Concerted Changes in Floral Colour and Scent, and the Importance of Spatio-Temporal Variation in Floral Volatiles

Abstract

Many floral phenotypes are naturally dynamic, changing shape, colour or reward availability as flowers age. Here we explore the dynamic nature of floral volatiles, with special reference to the potential for concerted changes in floral colour and scent and their functional integration as multimodal floral signals. We review the evidence for concerted changes in floral colour and scent in the literature and survey the most appropriate content- and efficacy-based hypotheses for multimodal floral function in such cases. Finally, we analyze unpublished data on concerted changes in floral colour and scent collected from the colour-changing flowers of the pan-tropical invasive weeds Lantana camara, L. montevidensis and Heliotropium amplexicaule. In each species, older, colour-changed flowers emitted reduced amounts of linalool and its oxides, along with common aromatic esters and alcohols, whereas abundant sesquiterpene volatiles emitted by floral calyces and the plants’ leaves and stems did not change. Although these compounds were not the most abundant floral volatiles, they are known to show behavioral and electrophysiological–olfactory activity for several species of butterflies. We conclude with a discussion of the conceptual and methodological reasons why dynamic patterns of floral scent emission often are overlooked, and identify opportunities for further progress in these areas.