Do flowers with specialized morphologies produce more nectar and pollen?

Premise: Flower morphology influences the architecture 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 interspecific 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: We compiled a global data set 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. 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 data set of 854 species that produce both pollen and nectar and 1040 species that produce only pollen as rewards for visitors. Genus explained more of the variation in nectar production and less of the variation in pollen production than family. Conclusions: Our findings suggest that visual signals associated with specialized flowers indicate 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 and symmetry as combined signals that guide their choices of nectar sources.