen-
trations, while other species show a significant variance: for example, 4-72%
in
Clintonia borealis
(Liliaceae) (Plowright, 1981), and 2-62% in
Echium
plantagineum
(Boraginaceae) (Corbet & Delfosse, 1984). Temporal varia-
tion in nectar sugar can be caused by nectary activity (secretion or
reabsorption) and/or removal by foragers. Variation in the water component
of nectar occurs through the same mechanisms, but nectar water is also af-
fected by equilibration with ambient humidity (Corbet, 2003). Because of
these sources of variation, it is clear that the nectar of a plant species should
not be characterized by single measurements of its volume and concen-
tration. In disregard of this variability, erroneous ecological significance is
sometimes attributed to nectar concentrations (especially when values for
many species are averaged), because plants with
similar pollination syn-
dromes tend to have similar nectar concentrations. In broad terms, insect-
pollinated flowers produce relatively concentrated nectars, whereas flowers
pollinated by birds and bats generally produce dilute nectars (Pyke & Waser,
1981; Baker & Baker, 1982a, 1983b).
2.2
Chemical and microclimatic influences on nectar
concentration
Percival (1961) carried out a semi-quantitative study of nectar sugars in 900
angiosperm species. Her study noted that plant families with deep or tubular
flowers tend to produce nectar rich in sucrose, whereas shallow flowers tend
to produce nectar rich in monosaccharides. Hexose nectars would be expec-
ted to evaporate more slowly than sucrose nectars of the same concentration
on a w/w basis, because more solute particles are present to lower the effec-
tive concentration of the solvent (water) (Corbet, 1978). For the same sugar
concentration, hexose nectars have much higher osmolalities than sucrose
nectars (Corbet, 1978; Corbet et al., 1979; Nicolson, 1994), and this leads to
slower evaporation and lower final concentrations that are in equilibrium
with the ambient relative humidity. Physical relationships are therefore in-
volved in the hexose dominance observed by Percival (1961) in shallow
nectaries, although phylogenetic effects will also be important.
The correlation between sugar composition and nectar concentration may
arise very early in floral development. Nectar originates from sucrose-rich
phloem sap or from sucrose synthesized in the nectary tissue, and the propor-
tion of monosaccharides depends on the presence and activity of various
nectary enzyme systems, including invertase. Hydrolysis of sucrose in-
creases nectar osmolality, thus drawing water into the nectar. This water
influx can potentially convert a 30% sucrose nectar into a 20% hexose nectar,
