Hauling field gear up Gothic Mountain…
Abstract Background and Aims Floral traits are key mediators of plant–pollinator interactions, and can be altered through changes in resources to plants. Although several studies have tested how nitrogen affects visual traits, only a few have examined nitrogen effects on floral scent, and to our knowledge none have evaluated the effect of nitrogen addition on nitrogenous volatile compounds. We test how nitrogen availability influences floral morphology, nectar traits, display size and floral scent (total emissions and individual compounds) during the day and night, with a special focus on a nitrogenous compound, indole, that influences reproductive isolation due to pollinator behaviour. Methods We investigated the effects of supplementary nitrogen on floral traits in two closely related species of Ipomopsis and their hybrids in greenhouse and field environments in the Colorado Rocky Mountains. In the greenhouse, we studied I. aggregata and I. tenuituba collected from natural populations, while in the field we studied I. aggregata and hybrids. By combining greenhouse and field components, we assessed the robustness of floral trait responses to nitrogen availability. Key Results Responses to nitrogen varied among taxa and between environments. In the field, nitrogen increased corolla width and nectar production in I. aggregata but not in hybrids. In the greenhouse, nitrogen decreased nectar production in I. tenuituba, but increased flower production and inflorescence height in I. aggregata. Emission composition showed a nitrogen effect in greenhouse I. aggregata but not in I. tenuituba or in either field taxon. Across both environments, total emissions and emission of indole were unaffected by nitrogen addition. Day–night differences explained most of the variation in total per-flower emissions, with compound-specific time differences contributing to this. Conclusions Our results suggest that, even for close relatives, floral traits respond differently to nitrogen addition. They also suggest soil nitrogen has weaker effects on floral volatiles than on flower number and some other individual floral traits.