The flowers of the genus Aristolochia (Aristolochiaceae) are known to trap specific groups of dipterans for pollination. Their flowers attract and trap pollinators at the female stage, while they release pollinators at the following male stage. Previous studies on the trap mechanism of Aristolochia in the subgenus Aristolochia have revealed that the presence or absence of trichomes on the adaxial surface of the flower mediates the trap/release functions. However, these trichomes were notably absent in the flowers of Aristolochia liukiuensis and A. shimadae in the subgenus Siphisia. In this presentation, we report the pollination system of A. liukiuensis and A. shimadae.
First, we tested if their flowers trap pollinators. Our bioassays showed that pollinators cannot climb up the inner surface of the female stage corolla tube but can climb up and leave the male stage flower. This suggests that the flowers of the two species trap pollinators in the female stage and release them in the male stage. Furthermore, our experiment revealed a significant correlation between the number of trapped pollinators and the changes of the height of the downward epidermal papillate cells.
We also examined volatile compounds emitted from the flowers and compounds contained in the secretion from the inner surface of the perianth. We found that the volatile compounds mainly consist of several terpenes, some of which are also detected in the secretion of perianth. The amount and composition of flower volatile are similar between the female and the male stages. In addition, we measured the amount of glucose in flower nectar secreted at the bottom of the perianth. both the female and the male stage flowers produce only a comparatively small amount of glucose, while the amount of the former is significantly larger.
This study showed that the two species also trap pollinators but using different mechanisms compared to species in the subgenus Aristolochia. The changes in the surface microstructure, comprised of papillate epidermal cells, may play a primary role in trapping and releasing pollinators. On the other hand, considering the similar chemical composition between the sexual stages of the flowers and the low amount of the glucose production throughout flowering, flower volatiles and nectar may play a relatively minor role in attracting and releasing pollinators.