Landscape scale variation in nectar amino acid and sugar composition in a Lepidoptera pollinated orchid species and its relation with fruit set

Understanding landscape scale variation in reproductive and pollination success is a major aim of plant population biology. A potential determinant of reproductive success that has received surprisingly little attention so far is variation in nectar chemical composition across plant individuals and populations. Using the Lepidoptera pollinated fragrant orchid (Gymnadenia conopsea) as a model species, we aimed at providing a first account of landscape scale variation in nectar amino acid and sugar composition, its environmental drivers, and how it may affect plant fruit set. Nectar was sampled from 986 flowers from 296 individuals across eleven discrete populations of the G.conopsea. The proportions of sugars and amino acids were determined using high-performance anion-exchange chromatography. Nectar composition was related to soil characteristics of the populations, and to fruit set, using linear mixed models. Approximately, 20% of the variance in nectar traits was situated between populations, whereas the highest proportion of variance (c. 45%) was found among flowers within individuals. Soil carbon and nitrogen content affected both nectar concentration and composition. Furthermore, fruit set of G.conopsea individuals was found to be significantly related to nectar amino acid and sugar composition.Synthesis. These results show that landscape scale variation in nectar amino acid and sugar composition should be taken into account in future studies of plant reproductive success. Furthermore, there is also high within-plant variation in both nectar amino acid and sugar composition. This variation may reduce geitonogamous pollination, but it may also limit rapid pollinator-mediated selection on nectar composition.