Biology Reference
In-Depth Information
For the well-researched North American hummingbird flora, it is gener-
ally accepted that hummingbird pollination is derived from insect pollination
(Stebbins, 1989; Grant, 1994). Hummingbird-pollinated plants north of Mexico
belong to different genera and families but are convergent in floral colour,
size, shape, and nectar rewards (Brown & Kodric-Brown, 1979). In addition,
their pollinators are a small group of closely related species, among the
smallest hummingbirds in the family Trochilidae (Bleiweiss, 1998). The
hummingbird plants consist of isolated species in otherwise insect-pollinated
genera such as
Aquilegia
(Ranunculaceae),
Ipomopsis
(Polemoniaceae),
Pen-
stemon
(Scrophulariaceae), and
Salvia
(Lamiaceae), and phylogenetic inertia
can explain the predominance of sucrose in the hummingbird nectars. Recently,
Perret et al. (2001, 2003) investigated nectar sugar composition in relation to
pollination syndromes in the Neotropical tribe Sinningieae of the Gesneri-
aceae, and found similar high-sucrose nectar (58-89%) in both hummingbird
and bee flowers, with composition changes occurring only in bat flowers.
Bee pollination is also the primitive condition in tribe Antirrhineae of the
Scrophulariaceae, where the sugar composition of 45 species was also rela-
tively constant, despite a variety of pollinators (Elisens & Freeman, 1988).
Similarly, in
Nicotiana
species pollinated by hummingbirds and hawkmoths,
increased floral size is accompanied by higher volumes and lower concentra-
tions, but the sucrose proportion and total energy change much less
(Kaczorowski et al., 2005). This is discussed further by Nicolson (2007,
Chapter 7 in this volume).
In conclusion, phylogenetic history appears to be the primary determinant
of nectar chemistry, but pollinators have a secondary effect. Interestingly,
Roulston et al. (2000) found no evidence of any association between pollen
protein content and the dietary demands of pollinators; rather, protein con-
centrations were highly conserved within plant genera and families (but
pollen, unlike nectar, is not solely a reward).
4
INORGANIC IONS
Published data on ion concentrations in nectar are scarce. Hiebert and Calder
(1983) measured K
+
and Na
+
concentrations in 19 species visited by hum-
mingbirds. Their findings suggested a phylogenetic component to patterns
of nectar ion composition, in particular high K
+
concentrations in the nectar
of Ranunculaceae. Heinrich (1989) found K
+
to be dominant in the nectars of
20 plant species. Higher K
+
than Na
+
concentrations in nectar are in agree-
ment with the relative concentrations of these ions in phloem sap (Ziegler,
1975; Pate et al., 1985, 1998), but Robards and Oates (1986), using X-ray
