Floral scents play a delicate balancing act, appealing to pollinators while repelling herbivores.
Researchers in Germany have found that the same floral scents that help plants attract pollinators can also deter herbivores. Rohit Sasidharan and colleagues reviewed previous studies on how floral scents influence insects like bees and pests like beetles. They also did statistical analyses to find relationships between fragrances and flower chemicals. Their research, published in the Annals of Botany, provides new insights into the complex role floral scents play in mediating the tradeoffs flowers face between appealing to pollinators and repelling florivores. Understanding these dynamics better could help manage both pollinators and agricultural pests.
The Scent Tug-of-War Between Pollinators and Pests
The new research found that some compounds in floral scents play a dual role in interacting with insects. Specific chemicals like the terpene linalool and benzenoid methyl salicylate attract pollinating insects like bees. But these same compounds deter herbivores like beetles that would eat or damage plant reproductive parts. This clever chemistry allows plants to target different insects with the same scent molecules.
However, across all floral scents tested, more compounds attracted both pollinators and pests than those that selectively repelled herbivores alone. This finding indicates an evolutionary dilemma for plants – evolving scents to draw in pollinators inevitably also appeals to unwanted freeloaders. With more shared attractive scents, plants cannot as easily communicate “pollinators welcome, pests beware.”
One finding that may point to an adaptive solution is the relationship between scent diversity and pollen toxicity. The research uncovered that plants producing more varied bouquets of floral scents often had fewer toxins in their pollen. This result suggests that greater complexity of odour molecules may indicate safer, more nutritious pollen for pollinators. Plants balance their chemical defences between repellent scents and toxicity, potentially signalling pollen quality through scent diversity.
Digging into the Details of Floral Chemistry
The researchers conducted an extensive literature review to analyse floral scents, nutrients, and toxins across published studies on various plant species. Focusing on 49 species, they specifically compared pollinator and herbivore responses to the floral scents of the same plants. This allowed them to pinpoint differences in how these visitor groups detect and behaviorally react to the bouquets of odour molecules flowers produce.
The studies also compared the diversity and complexity of floral scents to attributes of the pollen itself – both nutritional quality and any toxic chemicals present. Sasidharan and colleagues tested for potential relationships between scent diversity and composition and pollen chemistry. Specifically, they looked at correlations between the number of scent compounds emitted by flowers, the protein content of their pollen, and the number and types of toxins found in the pollen.
This multi-pronged approach allowed the researchers to untangle the complex chemical communication and signalling between plants and their insect visitors through floral odours and pollen chemistry. The focus on both pollinators and pests provides insights into how flowers evolve amidst these conflicting pressures.
The Bigger Picture: Floral Evolution and Chemical Diversity
The research uncovered that across floral scents tested, a higher percentage were detected by herbivores than pollinators. Sasidharan and colleagues suggest this may be because pollinators have evolved to specialise on key flower scents, ignoring other compounds. Meanwhile, herbivores may need to detect a wider array of scents to avoid toxins plants produce against them.
While floral scent diversity did not strongly correlate with pollen nutrient and toxin composition in this study, the relationships uncovered hint at important signalling functions. The authors point out that more data across diverse plant species is needed to explain these patterns.
Overall, the findings provide valuable insights into the tradeoffs plants face between attracting pollinators and repelling pests as floral traits evolve. The chemical complexity seen in floral scents, pollen nutrients, and toxins all affect how flowers communicate and interact with insect visitors. A better understanding of floral chemistry helps explain the relationships between plants and pollinators essential to both ecology and agriculture.
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Sasidharan, R., Junker, R.R., Eilers, E.J. and Müller, C. (2023) “Floral volatiles evoke partially similar responses in both florivores and pollinators and are correlated with non-volatile reward chemicals,” Annals of Botany, 132(1), pp. 1–14. Available at: https://doi.org/10.1093/aob/mcad064.