Beyond nectar sweetness: are aminoacid complements related to pollinator guilds

Floral nectar is undoubtedly a source of sugars, being these substances its most abundant components. Pollinator‘s guild visiting flower is related to the composition of nectar sugar. In fact, species sharing the same type of pollinator show a convergence in the nectar sugar profile. Although amino acids are the second most abundant compounds in nectar after sugars, a relationship between amino acid complements and type of pollinators has been not yet investigated. Amino acids are firstly important nitrogen integrators and more than sugars contribute to the taste of nectar. Nectar protein-amino acids can be distinguished into four classes basing on their effect on insect‘s labellar chemoreceptors. Proline is very frequently the most abundant amino acid in nectar, and it seems to have a special importance for insect. Proline not only contributes a taste preferred by insects but it stimulates their salt cell (class III), a labellar chemosensory receptor, resulting in increased feeding behaviour. In honeybees, proline is the most abundant amino acid in the haemolymph and is required for egg laying. Moreover proline can be metabolised very rapidly and its degradation provides an efficient, short burst of energy. Nectar contains also non protein-amino acids being sometimes more abundant then protein-amino acids. GABA, taurine and β-alanine are non protein-amino acids more frequently reported in floral nectar. They are all abundant in the nervous systems of insects where they function as inhibitory neurotransmitters. GABA acts in synergy with taurine, limiting excessive, potentially disruptive excitation states during stressful conditions. Non protein-amino acids are poorly investigated and, due to their multi-functional biological activities, may contribute to shape the complex ecological interactions between nectar and animals. We had analyzed nectar sugar and amino acid profile from 32 species of Boraginaceae (tribe Lithospermeae) and from 3 other unrelated species (Cucurbita pepo, Helleborus foetidus and Gentiana lutea). Most of the species studied are pollinated by large Apoidea (mainly bees and bumblebees) and their floral nectar is generally sucrose dominant with a large amount of non-protein amino acids. The only two exceptions are Buglossoides purpurocaerulea and Buglossoides calabra: their floral nectar is exose dominant with low amount of non-protein amino acids. Correspondingly the main pollinators of these two species are the dipterans Bombylius major and Empis pennipes. These results push us to widen the number of species investigated trying to unravelling the importance of non protein-amino acids in shaping the pollinator guild. Until now non protein-amino acids has been documented in nectar of few species and their ecological role was still awaiting further investigations. Most probably, being involved in the regulation of nervous system activity, they may affect insect's foraging behaviour during flower visits.