Stylar curvature and its influence on the reproductive assurance in Passiflora foetida

Reproductive assurance represents a key adaptive strategy in flowering plants, and many floral traits have evolved or been modified to ensure successful reproduction under environmental uncertainty. In Passiflora foetida L., styles exhibit varying degrees of curvature, giving rise to three distinct (fully curved, semi-curved, and erect) flower types during anthesis, each characterized by a different potential to ensure reproductive success. In this study, for the first time, the relationships among stylar curvature, pollen deposition, pollinator visitation, fruit set, and seed set were quantified under natural and controlled pollination treatments. The results showed that fully curved styles received the highest stigmatic pollen loads and exhibited the greatest fruit and seed set. Semi-curved flowers displayed intermediate reproductive success, whereas erect flowers recorded the lowest values across all reproductive parameters. The major pollinators, carpenter bees (Xylocopa spp.) and the giant honey bee (Apis dorsata), differed in visitation rates and single-visit effectiveness. However, pollination success depended strongly on stylar position. Stylar position played a decisive role in determining whether these visits resulted in successful fertilization. Notably, only fully curved flowers were capable of setting fruit through autonomous self-pollination, highlighting that stylar curvature serves as a mechanism of reproductive assurance under conditions of pollinator scarcity or unreliability.