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et al., 1992; O'Brien et al., 1996); in Impatiens glandulifera , however, it is
apparently without any major effects (Burquéz & Corbet, 1998).
Elevated CO levels significantly stimulate nectar secretion rates, in-
creasing nectar volume rather than sugar content in Tropaeolum majus
(Tropaeolaceae; Lake & Hughes 1999) and Cucumis melo (Cucurbitaceae;
Dag & Eisikowitch, 2000). However, Davis (2003) and Erhardt et al. (2005)
found that the effect of elevated CO levels and increased UV-B radiation on
nectar production varied from species to species, making it difficult to
2
2
generalize. The relationship between these two global changes (increased CO
2
and UV-B levels) and nectar characteristics is intriguing from the ecological
point of view because it may modify the behaviour of foraging animals.
Soil nutrients may also affect nectar production. Shuel (1955) demonstrated
that nectar yield per flower in Antirrhinum majus (Scrophulariaceae) was
comparatively good under conditions of low nitrogen supply and moderate
growth. Gardner and Gillman (2001) found that fertilizer treatments in-
creased the concentration of proline and glutamine in nectar of Agrostemma
githago (Caryophyllaceae) but had no effect on other amino acids.
8.2
Intraspecies variability
Broad intraspecies variability of nectar characteristics is reported by numer-
ous authors. Intraspecies variability of nectar features may be revealed at
different levels: in individual flowers, between flowers of the same plant,
between plants of a population, and between populations.
Variability within individual flowers occurs in several species of Bras-
sicaceae that have a compound heterogeneous nectary—the nectarium
(Davis et al., 1998)—composed of two lateral and two median nectaries. The
lateral nectaries have relatively rich quantities of phloem bundles that pene-
trate the secretory tissue, and produce much more nectar sugars than the
median nectaries (Davis et al., 1998). Intraflower variability is also observed
in Helleborus foetidus (Ranunculaceae), whose flowers generally have four
nectaries that produce nectar with different relative abundances of the three
main sugars (sucrose, glucose, and fructose) (Herrera et al., 2006).
Nectar variability among flowers of an individual plant can be expressed
in many different ways:
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