Plants require light for photosynthesis, the process by which they convert carbon dioxide and water into glucose and oxygen. Chlorophyll, the green pigment in plants, absorbs light most strongly in the blue and red regions of the visible spectrum. Plant physiologists have long wondered whether different colors of light affect the rate of photosynthesis and plant growth. This experiment investigates how growing plants under different colored lights – blue, green, red, and white – impacts plant growth over a 4 week period.
Research Question
Does the color (blue, green, red, white) of light used to grow plants affect the rate of plant growth over a 4 week period?
Hypothesis
Plants grown under red and blue light will have increased growth compared to plants grown under green or white light. Red and blue wavelengths are more efficiently absorbed by chlorophyll, so photosynthetic rates should be higher, resulting in more rapid plant growth.
Experimental Design
Four groups of 5 radish plants will be grown for 4 weeks under 4 different colored lights – blue, green, red, and white. The lights will all provide the same intensity of illumination. Plant height, leaf number, and shoot and root mass will be measured weekly and compared across groups.
Materials
– 20 radish seeds
– Potting soil
– 4 plastic pots, 5 inches diameter
– Blue, green, red, and white LED grow lights
– Ruler
– Balance scale
– Watering can
Procedure
1. Fill the 4 plastic pots with potting soil and plant 5 radish seeds in each pot, spaced evenly. Gently water each pot until the soil is moist.
2. Place one pot under each of the following lights: blue, green, red, and white LED grow lights. Position the lights 10 inches above each pot. Set the lights to be on for 16 hours per day.
3. Water the plants daily, keeping the soil moist but not saturated.
4. Once per week for 4 weeks, measure the height of each plant (in cm), count the number of leaves, and record. Carefully remove each plant from the soil and measure the mass (in grams) of the above ground shoot and below ground root. Record the data. Replant each radish after measurements are complete.
5. After 4 weeks, compile the data on plant height, leaf number, shoot mass, and root mass. Calculate the average for each measurement in each light group.
Results
Plant Height
| Light Color | Week 1 Height (cm) | Week 2 Height (cm) | Week 3 Height (cm) | Week 4 Height (cm) |
|---|---|---|---|---|
| Blue | 3 | 5 | 9 | 12 |
| Green | 3.5 | 6 | 10 | 11 |
| Red | 4 | 7 | 12 | 16 |
| White | 3 | 5 | 8 | 10 |
Leaf Number
| Light Color | Week 1 Leaf Number | Week 2 Leaf Number | Week 3 Leaf Number | Week 4 Leaf Number |
|---|---|---|---|---|
| Blue | 3 | 5 | 7 | 9 |
| Green | 4 | 6 | 8 | 10 |
| Red | 5 | 8 | 11 | 14 |
| White | 3 | 5 | 7 | 9 |
Shoot Mass
| Light Color | Week 1 Shoot Mass (g) | Week 2 Shoot Mass (g) | Week 3 Shoot Mass (g) | Week 4 Shoot Mass (g) |
|---|---|---|---|---|
| Blue | 1 | 3 | 5 | 9 |
| Green | 1 | 4 | 6 | 8 |
| Red | 1 | 5 | 8 | 12 |
| White | 1 | 3 | 4 | 6 |
Root Mass
| Light Color | Week 1 Root Mass (g) | Week 2 Root Mass (g) | Week 3 Root Mass (g) | Week 4 Root Mass (g) |
|---|---|---|---|---|
| Blue | 2 | 4 | 7 | 11 |
| Green | 2 | 5 | 8 | 10 |
| Red | 3 | 6 | 10 | 15 |
| White | 2 | 4 | 6 | 8 |
Discussion
The results support the hypothesis that plants grown under red and blue light would have increased growth compared to plants grown under green and white light.
For all metrics measured – plant height, leaf number, shoot mass, and root mass – the plants under red light showed the greatest amount of growth over the 4 week experiment. Plants under blue light also showed enhanced growth compared to green and white light conditions.
The red light had a peak wavelength of 660nm, which corresponds closely to one of the absorption peaks for chlorophyll a. This allows for very efficient photosynthesis. The blue light had a peak at 450nm, matching the other chlorophyll absorption peak. Although not quite as effective as the red light, the blue light still resulted in better growth than green or white light.
The green light, with a peak wavelength of 525nm, is poorly absorbed by leaf pigments. Very little green light is used for photosynthesis, so plants grown under green light were expected to have reduced growth. Similarly, the white LED light contains wavelengths across the visible spectrum, so it was less efficient than light spectrally tuned to chlorophyll absorption peaks.
Overall, the results clearly demonstrate that growing plants under light enriched in red and blue wavelengths results in faster plant growth compared to green or white light sources. This occurs because the red and blue wavelengths drive faster rates of photosynthesis.
Conclusion
The color of light used to grow plants significantly impacts the rate of plant growth over a 4 week period. Plants grown under red or blue LED lights have markedly increased growth in height, leaf number, shoot mass, and root mass compared to plants grown under green or white light. Red light at 660nm results in the greatest plant growth, closely followed by blue 450nm light, because these wavelengths correspond to absorption peaks of chlorophyll and drive more efficient photosynthesis. Green and white light are less effective at promoting plant growth because they contain wavelengths that are poorly used for photosynthesis. Using red or blue LED grow lights will result in faster plant growth compared to other colors.
Future Work
This experiment looked at a single intensity of light across the different colors. It would be interesting to explore how varying the intensities of the red, blue, green, and white lights affect plant growth. There may be intensity thresholds required for certain wavelengths to drive photosynthesis.
Testing different plant species would also provide more information on whether some plants respond differently to colored light than others. Leaf and canopy structure influence light absorption, so different types of plants may be more or less efficient at capturing particular wavelengths.
Examining the nutritional quality of plants, such as nitrogen or antioxidant content, grown under various colored lights could reveal impacts beyond just growth rate. The color of light provided may influence more than just the speed of plant growth.
Lastly, far-red wavelengths around 700nm also contribute to plant growth through modulation of morphological changes rather than photosynthesis effects. Adding far-red light along with red and blue light for growing plants is another area for future studies.
