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Pollination by passerine birds: why are the nectars so dilute?
1Department of Zoology and Entomology, University of Pretoria, 0002, Pretoria, South Africa. swnicholson@zoology.up.ac.za
Summary
Bird-pollinated flowers often have dilute, hexose-rich nectar, differing from sucrose-rich types. This nectar composition, influenced by invertase activity, impacts water influx and sugar secretion rates.
Area of Science:
- Plant biology
- Biochemistry
- Ecology
Background:
- Bird-pollinated flowers typically secrete dilute nectars (20-25% w/w).
- Southern African plants pollinated by passerine birds often produce hexose-rich nectars (10-15% concentration).
- Nectar composition, particularly sugar type, may correlate with nectar concentration.
Purpose of the Study:
- To investigate the relationship between sugar composition and nectar concentration in bird-pollinated flowers.
- To explore the role of invertase in nectar hydrolysis and its effect on nectar properties.
- To understand the factors influencing nectar dilution and sugar content.
Main Methods:
- Comparison of sucrose content with refractometer data for nectar concentration.
- Analysis of nectar composition in relation to plant phylogeny.
- Observation of nectar secretion and potential for dripping from flowers.
Main Results:
- Significant correlations were observed between sucrose content and nectar concentration.
- Hydrolysis of sucrose by invertase can increase nectar volume and dilute sugar concentration.
- Phylogenetic factors also contribute to the prevalence of hexose- or sucrose-rich nectars.
- Nectar evaporation leads to variability in concentration, influencing pollinator choice.
Conclusions:
- Nectar sugar composition (hexose vs. sucrose) is linked to nectar concentration and volume.
- Invertase activity plays a crucial role in modifying nectar properties for bird pollination.
- Both physiological and phylogenetic factors shape nectar characteristics in bird-pollinated plants.
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