Characteristics of Floral Volatiles and Their Effects on Attracting Pollinating Insects in Three Bidens Species with Sympatric Distribution

The transition from outcrossing to self-pollination is an evolutionary process in angiosperms. However, the changes in floral volatile composition during this process and their impacts on the behavior of pollinators are poorly understood. Therefore, this study investigated the potential differences in the floral volatile profiles and pollinator attraction capabilities of three invasive <i>Bidens</i> species. The results indicated that <i>Bidens pilosa</i> var. <i>radiata</i> (BH), which serves as a transitional species between facultative outcrossing and obligate outcrossing attracts a greater diversity and abundance of pollinators such as <i>Apis cerana</i> compared to the more self-compatible <i>Bidens frondosa</i> (DL) and <i>Bidens pilosa</i> var. <i>pilosa</i> (SY). Furthermore, a total of 37, 33, and 34 Volatile Organic Compounds (VOCs) were identified in the floral volatiles of BH, DL, and SY, respectively, with no discernible trend of decreased number of floral VOCs owing to increased self-pollination ability. Moreover, eleven significantly different compounds in the floral volatiles of the three <i>Bidens</i> species were obtained. Among these (E)-β-Ocimene (18.31 ± 1.10%) and (Z)-β-Ocimene (33.93 ± 3.49%) in the floral volatiles of BH (52.24 ± 4.59%) was significantly higher than that of DL (1.72 ± 0.50%) and SY (0.32 ± 0.19%). Additionally, Y-tube olfactometer behavioral assays indicated that (E)- and (Z)-β-Ocimene significantly attracted <i>A</i>. <i>cerana.</i> These findings suggested that (E)- and (Z)-β-Ocimene contribute to the attractiveness of BH to local pollinators. Furthermore, it can be inferred that within <i>Bidens</i>, stronger self-pollination ability reduces the relative content of VOCs-such as (E)- and (Z)-β-Ocimene-used to attract generalist pollinators.