Biology Reference
In-Depth Information
Table 3.
Nectar cation concentrations in southern African species of
Protea,
Leucospermum,
Erica,
and
Aloe
. Values are means ± SD*.
K
+
(mM)
Na
+
(mM)
Genus
No. species
Sugar (% w/w)
Protea
9
25.9 ± 2.8
17.3 ± 9.3
18.0 ± 3.7
Leucospermum
5
26.4 ± 12.8
9.7 ± 3.6
15.4 ± 5.2
Erica
5
21.5 ± 7.3
4.9 ± 3.4
3.5 ± 0.5
Aloe
7
12.4 ± 3.7
4.2 ± 3.1
3.3 ± 1.8
*S.W. Nicolson unpublished data, except for
Leucospermum
(Nicolson & W.-Worswick, 1990)
microanalysis of
Abutilon
nectary hairs, found that K
+
in the phloem appeared
to be excluded from the nectar. Table 3 presents cation concentrations for
four genera of southern African plants, showing that Na
+
levels may exceed
those of K
+
.
Protea
and
Leucospermum
are in the Proteaceae, and high nec-
tar Na
+
concentrations have also been recorded in two other genera in this
family,
Banksia
and
Adenanthos
(Bradshaw & Bradshaw, 1999).
Exceptionally high K
+
concentrations have been recorded in the nectar of
onion (
Allium cepa,
Alliaceae) flowers: up to 13,000 ppm, equivalent to 333
mM K
+
, when the nectar is concentrated by evaporation (Waller et al., 1972).
These authors suggested that these high K
+
concentrations may be responsi-
ble for the reduced attractiveness of the nectar to honeybees and resulting
poor pollination of onion crops. Onion flowers have concentrated nectar
(52-65%) with high hexose levels (Hagler et al., 1990), which may also in-
fluence their attractiveness to honey bees. The concentrations of Ca
++
and
Mg
++
have been measured in a few studies (Waller et al., 1972; Heinrich,
1989; Kronestedt-Robards et al., 1989; Barclay, 2002).
Four species of Fabaceae visited by carpenter bees have nectar with high
sugar concentrations but extremely low ion levels (Na
+
0.3 to 3.8 mM, K
+
1.4 to 6.4 mM). This suggests problems of ion conservation, especially when
high metabolic water production in these large bees contributes to water
excess (Nicolson, 1990).
5
AMINO ACIDS
The presence of amino acids in nectar has been known since the mid-1950s,
when Ziegler (1956) compared sieve element fluid to other plant fluids, in-
cluding floral nectar, and demonstrated ninhydrin-reactive material in nectar.
Subsequently, Lüttge, using descending paper chromatography, identified
