Biology Reference
In-Depth Information
GABA receptors in the insect herbivore, resulting in feeding satiation (Mullin
et al., 1991; Ozoe et al., 1999). The finding that this can be overcome in some
species by co-administration of GABA (Passreiter & Isman, 1997) suggests
a possible interaction between these compounds and the high level of accu-
mulation of GABA, a non-protein amino acid, in some plant nectars (section
5.1, above).
While terpenoids do occur in plant nectars, most are produced by cells
with specialized metabolic potential that are dispersed throughout the flower
(Bergström et al., 1995; Dudareva et al., 1998; McTavish et al., 2000). These
specialized cells produce the volatile compounds that serve as pollinator at-
tractants (Plepys et al., 2002; Andersson, 2003). Such attractants require the
nectar reward to be effective (Hammer & Menzel, 1995). Insect-learning
studies have demonstrated that associative learning requires both a condi-
tioned stimulus and an unconditioned stimulus. If insects are presented with
an attractive odour without a sucrose reward, the response to the odour soon
disappears or is extinguished (Bitterman et al., 1983). Many insects clearly
show a preference for one attractant over another (Honda et al., 1998; Natale
et al., 2003), driven by electrophysiological responses (Raguso et al., 1996),
and the presence of the feeding stimulus results in the reinforcement of
lesser attractants over dominant ones (Cunningham et al., 2004). Thus, long-
range factors such as terpenoids generally serve as attractants for visiting
pollinators, but are of little use if there is not an associated nectar reward to
reinforce the conditioned stimulus. As stated earlier for the phenolic com-
pounds, some volatile terpenoids are soluble in the aqueous nectar and their
presence in nectar may be due to passive absorption by the nectar. In a direct
test of this hypothesis, Raguso demonstrated that several floral scent com-
pounds, including geraniol, linalool, and jasmone, were taken up by artificial
nectars and subsequently volatilized (Raguso, 2004).
8
CONCLUSION
The rich composition of nectar suggests that this metabolic offering is a ma-
jor consideration both for the plant and for many animal visitors. Using
nectar as an energy source, pollinators move pollen from flower to flower,
but at the same time they carry a host of microbial contaminants. There must
exist mechanisms to maintain nectar in a microbe-free state, and indeed the
nectary expresses a number of protein-defence factors that function to pro-
tect the gynoecium and the nectar itself. Phenolic, terpenoid, and other
ingredients of essential oils are produced in flowers and may accumulate in
nectar, probably with additional antimicrobial benefits (Raguso, 2004).
