NECTAR CHEMISTRY - Nectaries and Nectar - page 230

morphological and chloroplast DNA characters, and among the morphologi-
cal (and chemical) characters that she used were nectar sucrose and amino
acid concentrations. Similar results were obtained from these two independ-
ent data sets, supporting the inclusion of the morphological data (for dis-
cussion see Weller & Sakai, 1999). The phylogeny suggests that transitions
from passerine to hummingbird pollination have occurred at least four times
in the genus, and these transitions involve different floral modifications in
each case and are not homologous (Bruneau, 1997). The pollinator shifts are
accompanied by major changes in nectar concentration, sugar composition,
and amino acid content (Baker & Baker, 1982b)—and probably nectar vol-
ume—indicating the absence of phylogenetic constraint on nectar chemistry
in this genus (Bruneau, 1997). Erythrina crista-galli is basal in the clado-
gram, and field studies have shown that it is mainly visited by honeybees
and carpenter bees, in spite of negligible sucrose in its dilute nectar, suggest-
ing a transition from insect pollination, which is typical of the Fabaceae, to
bird pollination (Galetto et al., 2000).
In relating nectar sugar composition to pollinator type, Baker and Baker
(1983b) were aware of phylogenetic constraints on the adaptation of nectar
sugars to pollinators: “In characterizing long-tongued bee-flowers as offer-
ing a sucrose-rich reward, we could fall into the trap set for us by
phylogenetic constraint. Many of the … flowers in our sample come from
the Lamiaceae …” It is increasingly apparent that some plant families show
characteristic nectar sugars, and the Lamiaceae is an example of a family
with high nectar sucrose (see also Petanidou, 2005). The taxonomic value of
nectar sugars in angiosperms can be demonstrated by extensive sampling of
closely related species, as for example within Scrophulariaceae, Proteaceae,
and Gesneriaceae (Elisens & Freeman, 1988; Nicolson & van Wyk, 1998;
Perret et al., 2001). As a clear demonstration, sugar composition in the
sub-family Alooideae (Asphodelaceae) is highly conservative within genera
and reflects taxonomic affinities rather than pollinator types (van Wyk et al.,
1993). In this study, 47 Aloe species averaged 1.0% sucrose, while 12
Gasteria species averaged 88.5% sucrose; both genera are bird-pollinated,
but Aloe flowers are much larger and produce higher volumes.
The sugar composition of nectars in the most advanced dicotyledon sub-
class, the Asteridae, has been analysed by Schwerdtfeger (1996). His 900
nectar samples were collected mainly from botanic gardens in Germany. In
agreement with Baker and Baker (1983b), the data show strong correlations
with pollinator classes, so that high sucrose nectars are found in flowers pol-
linated by bees, butterflies, moths, and hummingbirds, and high-hexose
nectars in flowers pollinated by small, unspecialized insects, passerine birds