Plant Growth

ABSTRACT Light has a profound impact on the growth of plants. In this investigation the effects that the colours of light have on plant growth and photosynthesis were explored. It was hypothesised that red and purple-blue light allow for the most growth in plants, whereas green light does not allow photosynthesis and plants don’t grow well in it. In the experiment, 15 mint plants were used with 3 trials being conducted for each colour of red, blue, purple, green and clear. The growth of these plants was monitored over a period of 3-4 weeks. After 3 weeks, it was observed that the plants under red did grow better and taller than the rest. Plants under blue light also grew fairly well, but as a profound contradiction to the hypothesis, the plants under purple light grew the least, even less than the plants under green light. By completing the experimental investigation, the hypothesis is supported in part, with more and further research and experimenting needed to prove these results.

RATIONALE FOR INVESTIGATION Plants and trees are the most important resources on our earth. Not only do they provide the oxygen needed by aerobic organisms such as ourselves, they also provide for the food requirement of heterotrophs which consume other organisms for food. However, these resources will not last forever unless we take certain measures to replace them as we use them. We need to sustain out current resources so that the needs of the present generation are met without compromising the ability of future generations to meet their own needs. Sustainable development depends on environmental protection, economic development and social progress and strategies include recycling, reduced consumerism and Figure 1: plants and trees are our most water and energy usage, growing our own fruit and important resources (Wikipedia, 2008). vegetables and sustainable practices in agriculture. In this report we are investigating the most successful ways of growing high quality plants in the backyard rather than waste energy in farms where food must be packaged and transported. There are many factors that affect the growth of plants including light, temperature, type of soil, availability of water, supply of minerals and pH. In this experiment we will investigate how the colour of light affects the growth of a plant. Light has a large affect on the ability of a plant to grow and photosynthesise. Plants usually tend to grow better in certain colours of light than other colours. This experiment will investigate which colours of light allow for the most growth in plants.

RESEARCH PROPOSAL Research questions: what factors effect the growth of plants and what are the best conditions under which a plant can grow? Why and how does light affect the growth of a plant? Aim: to investigate the relationships between the colour of light and the growth of plants. Hypothesis: green plants forced to grow in green light do not experience rapid and plentiful/proficient grow, whereas, these plants in red and blue–purple light grow better with more leaves and are taller. White light (clear) which is a mixture of all colours of light would allow for medium growth of the plant (not too little and too much). Red plants such as red algae grow more in green-blue light, whereas, they do not experience profuse growth in red and orange light.

HYPOTHESIS JUSTIFICATION Light can have a dramatic effect on the growth of a plant. Light is used by plants during photosynthesis with the help of water (H2O) and carbon dioxide (CO2) under the influence of chlorophyll to produce glucose (C6H12O6), oxygen (O2) and residual water. The equation for photosynthesis can be written as: Chlorophyll

6 CO2 + 12 H2O + light energy -----------à 6 O2 + C6H12O6 + 6 H2O Photosynthesis occurs in the thylakoid membrane of a chloroplast specifically using chlorophyll and is the process by which light energy is converted into chemical energy which is then stored in the bonds of the glucose produced (J. Stein Carter, 2004). Photosynthesis occurs in 2 stages: the light dependant stage and the light independent stage. The light dependant reactions occur in the first stage. In this phase chlorophyll traps light energy and uses it to produce ATP (adenosine tri-phosphate) molecules and split water into hydrogen ions and oxygen gas. Chlorophyll has a molecular structure that, when excited by photons of light energy, initiates a series of steps that result in the production of ATP molecules (Heinemann Education, 2004). The equation for these reactions could be written as: Chlorophyll

Water ------------à hydrogen ions + oxygen gas + ATP Light energy

The second stage of photosynthesis is independent of light. ATP made during the first stage provides the energy needed to combine CO2 with the hydrogen ions which are also from the light dependant reactions. These reactions form glucose, a molecule rich in energy and some water is reproduced. The equation for the light independent reactions can be writer as: ATP + H+ ions + CO2 à glucose + water + ADP (adenosine di-phosphate) (Heinemann Education, 2004). For photosynthesis to occur, this light energy must be provided to provide the energy needed for all the reactions to occur. Not all wavelengths of light can support photosynthesis however. The spectrum range that photosynthesis works in depends on the accessory pigments present in the plant. For example, in green plants the absorption spectrums for chlorophyll and carotenoids (these also play a part in photosynthesis) which peak for purple-blue and red light resemble the action spectrum, which shows which wavelengths of light are most effective in a specific chemical reaction. (Wikipedia, 2008). Chlorophyll which is abundant in green plants appears green because it reflects green light. This green light can not be absorbed and thus it cannot be used in photosynthesis in green plants. However, red and blue light are absorbed by chlorophyll and it is the energy from these colours that is used during photosynthesis (J. Stein Carter, 2004). The non-absorbed part of the light spectrum is what gives photosynthetic organisms their colour and Figure 2. The light spectrum and the rate of photosynthesis (Wikipedia 2008).

is the least effective for photosynthesis in the respective organisms (Wikipedia, 2008). In red algae, the action spectrum overlaps with the absorption spectrum of phycobilins. Phycobilins are unique photosynthetic pigments which are especially efficient in absorbing orange, yellow and green light, wavelengths which are not well absorbed by chlorophyll. This means that these algae can grow in deep waters where the longer wavelengths are used up by green plants.

EXPERIMENTAL DESIGN 15 mint plant set-ups will be used in this experiment. 3 trials will be conducted for each colour (blue, red, green, purple) and natural light will be used as a control. The growth of the mint plants will be observed over a period of a month, measuring the height of the plants on a weekly basis. Variables will have to be controlled accurately to increase the reliability of data obtained. Independent Variable: colour of light – red, blue, green, purple and white light Dep. Variable: how well the mint plants grow (growth in height as cm) Controlled Variables: • soil quality/ type of soil • amount of water/ nutrient solution provided • atmospheric temperature • amount of light provided (we are changing the colour, not intensity) • the type and size of container housing the mint plants • size of cardboard box • number of plants in each container – 5 individual plants in each container

The experiments conducted in this investigation have been controlled and mostly unbiased. The type of plants used has been the same. They are not all of different species. The type and amount of nutrient solution used are constant so the chance of one plant getting more or less provisions for photosynthesis is ruled out. The size, type and shape pf containers used to house the plants were constant. This was so that all plants would have equal space in the pot to spread their roots. Also, the soil composition used in each trial was the same so that the possibility of different types of soils providing different nutrients and minerals is avoided. Also, all plants are kept at constant temperatures. As temperature can affect photosynthesis and other biological processes, keeping the temperature constant ensures that the experiments are unbiased.

METHOD Materials:

15 mint plant set-ups (15 Styrofoam cups; 5 individual plants in each cup) 5 cardboard boxes Cellophane (blue, red, green, purple, clear) Measuring cylinders Nutrient solution Plastic containers to hold cups Small supports to hold up the boxes Stanley knives to cut out the holes in the boxes Vermiculite

Procedure: 1. Some mint plants in Styrofoam cups were grown by placing five seeds of mint into a cup nearly full of Vermiculite. 2. 15 of these were set up. 3. A few holes in the bottom of these cups were punched to allow water to get into the soil. 4. These plants were grown for 2 – 3 weeks prior to commencing the experiments. 5. On day one of the experimental investigation, small plastic containers were used to house each Styrofoam cup of plants. 6. 100ml of nutrient solution was poured into each container and the cup was placed in the water in the container. 7. The height of the plants were measured and averaged for the trials. 8. The sides and top from five small cardboard boxes were cut out. 9. These gaps were covered in cellophane, a different colour of cellophane for each box. 10. Three containers containing the mint plants were placed under each box. These will be the three trials for each experiment. 11. Under the corners of each box were placed little supports to suspend the boxes a cm or so above the table to allow air to get in. 12. Over the next four to five weeks, the heights of the plants in cm were recorded on a weekly basis. 13. This data was then tabulated and graphed.

RESULTS Table 1. Table showing the height of plants over a 3 week time period. Colour of light Clear

Date of measurement

Pot Number (trials)

Height of plant (cm)

11/8/08

1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2

1 2 1 1.7 1.7 1.2 3.5 2.5 2.5 7.5 6 6 1 1 1 2 2 1.5 2.5 2 3.5 8 7.5 5.5 1 1 1 2 2 2 2.6 2.8 2.5 4.5 6 6.5 1 1 1 1.5 2 1.5 3.2 2.6 1.6 9 5.5 4.5 1 1 1 1 2.5 1.5 1.5 3.2

18/8/08 25/8/08 1/9/08 Red

11/8/08 18/8/08 25/8/08 1/9/08

Green

11/8/08 18/8/08 25/8/08 1/9/08

Blue

11/8/08 18/8/08 25/8/08 1/9/08

Purple

11/8/08 18/8/08 25/8/08

Average height (cm)

1.3 1.53 2.83 6.5 1 1.83 2.7 7 1 2 2.6 5.7 1 1.7 2.5 6.3 1 1.7

3 1 2 3

1/9/08

2 4 8 4.5

2.2 5.5

Table 2. Table showing the height of plants over time. Day Day1 11/8/08 Day 8 18/8/08 Day 15 25/8/08 Day 22 1/9/08

Clear 1.3 1.53 2.83

Growth of plants under colour (cm) Red Green Blue 1 1 1 1.83 2 1.7 2.7 2.6 2.5

Purple 1 1.7 2.2

6.5

7

5.5

5.7

6.3

Figure 3. Graph showing the heights of plants under different colour light over a 3 week period. Average height of plants (cm)

8 7 6

Clear Red Green blue Purple

5 4 3 2 1 0 Day1 11/8/08

Day 8 18/8/08

Day 15 25/8/08

Time in days

Day 22 1/9/08

DISCUSSION Earlier it was hypothesised that red and blue-purple light allow for the most growth in plants. This hypothesis was proved right in part. Red light did allow the plants to grow more and actually allowed the most growth. However, the plants subjected to the blue and purple light did not grow as much as hypothesised. As an even bigger contradiction of the original hypothesis, for the first 2 to 3 weeks, the plants subjected to green light grew more than the plants in other coloured light. The plants under the green light experienced very rapid growth. From the first day to the end of the first week, they experienced an average growth of 1cm, much more than the rest which grew only 0.8 or 0.7cm. This trend continued until a quarter way through Week 2. After that the plants under green light did not grow as much and at the end of the experiment, they had growth the least after the plants under purple. During the last week, if you look at Figure 3, the rate at which the plants under green grow is slower than the rest, even purple. This rapid growth at the beginning could be because the plants perceived the lack of absorbable light as that they were being overshadowed by other taller plants. To evade this problem, the plants grow rapidly so that they can have their leaves above the rest of plants and be exposed to more light. But after extended periods of not getting enough or any light, the plants are still unable to photosynthesise and do not grow very much. On Day 1 of the experimental investigation, it was observed that the average height of the plants under the clear colour box were 1.3cm, 0.3cm taller than the other plants. This may suggest that those plants have grown faster or more already. However, on day 8, when the plants were measured again these plants had grown on average less than the other plants in the other experiments. While all other plants had grown at least 0.7 to 1 cm, the plants under white light only grew an average of about 0.2cm. At this point, the hypothesis was correct proving that plants in white light don’t grow as well as in red or blue light. However, over the next 2 weeks the growth increased rapidly. This could have been because there may have been inconsistency in the amount of water made available to the plants. Also, since there were other setups on the table on which the experiments were set up, it is possible that got in the way of the light and not all of the plants got an even amount of light. This could have also happened because of the bushes and trees in front of the setups blocking some light from reaching the plants. After the experiments conducted for the plants under the purple light, it is observed that the plants do not grow tall in this colour. Almost every time the heights were recorded, the heights for purple were less than those for all other colours. This heavily countered the hypothesis that plants grow well in purple light. However, this could be because of the quality of the cellophane used in the experiment. The cellophane used in this experiment was slightly thicker than the cellophane used in the other experiments and this could have caused some change in the light intensity rather than just in light. This could have heavily impacted on the efficiency of the experiments. To improve the experiment, we could do more trials than just 3 and instead of having just 1 box for every colour, more could be done and placed in direct sunlight without things blocking off the light. More experiments must be conducted under a stricter controlled environment to obtain more data. Further experiments can be conducted investigating other factors such as light intensity. CONCLUSION In these experiments the relationship between the colour of light and the growth of plants was investigated. The experiments were conducted in a fairly controlled environment to improve their validity such as the intensity of light, the environment, the amount of nutrient solution provided,

and temperature. It was hypothesised that plants forced to grow in red and blue light would grow taller and plants in green would not grow as much. The results supported the fact that plants grow well in red and blue light but not in green. However, the results heavily disproved out hypothesis that purple allows for good growth of plants. More research must be done to further support these results but it is easy to see that light has profound impact on photosynthesis and the growth of plants.

Bibliography Ames, Marianne; Johnson, S Wayne, 2007. ‘A Review of Factors Affecting Plant Growth’. Viewed on 13 September 2008. [Online resource]. Available at: http://www.hydrofarm.com/articles/factors_plantgrowth.php Carter, J. Stein, 1996. ‘Photosynthesis’. Viewed on 4 September 2008. [Online resource] Available at: http://biology.clc.uc.edu/Courses/bio104/photosyn.htm

Evans, Barbara; Ladiges, Pauline; McKenzie, John & Spenceley, Maggie 2004, Biology: A Contextual Approach, Heinemann Education, Melbourne, Victoria.

‘Photosynthesis: The Role of Light’ 2008, Author unknown. Viewed on 14 September 2008. [Online resource] Available at: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/L/LightReactions.html

‘Photosynthesis’ 2008, Author unknown. Viewed on 11 September 2008. [Online resource]. Available at: http://en.wikipedia.org/wiki/Photosynthesis

‘Phycobilin’ 2008, Author unknown. Viewed on 17 September 2008. [Online resource]. Available at: http://en.wikipedia.org/wiki/Phycobilin ‘Plant’ 2008, Author unknown. Viewed on 4 September 2008. [Online resource]. Available at: http://en.wikipedia.org/wiki/Plant