The Downward Floral Orientation in Polygonatum cyrtonema Enhances Pollination Efficiency and Reproductive Fitness

Floral orientation as a key floral trait for understanding how plants integrate biotic and abiotic selective pressures. Polygonatum cyrtonema is an economically significant medicinal species. The adaptive significance of its downward floral orientation remains poorly understood. We conducted manipulation experiments to explore the visiting time per flower, visitation rate and the pollination efficiency of bumble bees and honey bees between downward flowers and artificially reoriented upward flowers. We also assessed pollen viability and stigma receptivity under both sunlight and rainwater exposure in the two orientations. Furthermore, we quantified reproductive fitness components to compare the fruit set, seed set, and the qualities of fruit/seed between two orientation flowers. Our results showed that the downward orientation enhances reproductive success through integrated biotic and abiotic factors. Downward flowers attracted more effective bumble bee pollinators, resulting in higher visitation rates, longer visiting time, and higher pollen transfer efficiency compared to upward flowers. While the upward floral orientation increased the visitation of ineffective visitors, such as honey bees and other syrphid flies. Simultaneously, the downward orientation flowers provided protection from solar radiation and rainwater, maintaining higher pollen viability, and stigma receptivity while reducing pollen loss. Although the fruit set per plant showed no significant difference between orientations, downward flowers developed significantly larger fruits with greater fresh mass and fruit size. Both the seed number and the seed set per fruit were significantly higher in downward flowers than upward ones. These results collectively support the pollinator attraction, pollinator filtering and abiotic protection hypothesis, demonstrating that the downward orientation in P. cyrtonema is shaped by both biotic and abiotic selection.