Pollination and Pollen Tube Behavior in Apiaceae: Testing Hypotheses About Syncarpy and Sex Expression

Danielle Laberge, Vassar College ’10 and Prof. Mark Schlessman

Many species of Apiaceae are andromonoecious, with each plant bearing staminate and perfect flowers. The perfect flowers are syncarpous, with the two ovaries fused but the styles separate. The perfect flowers are also dichogomous: stamens dehisce and stigmas become receptive at different times. Variation in sex expression among the sequentially blooming orders of umbels is correlated with the form of dichogamy. We tested two hypotheses relating to these characteristic features of Apiaceae.

We predicted that one-stigma pollinations with two or more pollen grains would result in pollen tubes growing into both ovules. We examined pollen tubes in naturally pollinated Zizia aurea and hand-pollinated Angelica atropurpurea with fluorescent microscopy. Pollen tubes frequently crossed over from one style to the opposite ovule, but in some flowers all of the pollen tubes crossed, fertilizing the opposite ovule but not the closer one. This unexpected result indicates that some other factor, perhaps a difference in the amount of pollen tube attracting hormone released by the ovules, must have influenced the direction of pollen tube growth.

In the case of floral protogyny, the dichogamy hypothesis for expression of andromonoecy states that because the stigmas of early-blooming perfect flowers would become receptive when few other flowers were releasing pollen, the first-blooming (terminal) umbels should have few perfect flowers, and the ratio of perfect to staminate flowers should increase in later-blooming umbels. We tested two predictions of this hypothesis using Zizia aurea. While the total number of flowers did not vary significantly among sequentially blooming orders of umbels, both the percentage of perfect flowers and the proportion of successfully pollinated flowers were significantly lower in first-blooming umbels.

Our results provide the first empirical test of the prediction that floral dichogamy causes temporal variation in successful pollination. They also raise intriguing questions about the supposed advantages of syncarpy.