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Carrot Growth with Differing Wavelegnths of Light

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  • Pages: 11
  • Word count: 2635
  • Category: Light

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Daucus carota is a root that has small flowers but is capable of self fertilization and is biennial since it grows vegetatively in the first season and produces seeds in the second (Plants for a future 2008). It was originally native to Asia in colors ranging from purple to yellow to white with orange being the most prominent today (Carrots). The Daucus carota is capable of growing in many different conditions and regions from Asia to the Americas and Europe and Africa as long as there is direct sunlight (Carrots). This plant is hardy since it can grow in many temperatures from 10C to 25C, however from 15C to 20C is optimal, but can still grow in colder conditions, though much slower (Carrots). Daucus carota also prefers loose, deep soil that is well irrigated, but in more compacted soil it can grow however the root tends to be crooked or split. (Carrots). Lastly, this plant can grow in many pH ranges from 4.2 to 8.7 with 6.5 to 7.5 being the optimal range (Plants for a future 2008).

In order for photosynthesis to occur, there must be a light source to provide energy for the necessary reactions. This experiment will use different wavelengths of light to discover which one promotes growth the most for Daucus carota. Explained by the action spectrum diagram, different colors of light promote growth more or less efficiently than others. As shown, light in the violet-blue range and red range are absorbed by the plant the most and thus are most effective colors for promoting photosynthesis while the yellow-green range is rather ineffective since it is bounced off the plant and chlorophyll giving it a green color (Allot 2007). Every color in the action spectrum has a different wavelength. Green light is 510 nm, blue light is 475 nm, and red light is 650 nm. White light is a combination of all visible colors giving it a wavelength range of 400-700 nm. Even though the plant reflects much of the green light, some photosynthesis still occurs in this range (Allot 2007). While chlorophyll is the main light absorber in the plant, accessory pigments are able to absorb wavelengths that chlorophyll cannot (Allot 2007).

Photosynthesis is a process involving much more than pure light absorption. After the chlorophyll absorbs light energy, an electron in the chlorophyll molecule is raised to a higher energy level and is referred to as excited and photoactivated (Allot 2007). These electrons are passed from chlorophyll molecule to chlorophyll molecule until they reach the reaction center of the photosystem where they are transferred to electron carriers in the thylakoid membrane (Allot 2007). This is where ATP production begins. Each time the excited electron passes from one carrier to the next a little energy is released and eventually enough energy is released to create one molecule of ATP (Allot 2007). The electron flow causes a proton to be pumped across the thylakoid membrane into the fluid space creating a proton gradient (Allot 2007). ATP synthase lets the protons cross the membrane by going down the proton gradient and uses the energy to synthesize ATP (Allot 2007). Remember, that entire process is dependent on the amount of light energy absorbed.

Research Question:

How does the wavelength of light affect the growth of Daucus carota?


I hypothesize that white light will be most successful in promoting Daucus carota growth because it includes all colors of light, followed by red light which is more absorbed then blue light according to the action spectrum. Green light will be least effective since it is reflected by the chlorophyll.


Independent: Wavelength (color) of light

Dependent: Growth of Daucus carota

Constant: Temperature, type of dirt, type of plant, time exposed to light, wattage of the bulb, amount of water, size of container, pH.

Control: White light


* 4 lamps

* 1 red bulb (25 watts)

* 1 green bulb (25 watt)

* 1 white light bulb (25 watt)

* 1 blue bulb (25 watt)

* Power source for 4 plugs

* 80 Daucus carota seed

* 28 L of soil

* Watering container with metric measurements (I used a pipette with markings from .25, .5, .75, and 1 ml.

* Water source

* Microsoft excel (to record data in)

* Metric ruler (at least 20 cm)

* Camera (to record results)

* 80 50ml cups

* Thermostat

* Marker

* 3 dividers to keep each light color on only the specified experimental group. (I used stacked boxes)

* Timer (one that connects to a power source and can be plugged into by a power needing device)

* Power strip

* Four boxes that can contain 20 cups each

* Ph indicator strip


1. Gather all materials.

2. Put 20 cm of dirt (height- wise) in each cup. Measure with the metric ruler.

3. Place 20 cups in each box arranged 4 x 5 (four groups of five). Label each box so that you can remember which plant is which on your excel data table.

4. Gently push each seed 3 cm into the soil and cover the indentation so the seed is buried.

5. Keep the plants in natural sunlight until every plant is established (roughly 2 cm tall). You should make sure the soil is kept moist to stimulate germination and growth. When the soil feels dry add 1 ml at a time so you don’t overwater the plant. Remember to use the pH indicator to confirm that the water has a pH of 7.

6. When every plant is around 2 cm tall, move them inside to the dark space where the experiment will occur.

7. Each of the four lamps should be installed with a different color bulb (all 4 colors).

8. Make sure each lamp is plugged into the one power strip. Plug the power strip into the timer and the timer into the power source.

9. Set the timer to allow power through it for 10 hours a day. Now you do not have to worry about the lights- they will go on and off automatically and at the same time each day.

10. Now place the dividers in the given dark room so an experimental group can fit on either side of the divider.

11. Arrange each of the four experimental groups (they should be contained in the box in a 4×5 manner thus easy to move between the dividers. Now set them in the order box, divider, box, divider, box, divider, and box.

12. On the top of each divider, clip on the lights making sure each group has a light shining directly above them. Ex. The group on the far right has the green light shining on it, then on the left of that divider the red light shines down and then the third group has the green light shining down and the farthest left had the white light shining down

13. Now make sure the lights work and are all on (so that each light will be synced to turn on and off at the same time). You now can start the experiment.

14. Use the thermostat to maintain a constant temperature of 24C.

15. For 2 weeks, record the plant heights every two days. When measuring the plants you should also make sure the soil is still moist. When adding water, use 1.0 ml at a time so you do not drown the plants (since there is no way for the water to drain from the cup). You can determine if the soil is dry by touching it. If soil sticks to your finger it is wet, if the soil flakes off your finger like dust, it is too dry. Remember to use the pH indicator to confirm that the water has a pH of 7.

16. Process data

17. Report findings.

(See figures 3, 4 and 5 for reference)

t of living plants in each experimental group per day measured. When lines overlap, it means the groups have the same amount of living plants. The error is 0.1cm, 0.1 ml for water, .5 for ph.

Data Processing:

Table 1 is of the raw data recorded during the experiment of all 80 plants in all four experimental groups. Each color has four groups designated by the number one through four. As shown, the heights have an error of .01 cm since every metric ruler is not 100% accurate. Table 4 is the standard deviations of the final heights for each group. Table 3 uses the raw data to create average height/ growth for each color. Figure 1 is a line graph utilizing standard error bars. It shows the average height of each experimental group; however it does not include the amount of living plants at the end of the experiment. The lines do visually show how heights of the living plants differ since one can see which groups had taller plants by the height of the line.

As the lines move down, they show how plants in the experimental group withered (shrank) and when they died the lines move back up since that pant no longer affects the average since dead plants are not included in the average. As shown by table 4 and figure 2, blue light and green light groups have significantly more deaths then found in white and red light groups. This means that even if the green and blue groups had a few tall plants, the vast majority were dead as opposed to the white and red group which had very few plants die. When you take both the amount of living plants and the heights of the plants into account, the hypothesis is supported because the group under white light grew the tallest and also had the fewest deaths. The group under the red light was the third tallest but had significantly more plants survive then the group under blue light meaning that the red light is more effective in promoting growth. Then, although the plants under blue light mostly died, the surviving ones were rather tall meaning that blue light is the third most effective color to promote plant growth. Finally, the groups under green light had similar deaths to the blue group and also had very little growth meaning that green is the least effective at promoting plant growth.


Conclusion and Evaluation

My data supported my hypothesis that white light would be the most successful in promoting plant growth with green light being the least effective (Allot 2007). An explanation is that white light is what the plants are exposed to in nature meaning they are adapted to utilize all of the included wavelengths while green is naturally reflected from Daucus carota meaning it is the least utilized wavelength (Carrots). On the line graph of plant height, the order appears to be the white light group, the blue light group, the red light group and the green light group, but looking at the actual raw data and table 4/ figure 2 a different result is seen. Only five of the plants in the blue light group survived until the end of the experiment with six green plants surviving.

The plants exposed to both white light and red light however had very few deaths meaning that those colors were more effective in promoting plant growth (Allot 2007). The plants exposed to white light averaged 3.91 cm with a 1.06 cm .01 cm standard deviation, the red where 3.44 cm with a 1.02 cm .01 cm standard deviation, the blue 3.82cm with a 0.24 cm .01 cm standard deviation , and the green 2.50 cm with a 1.84 cm .01 cm standard deviation. These standard deviations are low with the highest being 1.84 cm .01 cm indicating solid and consistent data that can yield a reliable analysis. Only the plants exposed to the white and red light grew in a steady, linear manner, while the plants exposed to blue and green light show erratic growth which is characterized by little growth with periodic bursts as shown by the raw data and figure 1.

The results of this experiment are strong when considering both height and death rates together, but there are certain weaknesses and limitations in existence. The time limit of two weeks might not have been long enough for the plants to actually show their ability to grow, which is show by conflicting results between the red light and blue light when looking at the death rate chart in comparison to the Daucus carota growth chart. Also, since the plants were kept in a closet in order to keep outside light sources to a minimum, there might have been less circulating carbon dioxide in the room yielding less plant growth since it is a necessary factor of photosynthesis. In addition, the light from the lamps is different than nature since the light was constant and direct until they were turned off, as opposed to natural light which can be affected by cloud cover and can be shaded, which could have possibly denatured enzymes and burned the plants (although the bulbs were only 25 watts).

Since all of the Daucus carota plants used in the experiment did not all originate from the same parent plant(s) the plants each had a different genetic makeup leading to different pre- programmed growth rates, thus one plant might have been programmed to grow large as the energy is provided and another would be more likely to store energy and grow slowly. Furthermore, certain plants might be hardier then others allowing them to survive in adverse conditions which was shown by the few surviving plants in the green and blue light group. Also, one plant might require less water to grow then others which could alter data gained from this experiment using seeds only from one parent plant or even more accurate, using clones of one plant. Clones would allow for the same plant to be used in every experimental group eliminating speculations concerning genetic variation.

While this experiment was conducted as accurately as possible, there are a few improvements that could be made. Increasing sample size and the amount of days utilized would yield more specific results since there would be more data included in the study and there would be more time for the plants to show their growth potential. As previously mentioned, using seeds that all had the same genetic makeup would yield the most accurate results because each plant would then have the same traits which would eliminate the speculation of whether or not a plant was hardy or genetically not up to par. Another improvement would be to either rotate the plants or move the light source from the middle of the group to the sides so all plants would have equal opportunities to absorb light and grown (since plants on the edge get less light then plants directly under the lamp). Also, since this experiment was conducted in a closed closet with no ventilation, using a fan to move carbon dioxide around might yield a more conclusive result since then the gas would not be a limiting factor. Finally, measuring the plants at the same time would yield more consistent results because then there would be a steady, scheduled water supply.

Literature Cited:

Agriculture.kzntl.gov.za, Carrots, http://agriculture.kzntl.gov.za/downloads/files/Horticulture%5Ccarrots.pdf

Allot, A. (2007). Biology for the IB Diploma. New York: Oxford

Plants for a future, Daucus carota sativus, 2008, http://www.pfaf.org/database/plants.php?Daucus+carota+sativus

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