Nectar volatiles defend flowers against potentially harmful bacteria
Floral nectars offer nutrient-rich habitats for bacterial communities. Bacterial communities associated with flowers are often characterised by low diversity but high densities suggesting few species can utilise this resource. However high densities of bacteria can effect nectar quality and in turn, pollinator visitation, thus affecting plant fitness. In the North American bee-pollinated wild flower, Penstemon digitalis, the nectar volatile S-(+)-linalool has been shown to be under positive natural selection suggesting its importance for plant fitness. As earlier experiments by us showed that bees do not seem to respond directly to linalool concentrations, we hypothesized that high emission rates of linalool (as well as co-occurring nectar volatile methyl nicotinate) may inhibit the growth of bacteria. Thus mitigate negative interference with pollination. We isolated bacteria from four different floral and vegetative microhabitats to compare their responses towards different concentrations of linalool and methyl nicotinate.
Bacteria identified from P. digitalis nectar and tissues included strains from 6 genera including Pantoea, Pseudomonas and Acinetobacter that are known to colonize floral tissues in other species. Exposure to the two volatiles showed contrasting effects. The presence of linalool slowed bacterial growth and reduced the overall carrying capacity of nectar. Methyl nicotinate in low concentrations, however, allowed bacteria to reach higher densities and thus may be used as a carbon-source. Both volatiles increased the lag time of growth, meaning that bacteria increased their abundance much later than in media without volatiles. Our results strongly suggest that volatiles defend nectar against bacterial growth and add to the growing body of work that suggest floral volatiles mediate interactions beyond attraction to pollinators.