The dark side of plant defense: Comparing nocturnal and diurnal defensive induction in big sagebrush

Background/Question/Methods

Plants exposed to herbivory induce defensive responses that can both decrease their own palatability and attract natural enemies of herbivores. Inducible defensive traits allow plants to efficiently manage their limited resources and respond to current herbivore pressure. Defensive responses to herbivore or simulated attack have been documented in a growing number of plant species, including the focal organism of this study: big sagebrush (Artemisia tridentata). My primary research question was: can sagebrush induce defensive traits with equal effectiveness at different times of day?

In order to answer this question I systematically damaged 200 sagebrush plants within Tahoe National Forest in July of 2012. Plants were randomly assigned to one of five treatments: Day Damage, Night Damage, Morning Damage, All Time-Step Damage, No Damage. The distal third of 100 leaves were removed from an emitter branch on each plant during the designated time periods. This simulated an intense but realistic pulse of herbivore activity during the experimental time window. I then counted undamaged and damaged leaves on a neighbor branch in October 2012 as an assessment of season-long palatability to herbivores.

Results/Conclusions

Plants receiving simulated herbivory during the night had a higher proportion of damaged leaves than plants that received simulated herbivory during the day. Diurnal simulated herbivory was more effective in inducing defenses, and these plants received a lower proportion of damaged leaves over the course of the season. Damage received during the night or morning did not prevent later herbivore attack on neighbor branches compared to the control treatments, while damage received during the day did show improved herbivore resistance. These results demonstrate that when testing plant defensive responses, scientists should not only consider the identity of the herbivore but also the timing of the herbivore attack.