Biology Reference
In-Depth Information
The presence of unusual sugars in nectar may be a consequence of phloem
sap composition, with sugars passing untransformed through the nectaries
(Jackson & Nicolson, 2002). Unfortunately, studies that measure both nectar
and phloem composition of the same species are rare (Adler, 2000; but see
Pate et al., 1985). Xylose is present in the phloem sap of
Tanacetum vulgare
(Asteraceae), where it comprises 33% of phloem sugar and 14-89% of sugar
in the honeydew of four aphid species feeding on the plant (Völkl et al.,
1999). Raffinose is a characteristic phloem sugar of Myrtaceae, among other
plant families (Ziegler, 1975), and the mean molar ratio of raffinose to su-
crose is 0.29 in the phloem sap of
Eucalyptus globulus
(Pate et al., 1998).
The fact that this trisaccharide (also occurring in sugar beet) was not identi-
fied in the nectar of several
Eucalyptus
species (Nicolson, 1994) may merely
reflect the conditions used in HPLC analyses (B.-E. van Wyk, personal
communication). In orchid nectars analysed by thin-layer chromatography,
raffinose was a common constituent (Jeffrey et al., 1970). High mannose
levels in the nectar of lime trees (
Tilia
) during drought conditions may be
due to unusual phloem sap composition, and are toxic to honeybees (Crane,
1977), owing to low activity in these insects of the enzyme mannosephos-
phate isomerase (Sols et al., 1960).
3.1
Constancy of sugar composition within species
Although many older data were single analyses, the percentages of sucrose,
fructose, and glucose are often relatively constant when nectar samples from
different individuals of a species are analysed. Assuming that partial hydro-
lysis leads to the mixed sugar composition of most nectars, it is difficult to
see how the proportions of the three main sugars remain consistent in multi-
ple nectar samples from a given species. Consistency in sugar composition is
also surprising in view of the fact that microbes in nectar are carried from
one flower to the next by contaminating visitors (Willmer, 1980; Sandhu &
Waraich, 1985; Ehlers & Olesen, 1997; Antonovics, 2005). Such inoculation
is inevitable during pollinator visits, especially when they occur at high fre-
quency. As an example, Williams and Thomson (1998) observed a single
plant of
Penstemon strictus
, with fewer than 30 open flowers, and recorded
an average of 4.55 bumblebee flower visits per minute over the whole day.
Nectar is a nutritious fluid and an excellent medium for growth of bacteria
such as
Erwinia amylovora
, the causative agent of fire blight in apple and
pear orchards (Bubán et al., 2003). Colonization by yeasts may produce fer-
mentation volatiles in nectar that are important in flower-pollinator
relationships (Raguso, 2004). The likely result of microbial contamination is
a decreased proportion of sucrose as flowers age, as happens in
Citrus
flow-
ers (Loper et al., 1976). It has been suggested that exposure of extrafloral
