Biology Reference
In-Depth Information
pollinated flowers show attenuated nectar secretion and reduced sugar
content compared to unpollinated ones (Gillespie & Henwood, 1994).
In species with a long flowering season, such as
L. vulgaris
, which flow-
ers from June to November, nectar volume and sugar concentration vary
sharply between early and late flowers in response to variations in environ-
mental parameters, while the relative abundance of sugars (glucose, fructose,
and sucrose) remains almost constant (Nepi et al., 2003). In
H. foetidus
,
flowering from January to March, a sharp variation in the relative abundance
of sugars was found between the early and late flowers (Herrera et al., 2006).
In the same species within-plant variation was responsible for 86% of all the
variance of nectar characteristics in a population (Herrera et al., 2006). At
the opposite extreme there is the case of
Impatiens capensis
(Balsaminaceae),
the nectar properties of which were not found to vary significantly within
individuals (Lanza et al., 1995).
At population level, differences in floral nectar production among plants
may be genetically determined and may interact with environmental condi-
tions (differences in soil moisture, exposure, type of substrate, etc.) (Leiss &
Klinkhamer, 2005b). Because nectary traits are often very responsive to en-
vironmental variation, even substantial amounts of genetic variation may be
swamped out in the field (Mitchell, 2004). Most genetic studies on floral
nectar variability concern production rate and concentration, whilst we know
very little about the heritability of other major traits—such as sugar ratios,
amino acid composition, taste, and scent—that are probably less plastic in
response to environmental variation than are production and concentration
(Cruden et al., 1983; Leiss et al., 2004; Mitchell, 2004 and references
therein). These few studies indicate that there is abundant genetic variation
in nectar traits. According to Mitchell (2004), no studies concerning herita-
bility of extrafloral nectar traits have been published. The genetic control of
nectar traits has also been the subject of very little research. In
Mimulus
(Scrophulariaceae) and
Petunia
(Solanaceae), a minimum of two quantitative
trait loci (QTLs) are involved in controlling the amount of nectar produced
while the hexose:sucrose ratio in
Petunia
is under the control of a major
QTL which might code for an invertase (Galliot et al., 2006).
Dioecious plants often show differences in nectar features between flow-
ers of the two sexes, as demonstrated for
Silene latifolia
and
S. dioica
(Caryophyllaceae) (Shykoff, 1997; Hemborg, 1998). In a review of 19 dioe-
cious species, Eckart (1998) reported that ten exhibited higher nectar
production in pistillate flowers, the remainder yielding more nectar in stami-
nates. Although 19 species is not a very representative sample, it seems
