Brassica Rapa Leaf Area Index Responds

Published Date: 02 Nov 2017

Derek Cebula

IB 103 – Fast Plant Report

Brassica rapa Leaf Area Index Responds to Differing Light Wavelength

Abstract

The color of light that a plant is grown in will affect the plant greatly. Most plants grow in white light, but white light is a mixture of all visible light, so varying the levels of the colors of light making up white light can change how a plant grows. Plants grow in order to recieve more light, so knowing how they respond to each color is important to furthur our understanding of plants. In this study, Brassica rapa plants were grown under white, red, green, and blue light to determine how leaf area index is affected. Plants were grown under colored light filters and leaf area index was measured halfway through and at the end of the experiment. Red light promised the largest leaf area index after the control(white light), but they suddenly died after the first measurement. Using trends in the data, it is reasonable to assume red light showed the most leaf area index after the control, followed by blue and then green light with the lowest leaf area index. This data shows that red light is more beneficial for increasing leaf area index over green and blue light. This data can be used to improve light delivery systems for plants around the world.

Introducton

The origin of Brassica rapa has not been confirmed by scientists. It is most likely from the area from the eastern Mediterranean to Pakistan, also China has been suggested.( Toxopeus, H. & Baas, J.) It was spread around the world from those areas via trade routes over many years. Eventually it made its way to North America via cargo ships. Since this plant has been in many places around the world, it is extremely diverse and has many mutant phenotypes. While there are many phenotypes, the ygr phenotype will be focused on. This Brassica rapa mutant stems from the recessive gene "ygr".( Wisconsin Fast Plants: Seed Stock Profile) This is also called the Yellow-Green Leaf trait which was first detected as one pale-yellow seedling in a population made of all other green seedlings.( Wisconsin Fast Plants: Seed Stock Profile) This phenotype also has a purple stem near the base which results from the higher presence of anthocyanin.( Wisconsin Fast Plants: Seed Stock Profile) The main feature of this phenotype is its unique protein function. The function of the protein is unknown but is presumed to act upstream in the chlorophyll production pathway.

The more common name of Brassica rapa is the turnip.( Duke, James A) These plants are very commonly grown for human consumption but also in large amounts for feeding animals during the fall and winter.( Duke, James A) Brassica rapa is a biennial and annual herb with a very large white colored taproot.( Conservation Plant Characteristics) Brassica rapa is neither a tall plant nor a long living plant. The average plant height at 15 days is 15cm while they only live from 35 to 45 days.( Wisconsin Fast Plants: Seed Stock Profile) This short life can be useful for farmers to use the plant as a cover crop or to fill a field when they do not want to leave their soils bare.

Light is an extremely important factor in a plant’s growth. Different colors of light affect how a plant functions. Some colors of light will promote growth over other colors of light based on their inherent properties and based on what the plant needs to survive. Most plants are green since this is the color of light that they are reflecting. Plants reflect green light since they use it the least compared with other light colors. There are many receptors on a plant to recieve this light, but one receptor has been found to manage more plant functions that any, phytochrome. Phytochrome is a photoreceptor, or pigment that a plant uses to detect different colors of light.( Decoteau, Dennis R., and Heather H. Friend) It has been suggested that certain colors or wavelengths of light will promote a plant’s growth over other types of light. Red light is the most affecting color of light, with plants generally growing taller when exposed to more red light that other light colors.( Decoteau, Dennis R., and Heather H. Friend) Phytochrome detects two different kinds of red light: far-red light which is 710-850nm and red light which is 620-740nm. (Decoteau, Dennis R., and Heather H. Friend) While red light may be phytochrome’s main source of activity, it still detects all other colors of light. When red light is detected, phytochrome switches to far-red light, which is the active form of phytochrome. When phytochrome is activated, many things are controlled, including flowering time, seed germination, seedling elongation, and the size of leaves. This size of leaves in plants is what will be examined

The experiment will focus on how light wavelength influences growth in B. Rapa plants, focusing specifically on leaf area index(LAI). This will be done by covering groups of ygr B. Rapa plants with different colored plastic films to reduce a white light source down to a single color. The colors that will be tested are red, green, blue, and white which is the control. This is an important experiment to perform since much can be learned from it. After the experiment, it will be known how a plant might react to different colors of light being absorbed. This knowledge will help scientists better understand how plants react to the sun in different parts of the world since light cover is not uniform accross the Earth and how producers of the future may be able to increase the reproductive and yield possibilities of a plant.

Dennis R. Decoteau and Heather H. Friend of Clemson University have conducted research on how light quality effects plants. Their experiment involved treating plants with far red light compared to red lightusing various mulches to change the type of light that was hitting the plants. After much experimentation, they concluded that plants grown in areas with more far red light relative to red light are taller than plants grown in environments with lesser amounts of red and far red light. (Cite this article) This experiment tested the stem length after exposure to various amount of red light, while this report examines how leaf area index was affected by light color. It is important to test the leaf area index and not just the stem length. If a plant has a larger leaf area index, then it can photosynthesize more and produce more sugars and food. If more food is available, then the plant should be able to grow larger as a whole. This is the purpose of this report and experiment, to determine if different colors of light will affect leaf area index differently.

The basis for this experiment has now been laid out. After extensive research, a hypothesis was drafted: Leaf area index will be affected differently by different colors of light, with the control(white light) promoting leaf area index increases the most followed by the experimental colors red, blue, and finally green light. The only difference among the plant groups was the color of light grown in. Everything else was kept constant to minimize error. The leaf area index is dependent on the color of light the plant was grown in and will be used to make conclusions later.

Methods/Materials

Study System:

Brassicca rapa is a very fast growing plant. Flowering and maturity takes only about two weeks. B. Rapa was chosen for this experiment because it is well suited for lab work. It is easy to grow under flourescent lighting and in a standard potting mix. The seeds of this plant actually mature with no seed dormancy. The experiment will take place in a classroom which is not well suited for growing plants. This is why B. Rapa is being used, since it is small, matures quickly, and can be grown under flourescent light. B. Rapa was used to determine how light colors affect growth in leaves.

Seed Planting and Treatment:

B. rapa seeds were planted in a soil mixture that of vermiculite and potting soil that was mixed in a 3:1 ratio, ¾ potting soil and ¼ vermiculite. Osmocote fertilizer was added to each seperate pot of seeds. Seeds were planted at an average depth of 1/8 inches and covered with a light layer of the soil mixture. Each color of light had three small pots with four seeds in each pot, totalling 48 seeds. Pots were placed on a sealed, open-top tray to prevent water from spilling around. Seeds were consistently watered the same amount. Red, blue, and green colored films were used to make light filters. Each set of three pots was placed under a different filter: no filter, red filter, green filter, blue filter. A custom built tripod made from bamboo was used to hold colored films over the plants in order to block out natural light and expose the plant to one color of light. The tray containing the pots and tripods with filters was placed under flourescent lights. After two weeks, plants were cut back to thin out the quantity so more focus could be placed on a few plants.

Measurements:

Leaf Area Index was measured twice, once at the midpoint of the experiment and again at the end of the experiment. Leaf area index was measured using graph paper. A sample leaf was traced onto a grid and area obtained by totaling the number of squares occupied by the leaf, in milimeters.

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Results

Figure 1. – Light color has different effects on a plant’s leaf area index.

Figure 1 shows how leaf area index changed with different colors of light over a growing period of weeks. White light increased the most up to 281.3 mm2, followed by blue light at 145 mm2, then red light at 0 mm2, and green light at 46.4 mm2. Leaf area index generally increased with time except for plants grown in red light. Measurement 1 is 3 weeks into the experiment and measurement 2 is 6 weeks into the experiment.

Figure 2. - Light color has different effects on a plant’s leaf area index.

Leaf Area Index Related to Light Color

First Measurement

Second Measurement

Blue

66.7 mm2

145 mm2

Red

89.9 mm2

0 mm2

Green

29 mm2

46.4 mm2

White(Control)

182.7 mm2

281.3 mm2

Figure 2 shows the leaf area index of plants grown in different colors of light. White light had the highest result of 281.3 mm2, followed by blue, then green, and finally red at 0 mm2. Leaf area index generally increased with time except for plants grown in red light. Measurement 1 is 3 weeks into the experiment and measurement 2 is 6 weeks into the experiment.

Data and Trends:

Figure 1 and figure 2 show how leaf area index changed over the growing period of plants that were exposed to different colors of light. White light started at 182.7 mm2 at the first measurement and finished at 281.3 mm2 with a total increase of 98.6 mm2. Blue light started at 66.7 mm2 at the first measurement and finished at 145 mm2 with a total increase of 78.3 mm2. Red light started at 89.9 mm2 at the first measurement and finished at 0 mm2 with no total increse. Green light started at 29 mm2 for the first measurement and finished at 46.4 mm2 with a total increase of 17.4 mm2. All plants increased leaf area index over the time period except plants grown in red light, which all died. White light increased the fastest, with blue light in second place, green light in third, and red in last with the lowest leaf area index.

Discussion

After finishing the experiment, data was collected and analyzed. It was found that plants grown in white light increased leaf area index from 182.7 mm2 to 281.3 mm2. Plants grown in green light increased from 29 mm2 to 46.4 mm2. Plants grown in red light decreased from 89.9 mm2 to 0 mm2. Plants grown in blue light increased from 66.7 mm2 to 145 mm2. All of the plants leaf area indexes increased as expected except for plants grown in red light, which were all dead by the end of the experiment.

The white light plants showed the most leaf area index growth and total leaf area index. This was expected from the start of the experiment. White light contains all of the wavelengths of light that a plant needs to successfully grow. The plants used in this experiment evolved on earth which is covered in white light. White light is the type of light most living organisms rely on to either see or grow. Since we placed our control in its natural lighting environment, it grew far better than any of the other plants grown in only one color of light. The plants grown in red light started off with the second largest leaf area index, but ended up dead by the end of the experiment. This is not what should have happened to the plants grown in red light. These plants should have the second largest leaf area index growth and total leaf area index. This will be discussed more in the errors section. The plants grown in blue light showed the second largest leaf area index growth and total leaf area index. These plants were expected to be in third place, behind plants grown in red light. Despite this, the blue plants results are in the expected area. Blue light is important for plants, but it is not important for a plant to grow. Red light is the primary light color that is responsible for plants to increase their size. Therefore, it makes sense that blue light is below white light, but above green light. Green light is the least used light color by plants, resulting in them being green. Plants grown in green light showed the third largest leaf area index growth and total leaf area index. While the red light plants are underneath these, the green light plants still performed as expected. They started with a lower leaf area index and it did not increase as rapidly as the other colors of light, excluding red. Green light is the least productive color of light available to plants. It has been shown through other studies that plants grown in green light are smaller, but the focus has never been on the leaf area index of those plants. This experiment further confirms that plants grown in green light will be smaller, especially in leaf area index.

After examining the data, it can be concluded that the hypothesis was partially correct. The red light plants created a large error in the data, but if the data is examined except for the final measurement of the red light plants, the hypothesis can be accepted. For some unknown reason, all of the red plants were dead by the end of the experiment. From the graphs, it is apparent that the red light plants started growing well, but that decreased. If the trends of plants of the graph are followed for plants grown in red light, it could be concluded that red light had the second largest leaf area index growth and total leaf area index. This is the result that was expected and clearly should have been according to the graph and previous knowledge of the subject.

The tripods with colored film were a source of most of the error in this experiment. It was difficult to construct an object that blocked all outside light given the lab conditions. The resources available were adequate for this experiment, but not ideal. There was no way to get the colored film to cover the entire tripod and reach to the floor. Also, the fact that the plants had to be watered means that the tripods had to be removed in order to water the plants. There were a few instances while checking the plants that a tripod was not completely covering the plants which means they were exposed to white light for an extended period of time, skewing the results. Perhaps the red plants died as a result of the person watering the experiment, but there is no way to be sure. Another source of error is the way the leaf area index was measured. Each leaf was traced onto graph paper with known unit size. It is not easy traing a leaf accurately and counting parts of squares where the leaf is curved. Despite this, it did not induce much error since the same method was used for each sample of plants. These were the two major sources of error in this experiment. Other erros existed, such as seed spacing and decreased light exposure due to other experiments, but were negligible.

Despite the problem with the plants grown in red light, this was a successful experiment. If the experiment was done again, it would reasonable to expect that the red light plants would have the second largest leaf area index. This makes sense based off of the science of how plants absorb and use light. White light is the most productive followed by red, blue, and green. This has been shown in multiple experiments and was expected to happen in this experiment. Excluding the red light plants results, this experiment was flawless according to the results. White light had the largest leaf area index which is expected since it contains all necessary light colors for a healthy plant. Blue light had the second largest leaf area index, although it should have the third largest after red light plants. Red light is the next most productive color of light after white light. Blue light follows red light in terms of productivity and is more important in tropisms than growth. Green light had the lowest leaf area index which follows previous experiments. Based solely on results, red light plants performed the worst while white light plants performed the best. If the results include the expected data, red light plants would have the second largest leaf area index and not the lowest.

Literature Cited

Conservation Plant Characteristics. United States Department of Agriculture. Retrieved January 22, 2013, from http://plants.usda.gov/java/charProfile?symbol=BRRA

Decoteau, D., Friend, H. Plant Responses to Wavelength Selective Mulches and Row Covers: A Discussion of Light Quality Effects On Plants. Retrieved January 22, 2013. Print

Duke, J., (1997, December 30). Brassica Rapa L. Retrieved January 22, 2013 from http://www.hort.purdue.edu/newcrop/duke_energy/brassica_rapa.html

Toxopeus, H., Baas, J., Brassica Rapa L. Retrieved January 22, 2013, from http://www.prota4u.org/search.asp

Wisconsin Fast Plants: Seed Stock Profile. Retrieved January 22, 2013, from http://www.fastplants.org/pdf/seedstocks/ygl.pdf