ABSTRACT
Nectar is widespread amongst angiosperms and it is exploited as an alimentary resource by a great variety of animals (invertebrates and vertebrates) which may pollinate or defend the plant from herbivores (Nicolson 2007; Heil 2011). Until recently, most studies concentrated on the ability of nectar to attract foragers, largely due to its high concentrations of simple sugars, namely: sucrose, glucose and fructose, as well as its lower content of amino acids. Fundamentally an aqueous solution, nectar is easily ingested, digested and absorbed by the alimentary canal, and is thus a very cost-effective alimentary resource for a wide variety of animals (Nicolson 2007; González-Teuber and Heil 2009). This classical “alimentary” perspective of nectar has recently been challenged by other studies that have both detailed and shown the presence of compounds that are not directly related to the alimentary value of nectar. For example, it was shown that fl oral and extrafl oral nectar may contain a large and heterogeneous assemblage of defence proteins that are active against micro-organisms (Carter and Thornburg 2000, 2004; Naqvi et al. 2005; Carter et al. 2007; Kram et al. 2008; González-Teuber et al. 2009, 2010; Hillwig et al. 2011; Nepi et al. 2011). Some of these micro-organisms are phytopathogens (Bubàn et al. 2003; Farkas et al. 2007). Others are not dangerous for the plant but may interfere with the pollinator’s alimentary choice. Yeasts are known to inhabit fl oral nectar, changing considerably the chemical composition of nectar and consequently affecting the relationships with nectar foragers (Herrera et al. 2008, 2009). It was hypothesized that nectar inhabiting micro-organisms may act as a third party in the mutualistic relationships linking plants and nectar foragers, an emerging tripartite relationship whose ecological and evolutionary signifi cance is still far from being well established (Canto and Herrera 2012).
