Phenotypic plasticity in Petunia hybrida floral traits and its impact on pollinator behavior

Background/Question/Methods

Phenotypic plasticity in response to environmental cues mediates ecological interactions for many plant species. Phenotypic plasticity in response to abiotic factors and enemies has been well-researched, but fewer studies have explored the impact of floral trait plasticity on pollinator behavior.  In particular, plastic responses to abiotic factors like nitrogen level may lead to changes in floral morphological or chemical traits involved in pollinator attraction; such changes may alter predicted patterns of pollinator behavior derived from more static views of floral traits.  We sought to explore this possibility using cultivars of the plant species Petunia hybrida grown under two conditions: traditional fertilizer and traditional fertilizer + nitrogen enrichment.  Petunia is a model species for floral trait and pollinator studies, and the use of hybrid cultivars allows for us to control genetic variation in an ecologically relevant organism.  We measured corolla size, stigma/anther lengths and distance, and floral pigment across six different genotypes in both treatments; we predicted greater corolla size and altered pigment in nitrogen enriched treatment.  Plants were also exposed to pollinators in an experimental array of plants from both treatments; all visitations were recorded.  Data were analyzed using norm of reaction graphs, ANOVA, and correlation coefficient calculations.

Results/Conclusions

As predicted, corolla size was greater in the nitrogen enrichment treatment; additional resources likely led to the generation of additional floral tissue, as generally expected in such studies.  Reaction norms were in the same general direction for all 6 genotypes.  In contrast, stigma and anther traits were relatively unaffected across all lines, suggesting that these traits are more canalized and less responsive to nutrient levels.  Such an outcome is unsurprising given the relative importance of these reproductive characters.  We had predicted that anthocyanin content would increase with increased soil nitrogen, as anthocyanins are produced from phenylalanine as part of the shikimic acid pathway.  However, our results suggested that anthocyanin levels do not always vary among treatments; generally, deep purple and red genotypes displayed greater plasticity for anthocyanin content than pale or white genotypes.  This suggests that amount of nitrogen therefore may be a limiting factor in anthocyanin production for heavily pigmented flowers.  Observation of pollinators resulted in slightly greater visitation to flowers from the nitrogen enrichment treatment, suggesting that plasticity in floral traits to nutrient levels can impact pollinator response.  Future studies will seek to determine if plasticity in floral traits alters pollinator responses in natural systems.