Biology Reference
In-Depth Information
andalgalensis
(Cactaceae) contains so much lipid that it has a milky consistency
to the naked eye (Baker & Baker, 1975). The nectar of bat-pollinated
Dacty-
lanthus taylorii
(Balanophoraceae) has been especially well studied (Ecroyd
et al., 1995). These investigators have identified the ethyl and benzyl esters
of hexadecanoic acid and C
18
to C
23
polyunsaturated fatty acids. In addition,
the nectar contained C
21
to C
31
hydrocarbons.
Because lipids are a highly reduced form of carbon, on a molar basis they
are among the highest-energy compounds available in nature, and may pro-
vide pollinators with a rich energy source. However, oil nectars are also
more expensive for plants to produce. Among Patagonian species, lipids are
relatively common nectar constituents, about 30-50% of these species accu-
mulating nectar lipids, especially among the Fabaceae (Forcone et al., 1997;
Bernardello et al., 1999). These authors argue that the extreme conditions
found in Patagonia may necessitate a high-energy food resource for effective
pollination.
Finally, a wide variety of lipids are known to accumulate in extrafloral
nectar of cotton (
Gossypium hirsutum
, Malvaceae). Using gas chromato-
graphy, Stone et al. (1985) identified free fatty acids including palmitic,
stearic, palmitolenic, oleic, linoleic, and linolenic acids. They also identified
a number of phospholipids. The fatty acid concentration was greatest in the
extrafloral nectar from young plants and decreased as the plants matured.
7.2
Organic acids
Phloem sap and nectar both contain primary metabolites of the plant that are
involved in fundamental plant physiological processes (Pate et al., 1985).
These include amino acids, especially some non-essential amino acids such
as asparagine and glutamine, and organic acids that are Krebs cycle interme-
diates (fumarate, malate, oxaloacetate, and succinate). Apart from early stu-
dies demonstrating the presence of organic acids in various nectars (Baker &
Baker, 1975, 1983a), little further attention has been paid to these substances
in nectar. Pate et al. (1985) compared the composition of phloem sap and
two types of extrafloral nectar in
Vigna unguiculata
, and found that succi-
nate and malate were the main organic acids of both phloem and nectar, with
malonate also significant in the nectar. The energy value of organic acids to
insects has seldom been considered, but leafhoppers (Cicadellidae) metabo-
lize xylem organic acids with high efficiency, as assessed by comparing the
chemical profiles of xylem fluid and honeydew (Andersen et al., 1989).
