Biology Reference
In-Depth Information
•
Different position in the inflorescence.
Demonstrated in
Gaura mutabilis
(Onagraceae), where upper flowers contained three times as much nectar
as lower flowers (Cruden et al., 1983).
•
Differences in inflorescence size.
In
Asclepias quadrifolia
(Asclepiada-
ceae) larger inflorescences produce less nectar per flower than smaller ones
(Pleasants & Chaplin, 1983).
•
Differences in microenvironment around the flower.
A. castanea
flow-
ers directly exposed to sun have a lower volume and higher concentration
than flowers in the shade (Nicolson & Nepi, 2005).
•
Differences in flower age.
This may cause differences in nectar secretion
rate (nectar volume) and sugar concentration. These differences are docu-
mented in several species and it is common for older flowers to have a
lower volume of nectar. They may have higher or lower solute concentra-
tions than younger flowers (Wyatt & Shannon, 1986; Petanidou et al.,
1996; Navarro, 2001; Nicolson & Nepi, 2005).
•
Differences in the sexual phase of dichogamous flowers.
In
Delphinium
sp. (Ranunculaceae; Cruden et al., 1983),
Lobelia cardinalis
(Campanula-
ceae; Devlin & Stephenson, 1985),
Echium vulgare
(Boraginaceae; Klink-
hamer & de Jong, 1990),
Alstroemeria aurea
(Alstroemeriaceae; Aizen &
Basilio, 1998), and
Euphorbia boetica
(Euphorbiaceae; Narbona et al.,
2005) the total sugar content was higher in the male phase. On the other
hand, in
Metrosideros collina
(Myrtaceae; Cruden et al., 1983),
Polyscias
sambucifolia
(Araliaceae; Gillespie & Henwood, 1994) and
C. carvi
(Apiaceae; Langenberger & Davis, 2002), nectar with a higher total sugar
content was produced during the female phase. Carlson and Harms (2006)
formulated two sets of adaptive hypotheses about the evolution of patterns
of gender-biased nectar production: sexual selection hypotheses and in-
breeding avoidance hypotheses.
•
Different flower morphs in individual plants.
Linaria genistifolia
(Scrophulariaceae; Davis & Vogel, 2005) may have unspurred zygomor-
phic flowers, single-spurred zygomorphic flowers, and 3-5-spurred
actinomorphic flowers on the same individual. On average, multi-spurred
flowers produce three times more nectar sugar than single-spurred flowers,
and 400 times more than spurless flowers.
•
Differences in visitation by pollinators.
This is important when polli-
nation induces changes in nectar secretion; e.g., in
P. sambucifolia
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