Floral Color Change Mediated by Pollen Input–Output and Its Manipulation of Pollinator Foraging Behavior in Heterostylous Plant <i>Arnebia guttata</i> (Boraginaceae)

The phenomenon of floral color change (FCC) in angiosperms is frequently associated with pollination events. However, research on triggers, functions, and ecological effects of FCC still remains confined to phenomenological speculation or model-based validation due to the overlap between natural senescence and multiple ecological roles, as well as the difficulties in quantitative analysis of pollen transfer. <i>Arnebia guttata,</i> a typical distylous species characterized by five gradually fading purple spots on its corolla, serves as an excellent paradigmatic system for studying the triggers and adaptability of FCC, owing to its heteromorphic pollen-stigma morphology. Therefore, we investigated the flower syndrome, dynamic changes of spots under controlled pollination, pollinator visiting behavior and pollen deposition mediated by artificial spots. The results showed that emasculating and bagging, as well as inter- or intra-morph pollination, can accelerate or delay the time of spots fading. When artificial spots were added to some spotless older flowers to mimic natural ones, these flowers, whether with natural or artificial spots, attracted significantly more pollinators than those without spots. Additionally, the number of incompatible pollen grains on the stigmas of both L-morph and S-morph flowers increased significantly, while the compatible pollen grain counts remained unchanged. In conclusion, as a typical distylous plant with weak self-incompatibility in the S-morph, FCC in <i>A. guttata</i>, which is influenced by pollen input-output dynamics, plays a critical role in reducing self-pollen deposition. For the first time, the study systematically evaluated the induction and ecological function of FCC through employing the manual control experiment, quantitative analysis of stigma pollen deposition, and investigation of pollinator visitation behavior in a distylous population.