Secondary Metabolites: Attracting Pollinators

Abstract

Flowers present visual and chemical signals that mediate the interaction between plants and pollinators, who transfer pollen between flowers of different plants while foraging for nectar and/or pollen. This process favours the sexual reproduction of plants and covers the energy requirements of floral visitors. Colour and floral scent, as well as the chemical composition of nectar and pollen that constitute floral rewards, are among the attraction attributes. The expression of these attributes is the result of the production of a set of secondary metabolites such as pigments, volatile organic compounds and chemical compounds in nectar and pollen. In addition to attracting pollinators, these metabolites can repel natural enemies of plants (florivores, nectar robbers, microorganisms) or pollinators (pathogens), potentially affecting the outcome of pollination, and eventually the production of fruits and seeds.

Key Concepts

  • Most flowering plants that depend on pollinators for reproducing sexually have developed visual (colour) and chemical (scent) floral signals, as well as floral rewards (pollen and nectar), to increase the attraction of pollinators.
  • Secondary metabolites produced by flowering plants mediate the visual and chemical communication between plants and other organisms. While this communication is essential for pollination, it can also mediate the interaction of plants with florivores, nectar robbers, seed predators, microorganisms and pollinator pathogens that could affect pollination.
  • Pollinators are able to perceive variation in visual and chemical attributes of plants from the same or different species, affecting visitation rates and plant fitness.
  • Pigments are the secondary metabolites responsible for the synthesis of colour in different floral organs (petals, sepals, sexual organs) and floral rewards (pollen and nectar).
  • Floral volatiles are organic compounds emitted by flowers that mediate chemical communication in specialist and generalist plant–pollinators interactions.
  • Secondary metabolites in the nectar of various species include alkaloids, phenols and nonessential amino acids that can affect the preferences and foraging behaviour of pollinators, either by attracting or deterring them.
  • The impact of secondary metabolites in attracting different pollinators can result in pre‐reproductive isolation between plants and eventually contribute to their divergence.

Keywords: plant chemical communication; floral colour; floral nectar; floral scent; flower pigments; pollen colour; pollination; pollinator attraction; secondary metabolites

Figure 1. (a) Examples of a carotenoid pigment (β‐carotene) producing orange flowers in marigold (Tagetes estricta). Tiwary et al. . Reproduced with permission of Springer Nature. (b) An anthocyanin (cyanidin) producing a red colouration in Papaver rhoea flowers. Ghizlane and Aziz . Reproduced with permission of Elsevier. (c) A betalain pigment (betanidin) responsible for the purplish‐red colour of the common bougainvillea flowers. Gandía‐Herrero and García‐Carmona . Reproduced with permission of Elsevier.
Figure 2. (a) Variation in flower colouration of three petunia sister species and their respective pollinators: white flowers of P. axilaris are visited by hawkmoths, pink flowers of P. secreta are visited by solitary bees, and red flowers of P. exerta are visited by hummingbirds. (b) Wavelength and absorbance of the flowers of the species presented above. Source: https://academic.oup.com/aobpla/article/10/5/ply057/5113022. Licensed under CC by 4.0.
Figure 3. Pollen colour variation in flowers of Erythronium americanum, (a) yellow and (b) red anthers (a,b: Austen et al. . Reproduced with permission of John Wiley and Sons); Epimedium pubescens, bee pollinator visiting plants with (c) yellow and (d) green pollen grains (c,d: Wang et al. . Reproduced with permission of John Wiley and Sons); Campanula americana, (e) anthers going from white to deep purple (e: Koski and Galloway . Reproduced with permission of John Wiley and Sons).
Figure 4. Flowers of Nesocodon mauritianus produce red nectar (a) due to red aurones (b) and are visited by birds (c) (a–c) Olesen et al. . Reproduced with permission of Springer Nature. Flowers of Leucosceptrum canum produce purple floral nectar (d) due to the anthocyanidin 5‐Hydroxyflavylium (e) and are visited by birds (f) (d–f) Zhang et al. . Reproduced with permission of John Wiley and Sons.
Figure 5. (a) Flowers of the orchid Caledenia plicata emit a mixture of isomeric (S)‐β‐citronellol and 2‐hydroxy‐6‐methylacetophenone, mimicking the odour of sexual pheromones produced by females of the Zeleboria sp. thyninne wasp. Source: https://www.frontiersin.org/articles/10.3389/fpls.2017.01955/full#h12. Licensed under CC by 4.0. (b) Flowers of Stapelia asterias emit volatile compounds mimicking the smell of rotten meat, animal stools or carcasses of dead animals. Jürgens et al. . Reproduced with permission of John Wiley and Sons.
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Sosenski, Paula, and Parra‐Tabla, Víctor(Apr 2019) Secondary Metabolites: Attracting Pollinators. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000909.pub2]