What shapes amino acid and sugar composition in Mediterranean floral nectars

Petanidou, T. , Van Laere, A. , Ellis, W. N. and Smets, E. 2006. What shapes amino acid andsugar composition in Mediterranean floral nectars? Oikos 115: 155 169.We studied the amino acid (AA) composition of the floral nectars of 73 plant species occurringin a phryganic (East Mediterranean garrigue) community and investigated whether AA andsugar composition is shaped by evolutionary (plant phylogeny), ecological (flowering time as adirect effect of summer drought) and coevolutionary (pollinator partnership) constraints. Ourstudy utilised an extensive plant pollinator matrix compiled in the same area where the plantshad been sampled.Using HPLC we detected 22 AA compounds/groups of compounds, out of which 15 werecommonly present in almost all nectars. Among all AAs, phenylalanine was the mostabundant, especially in keystone (‘‘cornucopian’’) plant species visited by many insect species,such as the majority of the Lamiaceae. Amino acid quantities were transformed intopercentages (% of each AA over the total AA content of a flower). Sugar composition wassimilarly expressed as % of each of the three sugars (glucose, fructose, sucrose) over the totalcontent of these sugars; a number of other sugars, occurring in only a few plant species and invery low quantities were disregarded. The number of insect species of a particular family orguild was taken as a measure of the attraction of a nectar compound for such a family (guild). We found that taxonomical plant group had a weakly significant effect on nectarcomposition while neither life form nor flowering season had a discernable effect.Pollinators’ preference had the most important effect, with phenylalanine being the mostconsequent discriminatory compound for the response of the nectar consumers in phrygana,predominantly for long tongued bees, especially for Megachilidae. Gamma-aminobutyric acid(GABA) had a similar, even stronger influence on bees (long tongued bees, Anthophoridae,Andrenidae) and flies (Syrphidae and other Diptera), whereas asparagine behaved as a generalrepellent together with tryptophane (rather as repellent). Considering total sugar and AAcontents, as well as the volume of nectar, we found that total AA content was positively relatedto the number of species of long tongued bees and included families visiting the phryganicspecies; nectar volume was negatively related to flies (both hover flies and remaining Diptera),whereas total sugar content was not significant for any guild. We argue that due to the highlyconcentrated nectars in the dry Mediterranean communities that are characterised byoutstanding melittophily, sugars play a less important role as phagostimulants compared toAAs in floral nectars. This is why phenylalanine, a phagostimulant tested earlier on honeybees,appears to be of high importance in phrygana, especially with long tongued bees andMegachilids as the main selective agents for phenylalanine-rich nectars. The role of GABA, astrongly NaCl-dependent AA, may be similar, probably because of the associated presence ofNaCl.T. Petanidou, Laboratory of Biogeography and Ecology, Dept of Geography, Univ. of the Aegean,University Hill, GR-81100 Mytilene, Greece (t.petanidou@aegean.gr). A. Van Laere,Laboratory of Molecular Plant Physiology, Inst. of Botany and Microbiology, K.U. Leuven,Kasteelpark Arenberg 31, BE-3001 Leuven, Belgium. W. N. Ellis, Zoological Museum, Sectionof Entomology, Univ. of Amsterdam, Plantage Middenlaan 64, NL-1018 DH Amsterdam, theNetherlands. Erik Smets, Laboratory of Systematics, Inst. of Botany and Microbiology,Kasteelpark Arenberg 31, K.U. Leuven, BE-3001 Leuven, Belgium.Accepted 10 April 2006Subject Editor: Jane MemmottCopyright # OIKOS 2006ISSN 0030-1299OIKOS 115: 155 169, 2006