Petunia floral trait plasticity in response to soil nitrogen content and subsequent impacts on insect visitation

- Floral trait plasticity due to abiotic factors such as soil nitrogen is likely.
- However, little research has explored the impact of such plasticity on pollination.
- We find plastic response to nitrogen for floral size, display, and eugenol emission.
- Moreover, such plasticity alters insect visitation patterns in Petunia hybrida.
- This suggests floral trait plasticity may play a role in plant reproductive success.

Plastic changes in floral trait phenotypes in response to biotic factors (i.e., herbivory and pollination) are known to impact pollinator behavior. In contrast, research on abiotic factors suggests substantial plasticity in floral traits, but has rarely explored the implications that such changes have on the behavior of putative pollinators. We explored the possibility of floral trait plasticity in Petunia hybrida Vilm. genotypes in response to soil nitrogen concentration, and the potential impact that such plasticity might have on insect visitation. Given the role of nitrogen in plant growth and biochemistry, we predicted increases in floral scent, pigmentation, and floral size with increased nitrogen. We further predicted that insect visitors would respond to this phenotypic plasticity by altering behavior patterns. Our results suggested little plasticity in floral color and floral scent in response to nitrogen. We found no changes in anthocyanin concentration per unit of floral tissue under increased nitrogen conditions. Of nine benzenoid/phenylpropanoid volatile compounds examined, only one (eugenol) increased per unit floral tissue with increasing nitrogen concentration. In contrast, floral size followed the predicted pattern, with clear increases in both corolla size and display size with increasing nitrogen. All traits varied among genotypes, indicating genotype-specific trait expression. Field-based choice experiments using genotypes from control, low, and high nitrogen treatments suggested support for our behavioral hypothesis: insects visited plants with increased soil nitrogen at a significantly higher rate than those without additional nitrogen. This pattern suggests that putative pollinators are likely responding to plastic changes in display size, corolla size, and/or eugenol emission. We also found a significant difference in insect visitation between genotypes, as predicted by floral phenotype differences found for these true-breeding lines, and a strong interaction between genotype and treatment, suggesting that differences in genotype responses to nitrogen treatments result in differential insect behavior patterns.