4. Discussion


Discussion


4.1 Key Findings



Table 5: Summary table, average of plants growth for each species per day



Overall, in descending order, the three types of plants under the different type of coloured lights grew differently, from the best to the least is as follows:


BLUE  light
RED light
SUNLIGHT
YELLOW light
GREEN light

The Mung Green Beans had grown differently under the lights, the best light blue, followed by red, sunlight, yellow and lastly by green, but their differences were not significant.


The Fittonia plants, plants in the blue light grew twice as fast as the control, sunlight. Red was the 2nd best lighting that also grew faster than the control and lastly, the plants under the yellow and green light grew slower than the control and were very unhealthy.


The Syngonium plants exposed to the blue and red light grew much better than the control, yellow and green lightings.


4.2 Explanation of key findings

This is because plants require light of specific wavelengths to grow. Chlorophyll, a green substance in plant leaves that absorbs light energy to help make a plant's food, absorbs mainly red and blue light and reflects green light. This is what makes most plants look green, because the colour of the light they reflect is the colour that we see. Therefore, the plants grown under blue light were the healthiest, followed by red. The plants under sunlight, our control were the third healthiest, and the plants under the green and yellow light were the least healthy.


The colour of the leaves of the different coloured plants, red, purple and green, did not affect the process of photosynthesis because the substance chlorophyll, the green pigment in every plant is the factor that affects photosynthesis and not the colour of the leaf. For example, in the Fittonia plant, chlorophyll is present at the border of the leaves and it traps the light for photosynthesis. Colours with higher energy are best absorbed by chloroplasts, and blue and red light have wavelengths with the highest energy.


Since the greater the energy, the shorter the wavelength of the light and the larger the frequency.



Fig. 9: Photosynthesis Spectrum, Source grow.ars-informatica.ca


After obtaining the results from the growth of the plants, we concluded that the colour of lights (that we used in our experiment) from the best to worst for the growth of plants are as follows: Blue, Red, Yellow, Green. Before starting the experiment, we did research to see what scientists have discovered through their experiments and took the four primary colours that were shown in the colour to growth of plants graph (shown in conclusions). The research was used as a basis for our experiment, and our final results are in line with the results that professional scientists obtained. Our experiment proved our hypothesis in the end, plants that grown under blue, followed by red will have the greatest impact on plant growth.  Green light is least effective because of the reflection of green light due to the plant’s chlorophyll. Only the chlorophyll will affect photosynthesis (therefore affecting the growth rate too) and the colour of the leaf only had minimal effects.

Another factor is the relationship between light and plant growth can be demonstrated by exposing leaves to various colors of light. Light supplies the energy to carry out photosynthesis, creating  their own foods  in leaves. All colours of light, including sunlight, have their own wavelengths. The shorter a light wavelength, the more energy it has and this in turn would have a higher rate of photosynthesis. Since the amount of light energy affects the rate of photosynthesis, the greater the light energy, the faster the rate of plant growth. Blue light has a wavelength range of 450-495 nanometers while red light has a wavelength of 620-740nm. Chlorophyll reflects green light (so leaves appear green), therefore it is the worst for plant growth as none of the energy is absorbed by chloroplasts.  

Fig. 10: A graph of the absorbance of the colour light

However, leaves not only contain chloroplasts, but it also contain carotenoids and phototropins. Carotenoids have xanophylls and carotenes. Carotenoids are also yellow, orange, and red pigments. In general, carotenoids absorb wavelengths ranging from 400-550 nanometers and the key role that they serve is to protect chlorophyll from photodamage. Although red light has the longest wavelength out of all the colors of light, it is the most effective in photosynthesis because it is absorbed completely by chloroplasts, so all its energy is used for photosynthesis. Since carotenoids are also red pigments, they reflect red light and do not absorb it at all. In contrast, most of the energy from blue light is absorbed by carotenoids due to its short wavelength(450-495 nanometer), reducing the amount of blue light energy for photosynthesis.
Fig. 11: Phototropism taking place
Phototropins are blue light receptors for phototropism, which is the growth of organisms in response to light, chloroplast movement, leaf expansion, and stomatal opening. Although blue light is not the best colour of light for the process of photosynthesis itself, it improves the process through chloroplast movement and stomatal opening. Therefore, blue light is better for the growth of plants instead of actual photosynthesis itself, which can be seen through leaf expansion.

4.3 Evaluation of Hypothesis

Our hypothesis was "From our background research, we concluded that exposure of plants to blue light and red light will result in the fastest rate of photosynthesis and hence the greatest plant growth. Green light is least effective because of the reflection of green light due to the green pigment of the chlorophyll.  Therefore, the plants should grow best under blue light and red light followed by yellow light, and lastly, green light."

Through our experiment, we proved that our hypothesis was correct because the order of the height of the plants under different coloured lights were:
1. Blue light
2. Red light
3. Sunlight
4. Yellow light
5. Green light

This order coincides with our hypothesis.


4.4 Areas for improvement.




After obtaining the results from the growth of the plants, we concluded that the colour of lights (that we used in our experiment) from the best to worst for the growth of plants are as follows: Blue, Red, Yellow, Green. Before starting the experiment, we did research to see what scientists have discovered through their experiments and took the four primary colours that were shown in the colour to growth of plants graph (shown in conclusions). The research was used as a basis for our experiment, and our final results are in line with the results that professional scientists obtained. Our experiment proved our hypothesis in the end, plants that grown under blue, followed by red will have the greatest impact on plant growth.  Green light is least effective because of the reflection of green light due to the plant’s chlorophyll. Only the chlorophyll will affect photosynthesis (therefore affecting the growth rate too) and the colour of the leaf only had minimal effects.



Another thing that we observed is that although blue light is the best colour of light for the growth of plants and green light is the worst, all 4 boxes with the 4 different coloured lights had green bean plants shriveled up with leaves dropping off. We then realized that this does not mean that the experiment results are wrong, but rather that the intensity of the LED bulbs were too strong, or the distance between the plants and the LED lights was too little. The green bean plants are the weakest plants out of the 3 plants we used in our experiment, which is why they all shriveled up even after being under blue light. This showed us that when light is concerned, the intensity of the light is equally important, and not just the colour of the light.


There was another problem we encountered while doing the experiment. On 14 August, Thursday, we came in for science lesson and realized that the LED light strips had all fallen onto the green bean plants in each of the boxes. The heat and light intensity killed many of the plants and we had no choice but to restart the experiment for the green bean plants with a new set of plants on Friday.


One thing that our science teacher pointed out to us is that a green plant under red light will appear black. A red plant under green light will also appear black. This is because a single colour light source cannot be reflected off a plants' natural colour, which is green. This is due to the fact that the reason why plants appear to have colour is because it absorbs all the light colours except the colour which is appears to be, which also means that for example, green beans reflect green light. In our experiment, we did not have any blue coloured plants. Therefore, when the plants were placed in the box with blue light, they absorbed all the light as there is only one colour of light to absorb, which is blue. 


After going through this entire experiment, we found several areas for improvement. Firstly, although the results turned out accurate, we have to pay close attention to the different factors affecting the growth of plants which include amount of air, quality of soil, intensity of light, and amount of water. We tried our best to ensure fair results, but sometimes some factors such as the amount of air can vary despite the holes for air being the same size. Therefore, in future experiments involving plants, we should find a better option such as a better container and location. 

Secondly, it is tough to manage time. As we only have about one month to complete the project, we had to plan out what we were suppose to do each lesson. This is because our science lessons in the lab only last an hour. This is why we took about 6-8 weeks to plan and set-up our experiment and only had another 6-8 weeks to conduct the recording of data. That is why we have to improve our time management and plan well, which includes dividing the workload between team members.

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