Colored nectar as an honest signal in plant-animal interactions

Abstract

Many flowering plants obtain the services of pollinators by using their floral traits as signals to advertise the rewards they offer to visitors—such as nectar, pollen and other food resources. Some plants use colorful pigments to draw pollinators’ attention to their nectar, instead of relying on the appeal of nectar taste. Although this rare floral trait of colored nectar was first recorded by the Greek poet Homer in the Odyssey, it has only recently received the attention of modern science. This mini-review focuses on recent findings about some of the species that use colored nectar; topics include its function as an honest signal for pollinators, as well as the pigments responsible for the nectar coloration. Such research of the ecology and physiology of colored nectar expands our understanding of the role and evolution of pollinator signaling in plants.

As stationary organisms, flowering plants must overcome a host of challenges to reproduce successfully, including the transfer of pollen between individuals. While many species use water or the wind to carry pollen from one flower to another, others rely on animals to act as pollinators. Some of these pollinator-dependent plants provide food, shelter, or warmth as rewards to their floral visitors; during visits to the flowers of such species, these animals obtain pollen, intentionally or accidentally, which may subsequently be deposited on the flowers of another plant of that species.

Plants have evolved myriad mechanisms to attract pollinators. One of the most prevalent and well-studied of these floral traits is nectar production, which offers pollinators an energy-rich food source.¹ While the nectar of most nectariferous species is a clear liquid containing sugars and trace amounts of amino acids, a few plant species produce vividly colored nectar that can seduce pollinators even before the nectar is smelled or tasted.^2-4 Although this rare floral trait was first recorded by the Greek poet Homer and has been recognized in several plant families,⁵ it has not received adequate scientific attention. Indeed, current understanding of the function and evolution of colored nectar remains limited for two possible reasons: first, the nectar color may be overlooked by plant workers when they collect living plants, since it is not a well-known phenomenon; and second, initial descriptions of newly discovered species tend to be based on dry specimens, for which any colored nectar would have evaporated before the analysis could take place.⁵

Olesen and coworkers first studied colored nectar in Mauritius, an island distant from the east coast of Africa.³ Although these scientists initially believed that the three Mauritian species with colored nectar represented the only colored-nectar species in the world, a total of 68 species from 20 genera in 15 families have now been discovered. Among these species, the most common nectar colors were yellow, amber, red, brown, green, blue and black.⁵ Most of these species were found in the tropical and subtropical regions of the Southern Hemisphere, across a wide taxonomical and geographical distribution.

The first compound reported to be responsible for nectar coloration came from one of the Mauritian species, Nescodon mauritianus (Campanulaceae); this species’ scarlet-red nectar coloration was found to be caused by a red aurone.² One hypothesis of Olesen and colleagues was that the colored nectar may serve as an honest signal to pollinators, and recent ecological experiments have revealed that the plant did in fact employ the nectar color as a visual signal.^3,4

A more recent study investigated the colored nectar and pollination biology of Leucosceptrum canum Smith, a bird-pollinated plant distributed across the Himalaya Mountains and southwestern China;⁶ interestingly, this range lies in the Northern Hemisphere, contrary to the previously discovered colored-nectar species. We sought to understand what advantages the production of colored nectar imparts to this plant species.⁶ Our study also explored the ecological functions and chemical origins of L. canum nectar coloration, by combining chemical analyses and pollinator behavior experiments.⁶ The nectar color was found to act as a foraging signal for bird pollinators to increase pollination efficiency.⁶ Moreover, this study supported the observation that colored nectar tends to be correlated with bird pollination.^4-6

Chemical compounds in floral nectar can be abundant.^7-9 Previous studies of nectar chemistry have demonstrated associations between pollinators and the sugar and amino acid content of the nectar,^10-15 as well as between nectar phenolic compounds and plant defense.^16,17 More information is needed, however, regarding the connections between colored nectar and colored compounds.² We isolated the purple anthocyanidin 5-Hydroxyflavylium from the dark purple nectar of L. canum and concluded that the compound was responsible for the nectar color.⁶ The compound was found to be unstable, however, probably as a result of the high sugar concentration in the nectar of mature flowers, or because the compound had been oxidized to another compound in later floral stages.

In contrast to traditional signal theory, a lack of the signal would be costly to colored-nectar plants like L. canum; due to the link between signal and reward, floral visits by pollinators may be unlikely without the presence of the visible reward.⁵ The colored nectar benefits both the pollinator and plant, since it acts as a signal to attract pollinators, thereby increasing pollination efficiency while also providing nutrition for the visitor. Such plant-pollinator interactions reduce inbreeding for the plant, and lead to increased outcrossing.¹⁸ The signal value of the colored nectar is likely to change over the course of flower development: the longer a flower remains unvisited, the greater the volume of the colored nectar becomes and hence the larger the visual signal to attract the pollinator becomes as well.^5,6 The active regulation of nectar production by flowers has been suggested to be an adaptive trait to improve reproductive fitness.^19,20 Interestingly, the dark purple nectar of L. canum presents different tastes during developmental flower stages. Bird pollinators feed only when the nectar becomes palatable, which coincides with reproductive maturity in the flower and increases pollination efficiency, while also keeping immature flowers from being damaged or depleted of nectar by precocious visitation.⁶

Although signaling between animals has been the main focus of research into signaling theory in the past, future studies could benefit from the application of signal theory to the interaction between plants and animals. The colored-nectar plants and their pollinators provide an excellent model to study signal theory in plants, incorporating the fields of biochemistry and pollination ecology and animal and plant behavior.