Biology Reference
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glutamine, asparagine, methionine, serine, tyrosine, cysteine, proline, and
alanine in the nectar of plantain bananas and later in the nectars of five addi-
tional species (Lüttge, 1961, 1962).
It was not until the early 1970s, however, that the presence of amino acids
in nectar became apparent on a large scale. In those studies Baker and Baker
(1973) used a simple method to screen large numbers of nectars for ninhydrin-
reactive material. Nectar spotted onto filter paper was reacted in a ninhydrin
assay, and the intensity of the coloured spot was compared to a linear stan-
dard curve of histidine spots to give semi-quantitative information on the
level of amino acids present (but not their chemical nature). Of 266 species
tested, only six failed to show amino acids in this assay. The Bakers com-
pared the nectars from flowers pollinated by different guilds (i.e., bee,
butterfly, moth, fly, and bird). For the most part the coefficients of variation
for the presence of amino acids in nectar were so large that little information
could be obtained; the authors were, however, able to conclude that special-
ized flowers which attract carrion and dung flies were especially rich in
ninhydrin-reactive material, as were butterfly-pollinated flowers. They sug-
gested that plant nectars cannot be overlooked as a potential source of amino
acids in the nutrition of butterflies and flies. Based upon the phylogeny of
the plant species examined, they also suggested that amino acids have been a
constituent of nectars since the earliest stages of angiosperm evolution. All
20 of the normal amino acids found in protein have been identified in vari-
ous plant nectars, and the essential amino acids may be an important
nitrogen source for nectarivorous pollinators (Nicolson, 2007, Chapter 7 in
this volume).
Although all ten essential amino acids are commonly present in floral
nectars, some non-essential amino acids such as asparagine and glutamine
can occur in much higher concentrations. In
Erythrina
species pollinated
by passerine birds, the total amino acid concentrations are far higher than
in hummingbird-pollinated species (Baker & Baker, 1982b), and it is
non-essential amino acids that are largely responsible for the difference (S.W.
Nicolson, unpublished data). The passerine-pollinated
Erythrina
species
produce relatively dilute nectars (Baker & Baker, 1982b), so the warning of
Inouye et al. (1980) that non-sugar components may lead to overestimation
of nectar sugar concentrations is a valid one in the case of
Erythrina
.
Gottsberger et al. (1984, 1990) cautioned against attributing too much
significance to measured amino acid concentrations, because of the likeli-
hood of amino acids leaching from dislodged pollen grains. However, this
was disputed by Baker and Baker (1986). Pollen addition to nectar of
Aloe
