Biology Reference
In-Depth Information
nectars to the drying and contaminating effects of the environment may lead
to greater prominence of hexose sugars, as well as higher sugar concentra-
tions (Koptur, 1994). Conversely, protection from microbial contamination
may be a factor in the tendency noted by Percival (1961) for tubular flowers
to contain nectar rich in sucrose. The antimicrobial properties of nectar (see
“Nectar redox cycle” on page 241) no doubt play a major role in the re-
markably consistent proportions of the main nectar sugars in many plant
species.
Changes in sugar proportions with age (as well as variation among flo-
rets, inflorescences, and plants) tend to be minor compared to the distinct
differences observed between species (Nicolson & van Wyk, 1998). Nectar
sugar composition remains constant in the long-lived flowers of three spe-
cies of
Eucalyptus
, in spite of continual exposure to visitors (Davis, 1997).
In
Combretum fruticosum
(Combretaceae), there is no decrease in sucrose
with flower age (Bernardello et al., 1994). Progressive hydrolysis, if it
occurs, will be more obvious in species with nectars that are initially high in
sucrose. The proportion of sucrose, although higher in female flowers of
Cucurbita pepo
(Cucurbitaceae), does not vary with age in either male or
female flowers (Nepi et al., 2001), but these flowers were screened to ex-
clude pollinators.
Ipomopsis longiflora
(Polemoniaceae) has high nectar
sucrose, and in flowers exposed to visitors the percentage of sucrose declined
significantly with flower age (Freeman & Wilken, 1987). Further investiga-
tion of the nectar sugar composition of this species showed that the percen-
tage of sucrose also declined with increasing temperature, under both field
and laboratory conditions, but was unaffected by water stress (Freeman &
Head, 1990; Villarreal & Freeman, 1990). Since sealing the flowers pre-
vented this decline, it seems to be due to external factors such as animal
visitation, rather than to secretory changes with age. For two bat-pollinated
species of columnar cacti, nectar sampling throughout the night (not com-
monly done) showed that sucrose levels were lowest after midnight (Petit &
Freeman, 1997).
The idea that nectar sugars are relatively constant within species has been
challenged by Herrera et al. (2006), who measured nectar sugars of
Helle-
borus foetidus
(Ranunculaceae) in southern Spain, comparing variation on
different levels, including variation between the five separate nectaries
within flowers. Differences among plants accounted for 14% of total vari-
ance, and differences among flowers of the same plant were the most
important at 56%. Differences among nectaries of the same flower (the level
at which bumblebees forage) were responsible for the remaining 30%. The
common practice of pooling nectar from different flowers on the same plant
