NECTAR CHEMISTRY - Nectaries and Nectar - page 235

marlothii has no effect on the already high amino acid concentrations (around
50 mM; S.W. Nicolson, unpublished data).

5.1 Non-protein amino acids

In addition to the normal 20 amino acids found in proteins, plants also make
a large number of non-protein amino acids (Fowden et al., 1979). Many of
these compounds are toxic to protein biosynthesis and they frequently accu-
mulate in seeds where they serve as deterrents to insect feeding (Swain,
1977). Because of the function of nectar as an attractant, most of these non-
protein amino acids are thought to be excluded from nectar. However, a few
of the non-toxic non-protein amino acids, including β-alanine, ornithine,
homoserine, and γ-aminobutyric acid (GABA) are known to accumulate in
nectar. Baker (1978) found non-protein amino acids in the nectars of 36% of
283 California species examined and in the nectars of 55% of 69 tropical
trees and vines. In extrafloral nectars, Baker et al. (1978) reported the pres-
ence of non-protein amino acids in 22 of 33 species of tropical and
temperate-zone angiosperms. Consistent with this is the recent finding that
GABA is present in each of eight Nicotiana species examined and is even
the predominant amino acid in the nectars of N. plumbaginifolia and N. alata
(Kaczorowski et al., 2005). In Impatiens capensis , Lanza et al. (1995) found
especially high nectar concentrations of glutamine and hydroxyproline, a
component of plant cell walls. It is apparent that non-protein amino acids are
consistent and sizable components of certain floral nectars, but whether they
have any role in attraction of pollinators must await further studies.

5.2 Nectar amino acids are under the control

of environmental factors

In an early study of the role of environmental factors in the expression of
nectar amino acids, Baker and Baker (1977) showed semiquantitatively that
the amino acid complement of nectar from six different species was fairly
constant from sample to sample even though environmental conditions and
growth locations were widely divergent. Recent studies using improved
methods of quantitation such as HPLC have demonstrated that the amino
acid composition of Impatiens capensis nectar can vary significantly within
a single plant, within a population, and also between populations (Lanza
et al., 1995). Other investigators have found that amino acid concentrations
may vary with the age of the flower (Gottsberger et al., 1990; Petanidou
et al., 1996). Although the concentrations of amino acids in nectar from any
given species can be quite variable, Gardener and Gillman (2001a) found