Extension
Grow Plants Under Light of Different Wavelengths
DISCUSSION
• How could they isolate the electric current of each light source in order to deliver only a section of the Sun’s radiation spectrum?
(Answer: use coloured cellophane (red, blue, and green) to cover each light globe.
• How could the comparison be made with the plant’s growth in natural light? i.e. What will be the wavelength control?
(Answer: have one compartment providing the full spectrum of light, not masked with cellophane).
• How would they control for the variability in growth rate of seedlings?
(Answer: (a) have 3 seedlings in each pot, averaging the results at the end of the experiment. And (b), keep growing conditions identical, with equal volumes of water to each pot throughout the experiment.
Activity
AIM
To demonstrate the absorption wavelengths that plants use to photosynthesise.
MATERIALS
• Ask the school workshop to design and build a wooden box with 4 compartments, with each compartment large enough to accommodate the pot. Each compartment is fitted with a light source (see Figure). Make sure globes are labelled ‘warm’ – this category provides full artificial sunlight.
• cellophane paper, red, green and blue.
• 12 lettuce (or tomato) seedlings. Prepare the pots 2 or 3 weeks before. Each pot has the same soil, planted with 3 seedlings in each. Keep moist, with equal volumes of water applied to each before and during the experiment.
• At approximately 5-10cm of green growth, the seedlings are ready for the experiment.
• Water.
METHOD
1. Prepare the light sources as follows: In compartment 1, cover with red cellophane, compartment 2 with blue cellophane, compartment 3 with green cellophane, and leave the light source in the 4th compartment, the Control, free of cover.
2. Measure the heights of the seedlings, record, and photograph their colour with iPads.
3. Place a pot in each compartment and cover the front of the box so that no natural light can enter.
4. Switch the power source on during the school day and off during home time.
5. Water the plants every 2nd day with equal small amounts of water to each pot.
6. Note the changes in growth, both the height of the plants and their colour. When there are marked changes (perhaps by the end of the 3rd or 4th week), record the results.
RESULTS
1. Measure the new height of each plant exposed to each wavelength band.
2. Average the results, arriving at a growth rate under red wavelength, green wavelength and blue wavelength.
3. Remove seedlings carefully, rinse roots under running water and weigh each plant.
4. Average the results for each pot, and compare.
5. Which plants grew the most?
6. How did their growth compare with those grown under green wavelength?
7. How did their growth compare with the Control?