How drought stress affects the pollinator-mediated selection of floral functional traits?

Background/Question/Methods

In animal pollinated plants, fitness is influenced by functional plant floral traits that act as advertisements and rewards. Functional traits often vary among individuals in the same population due to heritable differences among individuals or phenotypic plasticity in response to the local environment. Moreover, abiotic heterogeneity can cause variation among individuals in the capacity to invest in sexual reproduction. In environments where resources that are essential to maintain levels of pollinator advertisements and rewards are pronounced, plants invest greater resources in floral displays to attract pollinators. These increased attraction rates ensure increased rates of pollen transfer. One such resource is water. Meanwhile, studies addressing the effect of water availability on plant resource allocation for floral advertisement and reward are frequent, however, their evolutionary consequences are less understood. Therefore, we designed a factorial experiment aiming to assess whether drought stress alters plant signal and reward traits and whether the pollinator-mediated selection of those traits changes in these abiotic contexts.

Results/Conclusions

In a controlled environment, we grew 400 individuals of Wisconsin fast plant, Brassica rapa in four blocks subjecting each block to either induced drought or well-watered conditions coupled with one from two levels of pollinator access (open/hand pollinated). We measured a suite of plant vegetative and floral traits including floral nectar reward quality and quantity in each individual. Preliminary analysis shows, generally vegetative and floral traits were lower in plants exposed to drought conditions, except rewards (nectar volumes and sugar concentration). Selection analyses showed pollinator-mediated selection also differed between the two conditions. In drought, there was selection to increase size traits (plant height, diameter), efficiency traits (petal length, stamen-stigma offset), rewards (nectar sugar concentration) while the selection to decrease petal length (size) and sugar concentration (rewards) in wet conditions. Our results show that drought stress can not only have significant effects on plant traits and reproductive success, but it can also change the strength and direction of pollinator-mediated selection. Thus, we suggest that there should be more detailed studies to understand how plant functional traits are affected by variation in water availability during their life histories and the overall effect on the evolution of floral traits.