Do flowers with specialized morphologies produce more nectar and pollen?
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Premise of the study
Flower morphology influences the wiring of plant-pollinator interaction networks. Flowers with deep corolla tubes and bilateral symmetry have a narrower pollinator range, hence are considered more specialized than shallow radial flowers. Past inter-specific comparisons revealed positive correlations between flower depth and nectar production rates in a few plant communities, suggesting that specialized flowers may allocate more resources into food rewards for pollinators.
Methods
To evaluate this hypothesis, we compiled a global dataset of flower morphology vs. nectar sugar production rates (n = 494 plant species) and per-flower pollen counts (n = 164 species). We applied phylogenetically controlled mixed models to examine the effects of symmetry and tube length on floral rewards.
Key results
Corolla tube lengths, symmetry type and their interaction significantly predicted nectar production rates, with the larger effect attributed to tube length. Neither tube length nor symmetry predicted pollen number. Both nectar and pollen production were affected by phylogeny in a larger dataset that included 854 nectar-producing and 1040 pollen-producing species. Plant genus explained more of the variation in nectar production, and less of the variation in pollen production, than plant family.
Conclusions
Our findings suggest that visual signals associated with specialized flowers honestly advertise nectar production, but not pollen rewards. Other visual and chemical family-specific floral displays potentially advertise pollen availability to pollinators. We propose experiments to test whether nectar foragers indeed use flower depth as a visual signal that guides their choices of nectar sources.
