Abstract:

Bombax ceiba is a tall tree species with predominantly red flowers and is normally pollinated by birds. In some populations, a yellow flowered variety occurs. Honeybees frequently visit these uncommon yellow flowers but how this adaptation affects the life history of this variant remains unexplored. In the present study, floral syndrome and pollination mechanism of yellow flowers were compared with red flowers of B. ceiba populations on Hainan Island, southern China. The results showed that main nectar components of two floral phenotypes are both glucose and fructose dominated, and the volatile chemicals are mainly alkanes, esters, phenols and acids with no significant difference between the two phenotypes. The relative spectral reflectance showed that both birds and honeybees could detect the yellow flowers, suggesting visitation by honeybees can compensate for times when bird visitation is low. Compared with red flowers, yellow flower had a lower degree of dichogamy, which might increase selfing possibilities and provide reproduction assurance when red flowers receive low bird visitation. Interestingly, yellow flowers had a relatively higher degree of herkogamy, i.e. stigmas are much higher than anthers when compared to the red flowers. Greater herkogamy in yellow flowers may reflect an adaptation to decreased dichogamy, avoiding autonomous selfing and interference between female and male organs. Fruit set of the yellow phenotype (1.08 ± 0.56)% was lower than that of red phenotype (3.27 ± 0.93)%, suggesting pollen-limitation in B. ceiba. We propose that yellow flowers, with greater herkogamy but lower dichogamy, promote pollination via attracting diverse pollinators and protect red flowers from disturbance of honeybees.

Key words: bird-pollination, sexual interference, reproductive assurance, dichogamy, herkogamy