A Comparison of the Amino Acid Complements of Floral and Extrafloral Nectars

Abstract

The amino acid complements of extrafloral nectars from 34 species of tropical- and temperate-zone flowering plants were analyzed. Comparisons were made between 33 of these and analyses of floral nectars from 248 species. Also, in 21 cases, direct comparisons were made between extrafloral and floral nectars from the same species. The extrafloral nectar always differed in its complement of amino acids from that of the floral nectar. Certain acids that are only moderately frequently represented in the floral nectars are more frequently represented in the extrafloral nectars, most notably the cysteine group and lysine, asparagine, and tyrosine. Differences between extrafloral and floral nectar complements presumably relate to the function of the latter in feeding ant or wasp "guards," which may have different nutritional requirements from those of pollinators. Nonprotein amino acids are represented more frequently in extrafloral nectars, possibly indicating chemical protection of exposed nectar from "thieves." In Campsis x tagliabuana (Bignoniaceae) there are four distinguishable groups of extrafloral nectaries-on petioles, sepals, petals, and developing fruits-as well as conventional floral nectaries. The amino acid complements were slightly different in each case, but no statistically significant differences were found between the frequencies of individual amino acids in nectars from 21 species where the producing nectaries are in a vegetative part of the plant and nectars from 12 species where the extrafloral nectaries are restricted to the inflorescence. Until the biological functions of extrafloral nectar can be elucidated for each species in which it is produced, the full significance of variation between the amino acid complements cannot be appreciated.