Biology Reference
In-Depth Information
6.4
CO
2
and nectar
Datura wrightii
flowers open at dusk and wilt by the following noon. These
flowers are pollinated by the hawkmoth
Manduca sexta
(Lepidoptera:
Sphingidae). Newly opened flowers secrete large amounts of nectar along
with high levels of CO
2
(Guerenstein et al., 2004). It is thought that this CO
2
release signals the availability of nectar to visiting pollinators. In studies
using artificial flowers, emitting either high (double the current atmospheric
concentration) or low (equivalent to current atmospheric concentration) CO
2
levels, moths overwhelmingly preferred the flowers emitting higher levels of
CO
2
(Thom et al., 2004). This suggests that plants use CO
2
emission to sig-
nal the presence of an adequate nectar supply to visiting pollinators. The
mechanisms that result in CO
2
release have not yet been elucidated, but it is
clear that at the time of anthesis, flowers have very high metabolic activity
and there are clearly biochemical pathways that could result in the emission
of CO
2
. The role of these pathways in floral development has, however, not
yet been investigated.
Recently, other studies have examined the influence of elevated CO
2
on
floral nectar production in a number of dicotyledon species (Lake & Hughes,
1999; Davis, 2003). Both studies concluded that the quantity of nectar changed,
but the quality of nectar (defined as the solute concentration) was not sig-
nificantly affected by elevated CO
2
.
In recent studies, the effect of elevated CO
2
on nectar production in
Epi-
lobium angustifolium
was investigated (Erhardt et al., 2005). These studies
showed that elevated CO
2
significantly increased nectar production in this
species; however, the authors also noted that these results are not consistent
with earlier observations, and are therefore likely to be species-specific.
In a world in which atmospheric levels of CO
2
are predicted to double in
the 21st century (Watson et al., 1996), it is clear that flowers and their polli-
nators will have to learn how to deal with elevated levels of CO
2
, if they are
to be as successful in the future as they have been in the past. Mechanisms
that can sense double the level of atmospheric CO
2
as an indicator of nectar
presence may no longer function a century from now. Whether plants and
pollinators can compensate for the doubling of atmospheric CO
2
is not clear.
If they cannot, this could result in a significant reduction in fecundity for
both the plants and their pollinators.
