NECTAR CHEMISTRY - Nectaries and Nectar - page 216

of nectar is often less clear than that of its volume and concentration, which
jointly determine the energetic reward available to foragers. However, there is
an encouraging trend towards a more holistic approach, such as investigations
of the nectar of related species with different pollinators, combining meas-
urements of nectar production and chemistry, and their variation in time and
space. Examples are recent studies of Lycium , Ipomoea, and Nicotiana
(Galetto et al., 1998; Galetto & Bernardello, 2004; Kaczorowski et al., 2005).
The importance of plant phylogeny in nectar studies is becoming more
apparent and the geographical focus is becoming wider.

2 WATER

Little is known about the carbon costs of nectar production (Pacini & Nepi,
2007), and even less about the impact on plant water budgets. High water
costs of flowering have been measured in the extreme case of the huge inflo-
rescence of Agave deserti (Nobel, 1977) and in Polemonium viscosum
(Galen et al., 1999). In general the water losses due to nectar itself are
probably minor compared to transpiration from floral structures, especially
in species with large, showy corollas. In other words, more water is needed
to produce and maintain the advertisement than the reward. Transpiration
from both may have a cooling effect: Patiño and Grace (2002) have shown
that high evaporation from corollas and sepals cools the flowers of Convol-
vulaceae, and evaporation from the dilute nectar of Fritillaria imperialis is
enough to lower the nectar temperature by almost 1 o C (Corbet et al., 1979).

The water investment in nectar may be more significant in plants that are
subject to water stress (Chapotin et al., 2003; de la Barrera & Nobel, 2004).
Drier conditions are generally reflected in fewer flowers produced and also
in smaller-sized flowers. This saves water, because smaller flowers contain
less water and produce less nectar (Plowright, 1981; Cresswell & Galen,
1991). In Mediterranean shrub communities, these changes can be seen in
perennial species of Lamiaceae as the flowering season advances, and are
thought to conserve water (Petanidou et al., 2000). Devoto et al. (2006) stud-
ied the pollination ecology of Embothrium coccineum (Proteaceae) trees
along a steep east-west rainfall gradient caused by the Andes in Patagonia,
and they found a significant increase in nectar concentration towards the
drier end of the gradient, but no consistent changes in volume. In Epilobium
angustifolium (Onagraceae), drought treatment leads to a substantial reduc-
tion in flower size with a concomitant reduction in nectar volume relative to
controls, although the concentration of nectar is largely unaffected (Carroll
et al., 2001). Water stress also affects the volume of nectar in Ipomopsis