Variation in water and pollinator availability alters floral signals and rewards in the common morning glory (Ipomoea purpurea).

Background/Question/Methods

Changes in the availability of abiotic and biotic factors such as water and pollinators can modify floral traits including floral signals (e.g., floral size) and rewards (e.g., nectar) that mediate plant-pollinator interactions, and in turn alter plant reproductive success. Moreover, limitation of water availability and of pollinator access to flowers have the potential to modify patterns of selection on floral traits, although information is still scarce. Understanding how variation in water and pollinator availability alter both floral signals and rewards is key to predict how plant-pollinator interactions may respond and evolve under the context of global change. In order to address these questions, we experimentally manipulated water and pollinator availability in the common morning glory, Ipomoea purpurea, and measured floral traits and female fitness. Five floral traits including floral size traits (corolla length and width), plant size (diameter of the main stem), nectar volume and nectar concentration were measured in an experimental garden. At the end of the flowering season, seed set was estimated as a measure of female fitness.

Results/Conclusions

Water and pollinator limitation decreased mean floral size and nectar volume, while nectar concentration and plant size did not show significant variation among treatments. Plants under water deficit showed the lowest seed set values, and plants from the pollinator limitation treatment also produced lower seed set than control plants. In addition, water deficit altered patterns of selection on floral traits, with selection for larger flowers and lower nectar volumes in water-stressed plants, in contrast with selection for flowers with higher nectar volumes in control plants. Our results show that variation in either biotic and abiotic factors can change floral signals and rewards that may impact plant-pollinator interactions and ultimately plant reproduction. Moreover, we found evidence of effects of water deficit on selection on floral traits, which may alter the evolution of plant species under drier conditions. In our experimental population, water deficit had a larger effect on selection than pollinator limitation, however the influence of these factors may differ in wild populations. Overall, our study highlights the importance of investigating the response of multiple floral traits to changes in biotic and abiotic resources in order to understand the effects of global change on plant pollination.