Sophie Meyers
Eathan Gentry
November 6, 2018
Honors POB Lab
Herbivory in Wisconsin Fast Plants
Introduction
Herbivory is the consumption of plants by herbivores, or animals that have adapted to eat plants. This is a common phenomenon in nature as there are countless herbivores in each ecosystem (Stevens, Allison). Herbivores are drawn to certain plants due to their Nitrogen content, which is a vital part in not only metabolic processes, but is vital to life itself (Mattson Jr, William J. 1980 ). It has been suggested that herbivores are not lacking in resources for nutrients, they are lacking in resources for quality nutrients. Thus, there is the idea that because insect herbivores are regulated by this restricted access, it is highly unlikely that they will become abundant enough to largely affect plants (Crawley, Michael J. 1989).
In past studies and observations, there is evidence that in nature, plants build up a tolerance to herbivory that allows them to continue to thrive. There are noticeable differences between the effects of mammalian herbivores and those that are insects. There are two clear responses that plants may have when presented with the problem of herbivory: resistance and tolerance. Resistance is any characteristic of a plant that reduces the “performance or performance” of herbivores. These traits can include secondary chemical responses, or the evolution of the plant itself. Tolerance is seen as the plant’s ability to maintain its fitness, despite the damage caused by herbivory (Ashton, Isabel W., and Manuel T. Lerdau 2007). One study on the effects of cryptic mollusk herbivory on a perennial herb displayed the negative effects of herbivory to be: “increased probability of dying or staying dormant, poorer growth, and a lower probability of flowering” (Johan 2003).
The Wisconsin Fast Plant, or Brassica rapa, is used in experiments due to their rapid growth rates and low maintenance needs. Infact, brassica rapa were created at the University of Wisconsin as a research tool. They have been proven to be extremely useful in academic experiments due to their five week life cycles (Stephen P. Tomkins & Paul H. Williams 1990). In this experiment, we used a control group of twelve Wisconsin Fast Plants, and an experimental group of the same. The act of herbivory was simulated by making clippings in the leaves of the experimental plants. The null hypothesis is that herbivory would have no effect on the fast plants. For our alternative hypothesis, e predicted that the control plants would grow the tallest, and have more numerous and large leaves than the experimental group due to the fact that they would not be experiencing tissue damage.
Methods
Two clear bins, two white lids, and four black planters were placed in a vinegar bath and then rinsed with tap water. Soil was mixed and each pod in the planting tray, as well as the clear bin and lids were labelled with tape and a black sharpie. For example, the control pods were labeled C1,C2,… and the experimental pods were E1, E2 and so on. The bins were labeled as experimental or control and with our names. Three osmocote pellets for each pod (72 total) and 1 diamond felt for each pod were counted out. The pods were filled ¾ of the way with soil and the diamond felt was placed in an upright position in the center of each pod. The osmocote pellets were placed on top of the soil, and then the pod was filled the rest of the way with soil. Seeds were put into a weigh boat, and we placed three seeds in each pod and patted them into the soil. We soaked two felts for each bin, filled the bins with water, and then covered the lids with the felts. Two anti-algal squares were put in each bin. We wetted the top of the soil pods to allow them to seal with the diamond felts. The bins were then taken to the light racks in the prep room and placed on top of lids until the plant trays were 10 centimeters from the lights.
We watered the plants weekly and performed our experiment. Forceps were used to replicate the effects of herbivory on the leaves of the experimental group. Each week, we used the forceps to poke small holes in each experimental leaf. We measured the height of all plants in centimeters, the number of leaves per plant, and the length of the longest leaf in millimeters of each plant weekly for five weeks. The heights, leaf numbers, and leaf lengths were averaged and compared using a t-test.
Results
Figure 1. Mean number of leaves of plants subjected to herbivory (experimental, blue, N=12) over five weeks compared to plants not subjected to herbivory (control, orange, N=12).
There was a significant difference between the average total number of leaves between the experimental and control groups during the fourth and fifth weeks. Experimental plants grew or maintained more leaves than the control plants over the course of the experiment.
Figure 2. Mean plant height of plants subjected to herbivory (experimental, blue, N=12) over five weeks compared to mean plant height of plants not subjected to herbivory (control, orange, N=12).
The mean change in height between the experimental and control groups was insignificant throughout the duration of the study.
Figure 3. Mean longest leaf length of plants subjected to herbivory (experimental, blue, N=12) over five weeks compared to plants not subjected to herbivory (control, orange, N=12).
The average length of the longest leaf of each experimental plant in comparison to the control plants proved to be insignificant all five weeks.
Control Mean
Experimental Mean
t-statistic
t-critical
Significant?
Week 1
3.75
3.5
1.256
2.074
No
Week 2
6
6.25
.820
2.074
No
Week 3
6.667
7.417
1.278
2.074
No
Week 4
7.167
10.333
4.221
2.074
Yes
Week 5
4.833
8.333
3.398
2.074
Yes
Table 1. Results of t-test on mean total number of leaves per plant per week. Alpha level was set to 0.05, df=22, and N=12
Week
Control Mean
Experimental Mean
t-statistic
t-critical
Significant?
1
4.12
4.04
0.14
2.074
No
2
14.75
16.84
1.04
2.074
No
3
26.63
24.83
1.00
2.074
No
4
28.08
28.67
0.26
2.074
No
5
28.04
29.92
0.608
2.074
No
Table 2. Results of t-tests on the mean total change of plant height each week. Alpha level was set at 0.05, df=22, and N=12.
Week
Control Mean
Experimental Mean
t-statistic
t-critical
Significant?
1
10.83
11.04
0.23
2.074
No
2
45
39.42
1.79
2.074
No
3
47.26
44.08
0.71
2.074
No
4
46.42
45.5
0.2
2.074
No
5
30.25
40.25
1.51
2.074
No
Table 3. Results of t-tests on the mean length of the longest leaf per plant per week. Alpha level was set at 0.05, df=22, and N=12.
Discussion
The experimental plants subjected to herbivory showed no significant difference in the mean number of leaves (Figure 1) when compared to the mean number of leaves of the control group until the last two weeks. The mean plant height also showed no significant change between the two groups (Figure 2). Additionally, the mean length of the longest leaf showed no significant difference from the control group (Figure 3). This supports my null hypothesis that the effects of herbivory would not cause significant difference between the control and experimental plants. This supports the hypothesis that plants are able to develop a tolerance for herbivory and continue to survive.
The results of our study support the idea that the degree of impact an herbivore may have on a plant depends on nutrient availability, timing, and plant association. During a 1980s study, researchers found little to no difference between the plants subjected to herbivory and those that were not. More recent findings have provided evidence that plants also have a tendency either to overcompensate or undercompensate as a response to herbivory. It has been shown that as competition levels increase, the amount of compensation decreases (Joyce Maschinski and Thomas G. Whitham 1989).
In conclusion, our study found that herbivory has no significant effect on the number of leaves, height, or leaf size of Brassica rapa. Had we tested a more extreme level of herbivory, mimicking that of a large mammal, our results may have been different. While herbivory will persist to be a significant issue, especially in dollar amount for crops, it is safe to say that the effects are minimal on other species.
References
Ashton, Isabel W., and Manuel T. Lerdau. “Tolerance to Herbivory, and Not Resistance, May Explain Differential Success of Invasive, Naturalized, and Native North American Temperate Vines.” Diversity and Distributions, vol. 14, no. 2, 2007, pp. 169–178., doi:10.1111/j.1472-4642.2007.00425.x.
Crawley, Michael J. "Insect herbivores and plant population dynamics." Annual review of entomology 34.1 (1989): 531-562.
Johan Ehrlén, "Fitness Components versus Total Demographic Effects: Evaluating Herbivore Impacts on a Perennial Herb.," The American Naturalist 162, no. 6 (December 2003): 796-810.
Joyce Maschinski and Thomas G. Whitham, "The Continuum of Plant Responses to Herbivory: The Influence of Plant Association, Nutrient Availability, and Timing," The American Naturalist 134, no. 1 (Jul., 1989): 1-19.
Mattson Jr, William J. "Herbivory in relation to plant nitrogen content." Annual review of ecology and systematics 11.1 (1980): 119-161.
Stephen P. Tomkins & Paul H. Williams (1990) Fast plants for finer science—an introduction to the biology of rapid-cycling Brassica campestris (rapa) L., Journal of Biological Education, 24:4, 239-250, DOI: 10.1080/00219266.1990.9655152
Stevens, Allison. “Predation, Herbivory, and Parasitism.” Nature News, Nature Publishing Group, 201AD, www.nature.com/scitable/knowledge/library/predation-herbivory-and-parasitism-13261134.