Floral biology of Bidens pilosa var. radiata, an invasive plant in Taiwan

Invasion of exotic plants is threatening global biodiversity. Bidens pilosa L. var. radiata, a member of Asteraceae, has successfully invaded Taiwan after being introduced to the island. Knowledge of its reproduction is necessary in developing effective ways to control the spread of this plant. The objective of this study was to investigate the floral biology including: (I) floral structure, (2) the process of secondary pollen presentation, and (3) pollen/ovule ratio (P/O ratio) of this invasive plant. Plants were grown in pots from seeds and placed at the experimental farm of the National Taiwan University. Besides describing floral morphology, number of disk florets per capitulum, number of pollen grains per floret, and P/O ratio were quantified. Furthermore, we defined six floral stages according to anther tube and style morphology to investigate the process of pollen presentation and assessed pollen viability and stigma receptivity. Significant differences were found in the number of disk florets per capitulum (range 19-61, average 44.1) and the number of pollen grains in each floret (range 6,556-11,378) among individuals. Secondary pollen presentation was observed as the growing style brushes and pumps pollen grains out of the anther tube. Incomplete protandry was found from the observation of the flowering process and the tests of pollen viability and stigmatic receptivity. High P/O ratio (means +/- se. : 8,827 +/- 464) was measured which suggested that B. pilosa var. radiata might be obligately xenogamous.