Biology Reference
In-Depth Information
in aged flowers: this exceptionally high pH is due to dissolved ammonia
produced after the primary secretion of nectar (Prys-Jones & Willmer,
1992).
In addition, some plant species produce mildly toxic or narcotic levels of
chemical constituents in their nectars. The nectar of
Epipactis helleborine
(Orchidaceae) was recently found to contain a number of narcotic substan-
ces, including oxycodone, 3-{2-{3-{3-(benzyloxy)propyl}-3-indol, and 7,8-
dehydro-4,5-epoxy-3,6-D-morphinan (Jakubska et al., 2005). The authors
suggest that after imbibing these narcotic substances, pollinators become
naturally intoxicated, and their more “sluggish” behaviour increases the time
spent within the flower and the chances of successful pollination. A similar
role was previously suggested for ethanol in the flowers of these orchids
(Ehlers & Olesen, 1997). However, this intoxication of pollinators could lead
to undesirable levels of geitonogamous self-pollination, i.e., between flowers
on the same plant (Klinkhamer & De Jong, 1993).
Before we deal with several structural categories of nectar constituents, it
should be mentioned that pesticides can accumulate in floral and extrafloral
nectars, so care must be taken in the chemical control of insect or microbial
pests (Jaycox, 1964; Lord et al., 1968; Barker et al., 1980).
7.1
Lipids
The presence of lipids has been reported in numerous plant nectars (Vogel,
1971; Baker & Baker, 1975; Bernardello et al., 1999; Vesprini et al., 1999).
Flowers offering fatty oils instead of or in addition to nectar are found in ten
different plant families, and oil flowers are visited and pollinated by special-
ized bees (Buchmann, 1987). Secretion of lipids is from highly specialized
epithelial cells termed elaiophores or glandular trichomes, and the progres-
sion from lipid-containing nectars to pure oils is not surprising in view of
similarities in the cells involved (Fahn, 2000). Elaiophores are metabolically
active secretory epidermal cells that generate large quantities of lipids under
a thin protective cuticle, forming lipid-filled blisters. These lipids have been
well studied in
Calceolaria
species (Scrophulariaceae) and in the rhattanys
(
Krameria
species, Zygophyllaceae). The major lipids to accumulate in these
nectars appear to be β-acetoxy fatty acids of varying chain length between
C
16
and C
20
(Vogel, 1971; Seigler et al., 1978). These are present as the
modified free fatty acids as well as in diglycerides. In addition to β-acetoxy
fatty acids, the oil nectars of
Mouriri myrtilloides
and
M. nervosa
contain
unmodified free fatty acids of C
14
to C
20
(Buchmann, 1987). Nectar of other
species such as
Jacaranda ovalifolia
(Bignoniaceae) and
Trichocereus
