Andrew Scott- Ecological Theory and Research

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Question

What, if any, are the fitness consequences for inducing defenses against herbivory in Brassica rapa? Does the species exhibit eavesdropping? If so is there a benefit eavesdropping? Is there a fitness cost to a "false alarm"?

Background:

Induced defenses against both herbivory and pathogen attack in the form of increased "volatile organic compounds" are well documented, but the exact costs of these responses are not rigorously documented. In particular I'm curious as to the nature of how plant to plant "communication" plays into the costs of induced defense. It is as of yet unclear as to what percentage of plant species world wide exhibit signaling behaviors, and thus warrants inquiry. Plant to plant signaling in case of herbivore attack has been demonstrated in closely related Brassica oleracea (cabbage), though never explicate tested for in B. rapa


This may be a little random, but do brassica plants, or do plants in general, have a damage threshold- an amount of damage they can withstand until it becomes super costly? Plants have to "assume" that they will encounter a degree of damage within their lifetime. Can they prepare for it ahead of time? Emikucki

Hypotheses

1. Plants that have previously had defenses induce via simulated herbivory will show increased resistance to herbivory as opposed to plants who have had prior herbivore exposure. 2. Based on the literature it seems reasonable to expect Brassica rapa to exhibit some plant to plant signaling and induced defenses in specimens receiving the over-air signal. 3. If Brassica rapa does exhibit plant to plant signaling I expect the plants who receive the signal to exhibit increased resistance to herbivory. 4. If B. rapa does respond to plant to plant over air signals I expect there to be decreased fitness in the responding plants due to the cost of producing additional chemical defenses.

Methods:

As my main concern is overall fitness I will most likely utilize Wisconsin fast plant's Brassica rapa cultivator which will allow me to look at overall seed production well within the time frame of the rest of term. I imagine my experimental design as follows:

Group 1 : Baseline controls consisting of one pure control subset and one subset on which herbivory has been simulated. This group exists as an intuitive control group : I expect the non-manipulated plants to have the highest seed production and the manipulated subset to produce fewer seeds for both the loss of resources from simulated herbivory and potentially from inducing defenses.

Group 2 The first experimental group: Consisting of plants on which herbivory has been simulated and plants that have not been exposed to simulated herbivory. I will then introduce liver herbivores (hopefully in a controlled manner) to both subsets and measure both seed output and foliage damage. If there are indeed significant induced defenses in this species I would expect to see better resistance/enhanced fitness in the subset of plants who were exposed to simulated herbivory before the introduction of the herbivores

Group 3 "Eavesdropping" and air based chemical communication group : "Eavesdropping" has (so far as I can tell) never been observed in this species, and by the same token, has never been explicate tested for in this species. To test for inter-plant defense induction I would simulate herbivory in one subset of this group in close proximity to other specimens and then perform the same herbivore introduction on these close-proximity plants..

Group 4 The actual costs of responding to the "warning shot". This group would be an attempt to quantify the actual costs of producing HIVCs as a secondary responder. Do the plants that have been "warned" of herbivores suffer a direct cost to fitness by responding and inducing defenses? The design for this group would be similar to group 3 - to measure the seed output of plants that were in close proximity to other specimens on which herbivory was simulated as compared to the pure control group.

Groups 3 and 4 may well be a complete wash, but the questions behind the design are, in my opinion, fairly compelling.

References

Ecology and Evolution of induced defense against herbivores

2.Engelberth, J., Alborn, H. T., Schmelz, E. A. & Tumlinson, J. H. Airborne Signals Prime Plants Against Insect Herbivore Attack. PNAS 101, 1781–1785 (2004).

3.Rodriguez-Saona, C. R. & Frost, C. J. New evidence for a multi-functional role of herbivore-induced plant volatiles in defense against herbivores. Plant Signal Behav 5, 58–60 (2010).


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