Stress And Defense Responses In Plant Secondary Metabolites: Damage Of Cucumber Plants

Some plants are able to change their chemical composition to contain plant secondary metabolites (PSM) and increases their fitness against herbivory. This chemical change occurs when the plant feels threatened and need to adapt to its environment. It was hypothesized that the longer the plant is threatened, the greater amount of PSMs will be produced. This is because, in nature the plants need to increase their fitness based on its environment. In an experiment to test this hypothesis, damage was inflicted on cucumber plants by bollworms for different lengths of time. Then, new bollworms were given several days to eat the plants and the surface area eaten was measured. There was no significant difference between the groups.

Introduction

Most plants are immobile and therefore cannot run away from predators, fight back, or defend themselves in a physical manner. Because of this, many plants are able to change their biochemical composition in an attempt to defend themselves against predators, specifically herbivores. Many plants produce PSMs in defense, since they can be viewed as toxic to some herbivores. There are several different types of PSMs that can be produced depending on the stressors presented to the plant and the plants energy or ATP availability. Many different factors can induce PSM production. These include: temperature, precipitation, canopy, herbivory and much more. These factors can cause fluctuations in PSM production. Production of PSMs in these plants comes at a very high metabolic cost. For example, if a plant was dehydrated and wilting, it will focus its energy on staying alive as opposed to producing PSMs. There are not only costs to make these PSMs, but there are costs associated with their storage within the plant.

A large factor in PSM production is the ability to signal nearby plants. Studies have suggested that PSMs can be synthesized due to the release of chemical signals from neighboring plants, even if there is no direct harm to that plant. In an attempt to test the variability in PSM production in cucumber plants (Cucumis sativa), damage was induced by bollworms (Helicoverpa armigera) on the plants for different amounts of time. The experiment took place in a greenhouse, so the plants were protected from precipitation, but not temperature. It was believed that the more damage induced on a plant, the less the bullworms would eat, signifying more PSMs produced. Gaining additional information on the amounts and reasons behind PSM production can help give insight on behavioral changes and aspects of plant-herbivory interactions.

Materials and Methods

A pilot study was used to determine the number of days allowed for the bollworms to induce damage. The experiment took place in a greenhouse. In the first part of the experiment, the induce damage stage, three bollworms were placed on a single leaf of one cucumber plant and left for either 1, 2, 3, or 4 days to eat the plant. The eating of the leaves was inducing damage and saliva onto the plant. The bollworms were isolated onto a single leaf using nylon bags that were tied off at the stem of the leaf. There were 5 replicated for each treatment. The plants were arranged in rows approximately 10cm apart from each other. After the damage was induced, the bollworms were removed from the plants. In the second part of the experiment, pictures were taken of a single leaf on each plant. The leaf used for this part was a different leaf on the same plant as part one. Two new bollworms, that were place in a 25-degree Celsius room to ensure they would grow and eat a measurable amount, were placed on the new leaf of every plant in every treatment and secured with a nylon bag. The plants were then blocked by randomizing the order they were placed in to ensure no environment factors were placed on specific plants. The worms were left for five days to consume the leaves. At the end of the five days, the bags and bollworms were removed, and another picture was taken of the leaf for comparison to the first photo. To calculate the difference in surface area, the computer software Image J was used. This software was able to measure the surface area of the leaves before and after and then the difference was calculated. The difference in surface area was the dependent variable and measured in centimeters squared, while the independent variables was number of days damage was induced. Since the damage and saliva induced is a single factor, a one-way ANOVA was used to analyze the data and determine if there is a significant difference between the groups. The program SPSS Statistics (V24) was used for the analysis.

Results

The results show that there is no statistical significance between any of the different damage amounts. The mean values of the area consumed for 0 days of damage to 4 days was 38. 97, 13. 43, 29. 56, 3. 48, and 26. 82 respectively. The standard error for each treatment was 14. 67, 4. 87, 11. 37, 0. 69, and 14. 02 respectively. All of the data points are independent due to the design of the experiment in using different plants for each data point. Independence is an important aspect of running a one-way ANOVA since it is a required assumption. When testing for normality and homogeneity, the data resulted to be normal but not homogeneous.

In an attempt to transform the data to result in a homogeneous set, the natural log was applied to the values for the area consumed. The natural log is used as a strong transformation to adjust the values to become more normal and homogenous. The transformation increased the Levene Statistic test p-value from 0. 000 to 0. 022. Although in order for the result to be significant, the p-value must be greater than 0. 050, a one-way AVOVA can still be performed. The one-way ANOVA was run using the transformed data set and resulted in an F-value of 0. 364 with a significance level at 0. 05, so we accept the null hypothesis. This concludes that there is no statistical difference in area consumed between any of the 5 treatments of damage. With these results, it can be inferred that due to the lack of significant, the amount of PSMs produced in each plant was not significantly different either.

Discussion

In conclusion, due to the design of our experiment, it was found that there is no significant difference in PSM production depending on the amount of damage done over time. In a previous study, it was found that undamaged plants have the ability to react to chemical signals released by nearby plants and begin production of PSMs. Within our experiment, the plants we only placed 10 centimeters apart from each other. This may have allowed the damaged plants to chemically signal the others, resulting in similar amounts of PSMs produced throughout. In order to improve the experiment in further research, it would be ideal to place the plants in distant location from each other, but still remaining under the same conditions, such as temperature and precipitation. Studies have shown that an increase in temperature has be directly related to an increase in secondary metabolites in some plants.

Over the course of the experiment, there was a large spike in temperature and the plants had wilted until more water was provided. This change in temperature was applicable to all of the plants and therefore would affect the PSM production. In the second half of the experiment, the bollworms were eating the leaves while the plants were trying to survive. Genetics have shown to play a huge role in plant secondary metabolite production. Depending on an individual plant’s genes, it may or may not be able to produce certain types or amounts of PSMs.