This is true for many of the 41 insect-pollinated plant species growing in a Phrygana scrubland habitat on the Greek island of Lesbos. An international research team published their findings Sept. 4 in Nature Ecology & Evolution.
The team investigated the way these plants communicate with a diverse assemblage of insect pollinators in the same community. They discovered a link between the color of the flowers and their fragrance, such that the two characteristics can be regarded, to a surprising extent, as one integrated signal.
This is the first study to demonstrate color-fragrance integration for an entire plant community.
“This result shocked us because we collected and analyzed the data in a blind and unbiased way, and because previous studies had not even considered the possibility of scent-color coordination,” said Robert Raguso, professor of neurobiology and behavior, who participated in the study.
The flowers use coordinated signals of color and fragrance to attract insects, which acquire pollen during floral visits and ensure pollination of the plants. In turn, the insects benefit by acquiring nectar and pollen as food.
By connecting visual and olfactory channels, the flowers render their signal stronger and more stable under the intense environmental conditions of the Aegean. On windy days, fragrances may dissipate but colors will remain viable floral attractants, whereas fragrance could be the primary attractant when flowers are concealed by the dense vegetation of the Phrygana scrublands.
According to Raguso, it is also likely that many insects learn to associate nectar or pollen meals with specific combinations of color and fragrance.
The researchers constructed a “social network” that illustrated the relationships between the 41 plant species and 351 fragrance compounds identified from the fragrances of these plants. The resulting network consisted of seven smaller modules of chemically similar plants. Surprisingly, nearly all of these modules could be characterized by specific odor-color combinations.
For example, one module featured red flowers dominated by waxy, long-chain hydrocarbon scents. Another contained plants with purple-pink flowers that emitted distinctive fragrances from a hydrocarbon class (sesquiterpenes) common to salvia, sagebrush and other aromatic herbs.
The plants in these groups are only remotely related, so the common occurrence of fragrances and colors does not reflect common ancestry. Instead, the authors suggest these patterns reflect adaptive compromises between pollinator attraction and other pressures, such as environmental stress and defense against enemies. The biochemical and genetic links between floral pigmentation and scent production remain poorly understood, so it is unclear whether color-odor combinations conserve energy for plants or reflect genetic factors that facilitate their integration.
Beyond the discovery of coordinated floral color and scent, the study raises additional questions and challenges for pollination ecologists. Flower-visiting insects in the Greek community are highly diverse, and can perceive floral stimuli in very different ways.
“Bees are the dominant pollinators in our study site, and they have trichromatic color vision – they see UV, blue, green,” Raguso said. “But butterflies and beetles have divergent visual systems, and can also see in red. We designed our study to account for these different forms of perception and selective pressure.”
The study provides a new direction for research on the interactions between plant signals and animal senses.
“Progress in our field has been hampered by the ways that we study plant-pollinator interactions – focusing only on one spatial scale or one sensory channel,” Raguso said. “With this study, we took a step closer to what I suspect is the reality for most pollinators, which seamlessly integrate across sensory channels as they approach a food item, just as we do.”
The researchers estimate the study will lower the barriers to reaching a more holistic understanding of pollination, by providing a blueprint for how to perform unbiased sensory-ecological analyses in any plant-pollinator community.