Sources of floral scent variation in the food-deceptive orchid Orchis mascula

Abstract

Intraspecific variation of floral signals, and particularly floral scent, is widespread in orchid species. Pollinator-mediated selection is often hypothesized to govern such polymorphism. For example, circadian rhythms of floral emissions have been associated to the daily activity patterns of pollinators in several plant species. However, most of the studies on floral scent variation usually consider only one factor of variation (diel variation, pollination, flower age …). In this study, we investigated simultaneously seven sources of variation susceptible to induce changes in floral scent emissions in the food-deceptive orchid Orchis mascula: circadian rhythm, flower age, pollination, inflorescence morphology, herbivory, leaf spots and habitat. We found that all the factors studied, except plant morphology (spots on leaves, inflorescence morphology), were associated with significant changes in the volatile profile of O. mascula inflorescences. Electroantennography experiments with Bombus terrestris, the main pollinator of O. mascula, revealed that bumblebees were able to detect almost all the compounds involved in floral scent variation. Whether pollinator behaviour is influenced by changes in floral odours induced by these different factors, and whether it can affect plant reproductive success will have to be tested in further studies.