Biology Reference
In-Depth Information
amino acid content of nectar constitutes a very significant trait to which
some pollinator guilds (anthophorids, megachilids, and apids) respond. This
supports the general ideas of Baker and Baker (1982, 1986) that amino acids
may have co-evolutionary significance in floral nectars. The novel finding
by Petanidou et al. (2006), however, is that this positive effect is also appli-
cable to solitary bees, as well as social honeybees and tropical stingless bees
discussed by earlier studies (Roubik et al., 1995; Gardener & Gillman, 2002).
The most important finding, however, is that total amino acid content of nec-
tars versus sugar content is the most significant factor in shaping plant-
pollinator interactions in the Mediterranean habitats. The explanation for this
is probably related to the Mediterranean climate, as in such hot and dry habi-
tats, characterized by very high sugar concentrations (Petanidou & Smets,
1995), the sweet taste of nectar may probably be too “strong” to function as
species-specific discriminator and allurement. In these habitats, the high con-
tribution of amino acid content, together with the presence of individual
phagostimulants (e.g., phenylalanine, and possibly NaCl-combined GABA)
may have been selected in addition and constitute the nectar traits that are
specifically important in attracting particular insect guilds (Petanidou et al.,
2006). It will be interesting to see if these trends are also found in other
mediterranean regions and habitats, such as Chile, South Africa, and the
Californian coastal scrub.
5.4
Nectar minerals and pollinators
Among all minerals present in floral nectars special attention was given to
potassium, which was found to discourage honeybees from visiting onion
flowers (Waller et al., 1972; Liu et al., 2004). Moreover, there is some evi-
dence today that sodium (in the form of NaCl) has a positive effect on
attracting honeybees (Taber, 1991; Fulton, 1997); NaCl may improve nectar
taste significantly (Gardener & Gillman, 2002).
Plants contain a high K/Na ratio, which is reflected in the haemolymph of
herbivorous insects, including the highly evolved bees, as a result of co-
evolution with higher plants (Boné, 1944; Duchateau et al., 1953). This may
also influence nectar-pollinator relationships (Hiebert & Calder, 1983). Given
the deterrent character of potassium opposed to the attractive character of
sodium (Waller et al., 1972; Liu et al., 2004), I hypothesize that, in general,
highly attractive nectars are selected on the basis of their high Na/K ratio,
especially those visited by highly evolved pollinators. Within
phrygana
, I
expect that this will mostly apply to plant species visited by long-tongued
bees, primarily Megachilidae and Anthophoridae. No doubt, future research
on nectar attractiveness will explore these questions and hypotheses.
