Sugars in Mediterranean floral nectars: an ecological and evolutionary approach

High-pressure liquid chromatography analyses of 73 plant species showed that the nectars of phrygana (East Mediterranean garrigue) mainly contain sucrose, glucose, and fructose, and traces of 10 minor sugars. Although the sucrose/hexose ratio was not related to plant life habit, ecological constraints had a detectable effect in shaping sugar composition. This was detected by distinguishing the phryganic plant species into "spring-summer" and "winter" flowering, with the distinction made on the basis of the water deficit in the study area. Plants flowering in spring-summer had a higher rate of "high sucrose" (i.e., sucrose/hexose ratio > or = 0.5; 60.8% of the plant species) vs. "low hexose" nectars (i.e., ratio <0.5; 39.2%). The ratio was reversed in winter flowering species (36.4% vs. 63.6% with "high sucrose" and "high hexose," respectively). Sucrose/hexose ratios were associated with plant family. The highest values were those of Lamiaceae, which differed significantly from the "low sucrose" Liliaceae and Apiaceae. Based on recorded plant-pollinator interactions in the community, the present data provide evidence of a partitioning of nectar resources by the existing pollinator guilds within the community, based on the sugar profiles of nectar (all sucrose/hexose ratios for all interactions). Among all major groups, bees and wasps (aculeates) preferred "high sucrose" nectars, which differed significantly from syrphids, anthomyid a.o. flies, and beetles that visited "low sucrose" nectars. Similarly, butterflies visited "lower sucrose" nectars compared to bees. Within families, only Megachilidae could be clearly characterized as "high sucrose" consumers, differing in this respect from all the remaining insect groups including most other bee families. This confirms previous findings that Megachilidae have a key position in Mediterranean communities where they probably constitute a selective factor for "high sucrose" nectars.