What shapes amino acid and sugar composition in Mediterranean floral nectars?

We studied the amino acid (AA) composition of the floral nectars of 73 plant species occurring in a phryganic (East Mediterranean garrigue) community and investigated whether AA and sugar composition is shaped by evolutionary (plant phylogeny), ecological (flowering time as a direct effect of summer drought) and coevolutionary (pollinator partnership) constraints. Our study utilised an extensive plantpollinator matrix compiled in the same area where the plants had been sampled. Using HPLC we detected 22 AA compounds/groups of compounds, out of which 15 were commonly present in almost all nectars. Among all AAs, phenylalanine was the most abundant, especially in keystone (‘‘cornucopian’’) plant species visited by many insect species, such as the majority of the Lamiaceae. Amino acid quantities were transformed into percentages (% of each AA over the total AA content of a flower). Sugar composition was similarly expressed as % of each of the three sugars (glucose, fructose, sucrose) over the total content of these sugars; a number of other sugars, occurring in only a few plant species and in very low quantities were disregarded. The number of insect species of a particular family or guild 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 nectar composition while neither life form nor flowering season had a discernable effect. Pollinators’ preference had the most important effect, with phenylalanine being the most consequent 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 general repellent together with tryptophane (rather as repellent). Considering total sugar and AA contents, as well as the volume of nectar, we found that total AA content was positively related to the number of species of long tongued bees and included families visiting the phryganic species; 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 highly concentrated nectars in the dry Mediterranean communities that are characterised by outstanding melittophily, sugars play a less important role as phagostimulants compared to AAs 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 and Megachilids as the main selective agents for phenylalanine-rich nectars. The role of GABA, a strongly NaCl-dependent AA, may be similar, probably because of the associated presence of NaCl.