Abstract
Summary The ecological function of secondary metabolites in plant defence against herbivores is well established, but their role in plant–pollinator interactions is less obvious. Nectar is the major reward for pollinators, so the occurrence of defence chemicals in the nectar of many species is unexpected. However, increasing evidence supports a variety of potential benefits for both plant and pollinator from these compounds. Beneficial effects may include: (i) mediating specialization in plant–pollinator interactions, (ii) protecting nectar from robbery or larceny and (iii) microbial activity including preservation of nutrients in nectar from degradation and reduction in disease levels in pollinators. Secondary metabolites in nectar can be toxic or repellent to flower visitors, but equally they can go undetected or even make nectar more apparent or attractive. These biological effects are concentration dependent, so must be considered at a range of ecologically relevant doses. For example, caffeine occurs in nectar and improves honeybee memory for odours associated with food rewards, which enhances pollen transfer at naturally occurring concentrations but is repellent to honeybees at higher concentrations. This review synthesizes evidence from recent literature that supports selection for secondary metabolites in floral nectar as an adaptation that drives the co‐evolution between plants and their pollinators. However, their presence in nectar could still simply be a consequence of their defensive role elsewhere in the plant (pleiotropy). We highlight the need for more studies demonstrating measurable benefits to the plant, the importance of exposure levels and effects on target species beyond the current emphasis on alkaloids and bees. A Lay Summary is available for this article.
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Claims 11
Claim · model-written summary
The review finds evidence that nectar secondary metabolites can mediate specialization in plant–pollinator interactions.
Verbatim
e0“mediating specialization in plant–pollinator interactions”
t0“Plant secondary metabolites in nectar”
Claim · model-written summary
The review finds potential for nectar secondary metabolites to protect nectar from robbery or larceny.
Verbatim
e0“protecting nectar from robbery or larceny”
t0“Plant secondary metabolites in nectar”
Claim · model-written summary
Nectar secondary metabolites may preserve nectar nutrients against microbial degradation and reduce disease levels in pollinators.
Verbatim
e0“microbial activity including preservation of nutrients in nectar from degradation and reduction in disease levels in pollinators”
t0“Plant secondary metabolites in nectar”
Claim · model-written summary
Nectar secondary metabolites can be toxic or repellent to flower visitors.
Verbatim
e0“Secondary metabolites in nectar can be toxic or repellent to flower visitors”
t0“Plant secondary metabolites in nectar”
Claim · model-written summary
Nectar secondary metabolites can go undetected or make nectar more apparent or attractive to flower visitors.
Verbatim
e0“they can go undetected or even make nectar more apparent or attractive”
t0“Plant secondary metabolites in nectar”
Claim · model-written summary
The biological effects of nectar secondary metabolites depend on concentration.
Not reportedDirection
Verbatim
e0“These biological effects are concentration dependent”
t0“Plant secondary metabolites in nectar”
Claim · model-written summary
At naturally occurring concentrations, nectar caffeine improves honeybee memory for food reward odours.
- Taxon
- Plantt0
- Interactor
- honeybee
_Apis_mellifera_(female)_-_on_viburnum_tinus_flowers.jpg?width=60)
Apis melliferae0pollinator - Compound
- caffeinee0
- Outcome
- honeybee memory for odours associated with food rewardse0
- Direction
- improvese0
- Qualifier
- naturally occurring concentrationse1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
_Apis_mellifera_(female)_-_on_viburnum_tinus_flowers.jpg?width=500)
Verbatim
e0“caffeine occurs in nectar and improves honeybee memory for odours associated with food rewards”
e1“at naturally occurring concentrations”
t0“Plant secondary metabolites in nectar”
Claim · model-written summary
Nectar caffeine enhances pollen transfer at naturally occurring concentrations.
- Taxon
- Plantt0
- Interactor
- honeybeeApis melliferae0pollinator
- Compound
- caffeinee0
- Outcome
- pollen transfere0
- Direction
- enhancese0
- Qualifier
- naturally occurring concentrationse0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“caffeine occurs in nectar and improves honeybee memory for odours associated with food rewards, which enhances pollen transfer at naturally occurring concentrations”
t0“Plant secondary metabolites in nectar”
Claim · model-written summary
At higher concentrations, nectar caffeine repels honeybees.
- Taxon
- Plantt0
- Interactor
- honeybeesApis melliferae0pollinator
- Compound
- caffeinee0
- Outcome
- repellent to honeybeese0
- Qualifier
- higher concentrationse0
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“caffeine occurs in nectar and improves honeybee memory for odours associated with food rewards, which enhances pollen transfer at naturally occurring concentrations but is repellent to honeybees at higher concentrations”
t0“Plant secondary metabolites in nectar”
Claim · model-written summary
The review supports selection for nectar secondary metabolites as an adaptation driving co-evolution between plants and pollinators.
- Taxon
- plantsViridiplantaee0
- Interactor
- pollinatorse0pollinator
- Outcome
- co‐evolution between plants and their pollinatorse0
- Qualifier
- secondary metabolites in floral nectare0
Verbatim
e0“supports selection for secondary metabolites in floral nectar as an adaptation that drives the co‐evolution between plants and their pollinators”
Claim · model-written summary
Nectar metabolites may instead reflect their defensive role elsewhere in the plant.
Verbatim
e0“their presence in nectar could still simply be a consequence of their defensive role elsewhere in the plant”