Floral scent emitted by white and coloured morphs in orchids.

Polymorphism of floral signals, such as colour and odour, is widespread in flowering plants and often considered to be adaptive, reflecting various pollinator preferences for particular floral traits. Several authors have recently hypothesized that particular associations exist between floral colour and scent, which would result from shared biochemistry between these two floral traits. In this study, we compared the chemical composition of floral volatiles emitted by white- and purple-flowered morphs of three different orchid species, including two food-deceptive species (Orchis mascula and Orchis simia) and a food-rewarding species (Anacamptis coriophora fragrans). We found clear interspecific differences in floral odours. As expected from their pollination strategy, the two deceptive orchids showed high inter-individual variation of floral volatiles, whereas the food-rewarding A. c. fragrans showed low variation of floral scent. Floral volatiles did not differ overall between white- and coloured-flowered morphs in O. mascula and A. c. fragrans, while O. simia exhibited different volatile profiles between the two colour morphs. However, a detailed analysis restricted to benzenoid compounds (which are associated with the production of floral anthocyanin pigments) showed that white inflorescences emitted more volatiles of the shikimic pathway than coloured ones, both for O. mascula and O. simia. These results are consistent with the current hypothesis that shared biochemistry creates pleiotropic links between floral colour and scent. Whether intraspecific variation of floral signals actually affects pollinator attraction and influences the reproductive success of these orchids remains to be determined.

Rare white-flowered morphs increase the reproductive success of common purple morphs in a food-deceptive orchid.

How floral colour polymorphism can be maintained in evolutionary time is still debated. In rewardless orchids, it is unknown whether rare white-flowered morphs differ in scent chemistry from pigmented morphs, and whether such intraspecific variation in floral signals may have an impact on reproductive success. We compared the chemical composition of floral volatiles emitted by white- and purple-flowered morphs of Orchis mascula, and recorded the fruit set of both colour morphs. We also used white ping-pong balls to mimic white-flowered morphs in field bioassays. We found that colour polymorphism was not associated with floral odour polymorphism. Surprisingly, when populations of purple-flowered plants included a few white-flowered individuals, the fruit set of the purple morph increased significantly (from 6 to 27%), while that of the white morph remained low. We obtained the same fourfold increase in fruit set when using ping-pong balls as visual lures, demonstrating the association between colour variation and fruit set, and the key role of visual signals in pollinator attraction. Our results are incompatible with negative frequency-dependent selection, a hypothesis invoked to explain colour polymorphism in other rewardless orchids. We propose several hypotheses to explain the maintenance of white morphs in O. mascula.