Biology Reference
In-Depth Information
evaporation in exposed flowers. The nectar concentration determines its vis-
cosity and hence influences the feeding responses of animals; water in nectar
may also be an important reward for pollinators in dry conditions.
Carbohydrates.
The main nectar solutes are the sugars sucrose, glucose and
fructose, and their total concentration ranges from 7% to 70% w/w. Invertase
activity in the nectary determines the proportion of sucrose to hexoses. Con-
siderable attention has been paid to the question of whether the relative
proportions of these three sugars in nectars are a result of adaptation to polli-
nators (Baker & Baker, 1983; 1990) or phylogenetic history (e.g., Nicolson
& van Wyk, 1998; Galetto & Bernardello, 2003). Other monosaccharides
and disaccharides may be present in minor amounts, as well as oligosaccha-
rides such as stachyose, and sugar alcohols such as sorbitol. However,
oligosaccharides are much less abundant in nectar than in honeydew. Some-
times polysaccharides may be responsible for a jelly-like consistency of
nectar (Sazima et al., 2001). The sources of nectar carbohydrates are phloem
sap (in which case nectary parenchyma is reduced or absent); photosynthe-
sizing nectary parenchyma, starch stored in parenchyma and derived from
photosynthesis in that tissue or other floral parts, or the degeneration of cer-
tain nectary parts (Pacini et al., 2003). Sugars in nectar are usually the
primary energy source for consumers, and the study of plant-pollinator rela-
tionships has long been based on energetics, with clear correlations between
the sugar content of flowers and the energy requirements of the animals pol-
linating them (Heinrich, 1975).
Amino acids and proteins.
Amino acids are the most abundant nectar sol-
utes after sugars, and include a wide array of both essential and non-essential
amino acids, as well as some non-protein amino acids (e.g., Petanidou et al.,
2006). Proteins occurring in nectar include enzymes and preservatives
(Carter & Thornburg, 2004). These nitrogenous components are derived
from one or more of the following sources: phloem sap, protein bodies in the
nectary parenchyma, cytological activity or degeneration of certain parts of
the nectary, or the epidermis of the nectary parenchyma. Nectar amino acids
may play a role in taste preferences of insects (Gardener & Gillman, 2002)
and in their nutrition, depending on other food sources (Mevi-Schütz &
Erhardt, 2005). Proteins appear to have various homeostatic and regulatory
roles.
Ions.
These are derived from xylem and/or phloem sap, although informa-
tion on ion concentrations in floral nectars is scarce. Again, the nutritional
benefits to pollinators will depend on other food sources. High K
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