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The Effects of Specific pH Levels on the Growth of Brassica rapa (Wisconsin Fast Plants)

Abstract

The purpose of this experiment was to test the effects of various pH levels on the growth of Brassica rapa (Wisconsin fast plants). It was hypothesized that the B. rapa plants will experience negative growth (in terms of height, number of leaves, and number of flowers) with the pH levels above 8 and the pH level of 2. For this experiment, 5 groups of 6 trials were used. The B. rapa seeds were planted and allowed to grow for about two weeks. After this period, the heights, leaf count, and flower count of the plants were taken, and the pH solutions were added. In the first group all the plants were watered with a solution of pH 2 and water in a ratio of 1:4. The second group was watered with a mixture using pH 8. The third group was watered with a mixture of pH 11, and the fourth group was watered with a pH 13 solution. The fifth, the control, was watered with no pH added. After a week the heights, leaf count and flower counts were taken again. These results were compared with the initial measurements. Upon analysis, it was seen that, in the B. rapa plants, the control group had the most growth followed by the pH 8 group, the pH 11 group, the pH 2 group, and finally by the pH 13 group. This trend held true for flower growth and leaf growth. These results seem to support the hypothesis that the higher pH levels and the pH level 2 would negatively effect the growth of the plants.

Introduction

The purpose of this experiment is to determine if substances that raise the pH of water, will have a negative effect on Brassica rapa plant growth. It is known that too much acidity will kill plants. Regular precipitation has a pH of about five. This experiment concentrates on the higher pH levels, the bases. The reason for this, is to see the effect that alkalinity has on plant growth. By doing this, it can be determined whether or not materials that contain bases will negatively affect plants, when these materials leech or drain into water.

This experiment deals with Brassica rapa, commonly known as Wisconsin fast plants. B. rapa is in the class of Angiospermae, meaning it produces flowers. B. rapa is in the family of Brassicaceae, also known as Cruciferae because Brassicas have 4 petals in a cross-like shape. They also contain 4 sepals and 6 stamens (Department of Plant Pathology, 1995).

B. rapa plants are good plants to use in experiments because they have a short life cycle, their height is moderate, and they are easy to breed. Although they are good plants to use, B. rapa plants are effected by many factors of the environment. The most important requirement to growing B. rapa, is sufficient lighting. B. rapa plants grow best under continuous flourescent lighting. To ensure the B. rapa will grow well, it is best to place the plants 5 - 10 centimeters away from the light source. The reason lighting is so important to B. rapa, is the plant uses energy from the light to carry out growth and reproduction. The importance of energy for photosynthesis is even more crucial in B. rapa plants because of their rapid growth. Light also helps the form and color of B. rapa plants.

Another factor affecting growth is temperature. Temperature can affect the timing of the plant’s development. The temperature should be between 72º F to 82º F (22º C to 28º C). For this reason, it is wise to use fluorescent lighting because it doesn’t emit alot of heat. Water also effects the plants growth because B. rapa plants get their inorganic nutrients from the water they pull from the soil. Relative humidity can affect the rate in which the plants take up water. If they take up water too slowly, they can dry out because the rate of transpiration could surpass the rate of water uptake ("Wisconsin Fast Plants", 1995). Another factor effecting B. rapa is competition. Competition has an effect on leaf production, plant size, and flower production (Gurevitch, Taub, Morton, Gomez & Wang, 1996).

B. rapa has an average life cycle of about 35 days. By the 3rd day, the seedling is visible above the soil and it’s color can be clearly seen. By the 8th day, leaves have developed and buds begin growing. By day 12, the stem becomes longer, raising the flowers above the leaves. During this time the leaves and flowers continue to grow. By day 17, the flower structure can be seen as the flower buds begin to open. By day 22, the pods grow while the embryo begins to develop in the seed. Also during this time the flowers begin to lose their petals. By day 35, embryo development in the seed is complete and the pod begins to dry. At this point seeds can be gathered from the pod (Department of Plant Pathology, 1995).

An acid is any substance that yields hydrogen ions when dissolved in water. A pH below 7 is an acid and a pH above 7 is a base (alkaline). Regular precipitation usually has a pH of 5. The affect of acidity on plants significantly depends on the type of soil the plant is grown in. Some soils have the ability to neutralize acids that are introduced to them. Soils with lime are good neutralizers. Acids can cause a shortage of plant nutrients by causing them to leech. It causes aluminum to be more active, which damages roots. Many organisms can’t live in soils with a pH less than 6, and without these organisms plants can die. Acid precipitation can also harm foliage. Some acids can cause leaves to lose their ability to retain water, and they make plants more vulnerable to disease and insects (Grow & Pidwirny, 1996).

Dry deposition is the process in which pollutants come into contact with plants by settling, due to gravitational forces. Acid can reduce plants in number and variety, and production and decomposition can be affected. Changes in pH have also caused the composition of some plants to alter. Acid rain is believed to change nutrient conditions, therefore effecting plants. Depending on factors like location or nutrients, the acid may have no effects. Some soils with higher acidity contain lower levels of nitrates and phosphates (Goldberg, 1985). Acids increase rates of mineral loss in soils, but some soils can buffer acid depending on factors like pH, salinity, moisture level, structure, texture, minerals, and soil permeability (Bittenbender, Latendresse, Martysz & Mood , 1998). Since much information is given about the affects of acids on plants, this experiment will focus more on the higher pH levels.

It is hypothesized that the B. rapa will experience negative growth results (in terms of height, number of leaves, and number of flowers) with the pH levels above 8 and with the pH level 2.

Methods and Materials

To conduct the experiment, first the plant box in which to grow the plants was made. A 1 inch (2.54 cm) hole was cut in the center of a plastic plate. The edges of the plate were trimmed to make a 4-5 inch (10.16-12.7 cm) disk with a center hole. Several 4x14 cm ventilation slots were cut in the top, the upper sides, and in the back of an empty copy paper Xerox box. A 1 in (2.54 cm) diameter hole was cut through the center of the top of the box. Glue was then applied to each inner surface of the box and aluminum foil was pasted in so that it covered the entire surface of the inside of the box. The corners and edges were reinforced with duck tape. The plastic disk was glued to the inside of the top of the box, fitting with the hole in the top. A light fixture spliced with an extension cord was inserted through the hole in the paper plate and through the hole in the top of the box. Aluminum foil was taped over the front of the box from the top front edge to act as a curtain. The edges of the curtain were reinforced with duck tape and nails were taped at the bottom to act as weights. The lid of the box was put on the back for extra support. A 75 watt GE fluorescent circle light was screwed in the socket.

Figure 1. The Plant box with 5 containers representing the different pH levels. One of the disks is pH 2, one is pH 8, one is pH 11, one is pH 13, and one is the control of pH 7.

To setup the place to hold the plants, five 500 ml containers were obtained. One was labeled "pH 2", one was labeled "pH 8", one with "pH 11", one with "pH 13", and one with "pH 7 (control)". One 250 ml container was placed into each of the five 500 ml containers. A circle of pelon was cut to fit the bottom of each 250 ml container and was laid in each. A slit was cut into the bottom of the 250 ml container and an inch wide strip (2.54 cm) of pelon was cut and put through the slit so that it touched the bottom on the 500 ml container. At the bottom of thirty separate 35 mm film containers, a hole was punched. Cotton string was then pulled through the hole to act as a wick. Six film containers were put into each 250 ml container.

Figure 2. The 250 ml containers each with 6 film canisters inside. Each canister represents one trial.

One film canister was labeled "trial 1", one "trial 2", one "trial 3", one "trial 4", one "trial 5", and one "trial 6". This was repeated with the six film canisters in each 250 ml container. Premoistened potting soil was then put into each film container. The soil was not packed. The sheet of pelon was saturated in water with a drop of detergent. It was then put back in the bottom of the 250 ml containers. 280 ml of water with Miracle Gro fertilizer mixed in, according to the directions on the box, was put into the 500 ml container to act as a water source. The 250 ml container was put back inside the 500 ml one. Three Brassica rapa seeds were planted in each film container. The 5 containers with the seeds planted and the water were set inside the plant box. The light was plugged in and the plants were left to grow. Three seeds were planted to assure one would grow, so any extra plants besides one in each film container, were cut once they sprouted.

To make the pH 2 solution, 10 ml of .1 M HCl acid was combined with 90 ml of distilled water. The base that was used was NaOH. To make the pH 8 solution, 10 ml .00001M of the base was added with 90 ml of distilled water. To make the pH 11 solution, 10 ml .01M of the base was added to 90 ml of distilled water, and to make the pH 13 solution, 10 ml 1M of the base was mixed with 90 ml of distilled water. 70 ml of each solution was measured out and put into jars.

After about 2 weeks, the water was poured out of 4 of the five 500 ml containers and 210 ml of water and fertilizer solution was put back into them. To one of the containers, 70 ml of the pH 2 solution were added, to another 70 ml of pH 8 solution was added. To the third, 70 ml of pH 11 solution was added and to the fourth, 70 ml of pH 13 solution was added. The last container was poured out and refilled with 280 ml of regular water and fertilizer solution to act as the control. The height, leaf count, and flower count were taken at that time. Five days later, the effects of the different pH levels were observed. The height of the plants in all the film containers were measured with a ruler. The number of leaves and the number of flowers were also counted as a quantitative measurement. The differences were calculated and the means were found. T-tests were also performed to observe the statistical significance of the data.

Results

The results of the experiment show a clear trend. Tables 1 through 3 show the raw data set of heights taken from the B. rapa plants. Table 1 shows the heights of the plants before the pH solutions were added. Table 2 shows the height of the plants after the pH solutions were added, and Table 3 shows the total growth of the B. rapa. These numbers were obtained by subtracting the before height from the after height. The difference is the number of centimeters the plant grew. Since these values are raw data, they are presented in tables in the Appendix.

Tables 4 through 6 deal with the leaf count of the plants. Table 4 presents the number of leaves before the pH solutions were added for each trial. Table 5 shows the count of leaves after the pH solutions were added, and Table 6 shows the number of leaves gained or lost for each trial. The negative numbers respresent leaves that died or fell off. This data is also raw and can be found in the Appendix.

Tables 7 through 9 present the data for the third set of quantitative measurements. Table 7 presents the data from the number of flowers before the pH solutions were added. Table 8 shows the number of flowers after the pH solutions were added. Table 9 shows the number of flowers grown or lost because of the pH solutions.

The means of the raw data are shown in Tables 10 through 12. Table 10 shows the means for the heights of the plants. The B. rapa watered with the pH 2 solution, grew an average of 2 cm. The control that grew an average of 7 cm. The plants watered with pH 8 solution grew an average of 6 cm, while the plants watered with the pH 11 solution grew an average of 2.9 cm. Finally, the plants watered with the pH 13 solution, only grew an average of .2 cm (see Graph 1). Table 11 shows the averages for the number of leaves gained or lost for each pH level. The plants watered with the pH 2 solution lost an average of 2 leaves, the pH 8 plants grew and average of 1 leaf, the pH 11 plants lost an average of 1 leaf, and the pH 13 plants lost an average of 4 leaves. Losing leaves means that the leaves either died or fell off. These results make sense when compared with the control that grew an average of 1 leaf (see Graph 2).

Table 12 presents the averages for the flower counts of the B. rapa plants. The plants watered with the pH solution 2 grew an average of 2 flowers, the plants watered with the pH 8 solution grew and average of 5 flowers, the plants watered with the pH 11 solution grew an average of 5 flowers, and the plants watered with the pH 13 solution grew an average of 2 flowers. Finally, the plants in the control grew an average of 8 flowers (see Graph 3).

Table 13 gives the p-values from the t-tests ran. P- values below .05, resulted in the rejection of the null hypothesis. All the p- values were below .05 except for the ones comparing the control with the pH 8 group. The p- value for height comparing those two groups was .21, the p-value for leaf count was .11, and the p-value for flower count was .06. The control compared with the pH 2 group and the control compared with the pH 13 group had the lowest p-values. Refer to the Appendix for the table of this data.

Discussion

The data collected in this experiment supported the research hypothesis that the pH levels of 2, 11, and 13, would cause negative growth in the plants, but the pH 8 would not. The average growth of the plants in each group clearly expresse this trend. The control group grew the most, as was expected, followed by the pH 8 group, the pH 11 group, the pH 2 group, and finally, the pH 13 group. This shows a clear trend of higher mortality with the pH levels that are extremely high or extremely low. The pH level 8 is almost neutral and the group watered with that solution grew almost as much as the control. This makes sense because the control was neutral. It also supports the research hypothesis.

These trends were also observed in the number or leaves grown. The extreme pH level groups lost leaves, while the pH 8 and the control group grew leaves. The flower counts also clearly presented the same trends that the control grew the most, and the extreme pH levels grew the least according to their acidity or alkalinity.

Descriptive data analysis techniques were used to determine the statistical significance of the data collected. Specifically, the t-test was used. If the p-values from the t-test were above .05, then the null hypothesis was accepted. This means that the results occurred by chance, rather than by the independent variable. If the p-values were below .05, then the research hypothesis was accepted, giving the independent variable credit for the results produced. In this experiment, the p-values for the overall growth of the plants in the pH 13 group, were all very low. For example, the p-value for the height was .0000347. This means there is a very low chance that the reason the plants died was coincidence. The p-values for the pH 2 and pH 11 growth were also very low. The only p-values that were greater than .05, were the p-values for the pH 8 group. These p-values were .21, .11, and .06. This means that these plants did not die because of the pH that was added to them. That supports the part of the research hypothesis that says the pH level 8 will not result in the death of the plants. Because of the favorable p-values, the research hypothesis stating the pH levels above 8 and the pH level 2 would result in little or no growth compared to the pH level 8 and the control, was accepted. The null hypothesis was therefore rejected.

The only discrepancy in the experiment, would be time. Had the final measurements been taken two weeks after the pH was added instead of one week, the results may or may not have been different. By allotting more time, maybe the pH 8 groups would have experienced more neagtive effects than what was observed. The limitations of the experiment include the type of plants the results pertain to. The results may be different in other plants, like trees. The experiment was only done on one type of plant, so the results can only be depended on for that type of plant.

The sample size was efficient. There were 6 trials for each pH level, and 3 plants were planted in each trial. The recommendations for a person continuing the experiment would be to allow more time between the introduction of the different pH solutions, and when the final measurements were taken. Also, it is recommended that all the pH levels be tested. That way, the affect of each specific pH level on the plants could be observed. Then there would be substantial evidence to what specific pH levels are detrimental to plants, and what pH levels they can withstand.

Bibliography

Bittenbender, Brett, Latendresse, Keith, Martysz, Ivan, & Mood, Paul. (1998) Acid Deposition and Its Ecological Effects. Retrieved October 20,1999 from the World Wide Web: http://bigmac.civil.mtu.edu/public_html/classes/ce459/projects/t17/r17.html

Department of Plant Pathology. (1995). Wisconsin Fast Plants Manual. Burlington: Carolina Biological Supply Company.

Goldberg, Deborah E. (1985). Effects of Soil pH, Competition, and Seed Predation on the Distribution of Two Tree Species. Ecology, 66(2), 503-511.

Grow, Tracy, & Pidwirny, Michael. (1996). Acid Deposition and Precipitation. Retrieved September 25,1999 from the World Wide Web: http://royal.okanagan.bc.ca/mpidwirn/atmosphereandclimate/acidprecip.ht ml

Gurevitch, Jessica, Taub, Daniel R., Morton, Timothy C., Gomez, Proserpina L., & Wang, Ing-Nang. (1996). Competitive and Genetic Background in a Rapid-Cycling Cultivar of Brassica Rapa (Brassicaceae). American Journal of Botany, 83(7), 932-938.

Wisconsin Fast Plants. Retrieved November 29,1999 from the World Wide Web: http://fastplants.cals.wisc.edu/

Appendix

Table 1. Height of Brassica rapa Before pH Solutions Were Added