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that the overall amino acid composition of nectar is generally more highly
conserved than the individual amino acid concentrations.
Other factors known to affect nectar composition include the response to
elevated CO 2 levels. A series of studies showed that doubling CO 2 levels in
the greenhouse for 60 to 80 days resulted in reduced levels of amino acids in
the nectar of four of five species tested (Rusterholz & Erhardt, 1998; Erhardt
et al., 2005). These authors conclude that elevated CO 2 levels could have
detrimental effects on the interactions between flowers and visiting pollina-
tors (such as butterflies) that utilize nectar as the primary source of amino
acids in their diet.
The amino acid composition of nectar is also known to be affected by
growth conditions. Gardener and Gillman (2001b) demonstrated that increas-
ing fertilizer availability to Agrostemma githago significantly altered the
amino acid composition of floral nectar, with specific increases in glutamine,
proline, and asparagine. One non-protein amino acid, GABA, showed a de-
crease with increased fertilization. Local differences in nectar composition
may lead to a greater variety of visiting pollinators and higher cross-
fertilization among local populations, with important consequences for plant
genetic diversity. Modern farming practices cause the drift of applied fertil-
izers at cultivated field margins, and an unexpected consequence may be
nectars with significantly improved nutritional qualities, as postulated by
Kleijn and Snoeijing (1997).
Extrafloral nectar production is also known to be under environmental
control. Smith et al. (1990) examined the production and composition of
extrafloral nectar following simulated herbivory of Impatiens sultani . The
volume and carbohydrate concentration of nectar were unchanged between
intact and defoliated plants, but HPLC analysis showed a dramatic increase
in the amino acid concentrations in extrafloral nectars 24 h after experimen-
tal defoliation. This may be a plant response to attract additional defensive
insects. The plants apparently return to homeostasis rather rapidly—72 h
after defoliation, the amino acids of extrafloral nectar were restored to nor-
mal levels. Cowpea ( Vigna unguiculata ) has two types of extrafloral nec-
taries, and those on the inflorescence have much higher levels of amino acids
than those on the leaf stalks (Pate et al., 1985). Pitcher plants produce extra-
floral nectar to attract insect prey, and Dress et al. (1997) suggest that
variation in its amino acid composition may be related to previous capture
success.
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