Pollen aggregation by viscin threads in Rhododendron varies with pollinator.

Pollen grains can be dispersed singly or variously aggregated in groups. Whether the evolution of pollen aggregation is driven by the pollinator remains unexplored. We hypothesize that an extensive pollen aggregation is favored under a scarcity of pollinators. Variation in pollen aggregation by viscin threads in 13 Rhododendron species was measured as it is related to pollen removal in a visit. Visitation rates of functional pollinator groups that vary in their grooming behavior were investigated in each species. Pollen deposited on stigmas in the field was also sampled. Seven Rhododendron species were infrequently pollinated by low-intensity grooming animals, including birds, butterflies and moths. The other six species were more frequently pollinated by bees with a high intensity of pollen grooming. Bird- and Lepidoptera-pollinated species produced longer pollen-connecting threads that connected more pollen grains. Phylogenetically independent contrast analysis of the 13 species showed that pollinator visitation frequency was negatively related to amounts of pollen removal per visit but not to stigmatic pollen loads. The finding of interspecific patterns in pollen removal related to pollinator visitation frequency suggests pollinator-mediated selection on pollen packaging strategies, supporting the hypothesis of floral evolution via pollen export.

Nectar properties and the role of sunbirds as pollinators of the golden-flowered tea (Camellia petelotii).

PREMISE OF THE STUDY: Properties of floral nectar have been used to predict if a plant species is pollinated by birds. To see whether winter-flowering plants evolve nectar properties corresponding to bird pollinators, nectar properties of several Camellia species (including the golden-flowered tea), as well as the role of floral visitors as effective pollinators, were examined. METHODS: Potential pollinators of Camellia petelotii were identified at different times of day and under various weather conditions. A bird exclusion experiment was used to compare the pollination effectiveness of birds and insects. Nectar sugar components (fructose, glucose, and sucrose) from C. petelotii growing wild and another seven Camellia species and 22 additional cultivars (all in cultivation) were examined by high-performance liquid chromatography (HPLC). KEY RESULTS: The sunbird Aethopyga siparaja and honeybees were the most frequent floral visitors to C. petelotii. Honeybee visits were significantly reduced in cloudy/rainy weather. The fruit and seed set of flowers with birds excluded were reduced by 64%, indicating that bird pollination is significant. For the wild populations of C. petelotii, a bagged flower could secrete 157 µL nectar; this nectar has a low sugar concentration (19%) and is sucrose-dominant (87%). The eight Camellia species and 22 cultivars had an average sugar concentration of around 30% and a sucrose concentration of 80%, demonstrating sucrose-dominant nectar in Camellia species. CONCLUSIONS: The nectar sugar composition of Camellia species was characterized by sucrose dominance. In addition, the large reduction in seed set when birds are excluded in the golden-flowered tea also supports the suggestion that these winter-flowering plants may have evolved with birds as significant pollinators.

Evidence for passerine bird pollination in Rhododendron species.

When insect activity is limited at low temperature, birds may be comparatively more important pollinators than insects for flowering plants. It has been thought that many large-flowered Rhododendron species are pollinated by local birds in the Himalayan regions because most of these species flower in spring at high elevation with cool atmospheric temperature. However, experimental evidence for the role of bird pollination in this hyperdiverse genus remains scarce. To examine the role of birds and insects in pollination, we observed floral visitors to 15 Rhododendron species with different floral sizes and abundant flowering individuals in the eastern Himalayas, Southwest China. To examine the role of birds and insects in female reproductive success in each species, cages were used to exclude birds but not insects from visiting flowers and net bags were used to exclude all floral visitors. Inflorescences where visitation was excluded did not produce fruits in any of the Rhododendron species, indicating that sexual reproduction in these species depended on pollinator visitation. Bird visits were generally less frequent than bee visits in the studied species. However, in the nine species on which bird visitors were observed, fruit and/or seed set were greatly reduced in inflorescences caged to exclude birds but not bees, compared to open-pollinated inflorescences. In the other six species on which bird visitation was not observed, fruit and seed set did not differ significantly between caged and open inflorescences except in one species (R. wardii). Manipulations to achieve selective exclusion of visitors demonstrated that birds could be effective pollinators for 10 out of 15 studied Rhododendron species in the eastern Himalayas. Floral characteristics of these Rhododendron species and weather conditions might favour the evolution of bird pollination systems in the East Himalayas.