Grass Turning Yellow: Chlorosis
Does your grass like to have a top hat, or does it turn a little yellow at the thought? Are you in the dark about what plants need to grow well? Add and subtract light from your lawn and see if the grass keeps growing green.
Problem: Why does grass turn yellow?
- 7 large yogurt containers
- Hobby knife
- Cutting mat
- 1 square foot each of red, blue, and colorless cellophane
- Clear packing tape
- Large nail
- Safety goggles
- Recruit an adult who can help you cut off the bottoms off three of the seven yogurt containers using a knife and a cutting mat. (Make sure that your adult wears proper safety gear!)
- Cover one hole with red cellophane, one with blue cellophane, and one with colorless cellophane. Use the packing tape to secure the cellophane to the containers. Set them aside.
- Take two of the remaining containers. Using your large nail, press it through the bottom and sides of one of the yogurt containers until the container is full of holes. Set this container aside as well.
- Finally, ask an adult to use the knife one more time to cut semicircular flaps on the sides of one of the two remaining yogurt containers. You should be able to open these flaps a little, but they should not open so wide that a lot of light comes into the container.
- Now, choose a particularly green piece of lawn. Make sure that the grass is short enough that the containers can be placed solidly on the ground without falling over.
- Place your containers upside down in a row on the lawn, beginning with the single container that you didn’t change. This container is your control—it has no hole in the bottom to let light in. When you’re finished lining up the containers, you’ll have a row of containers with their bottoms up, some with light shining through the cellophane and some with air moving through holes in the containers.
- Leave your containers on the lawn for two weeks. After that time, come out and write down your observations of the grass under each container. Take a photo if you wish. What happened to the grass under each container?
The grass under the colorless cellophane will grow best, as it receives the full spectrum of light. Plants growing under red light are thin and spindly, while the grass grown under blue light is short and stocky. Grass that doesn’t receive any light will eventually turn yellow.
If you leave grass alone underneath a yogurt container, it will turn yellow. Why does this happen? While people go to the grocery store to buy food so that they can survive and grow, plants can make their own food through a process called photosynthesis. This process involves water, carbon dioxide, and light, and is controlled by chloroplasts, tiny organ-like structures (or organelles) that contain a green pigment called chlorophyll. Chloroplasts also contain other pigments like xanthophylls (yellow) and carotenes (orange).
You’ve likely seen these non-green pigments at work before! If you live in a place with deciduous trees, in the fall you will see the leaves changing color. While they’re working on making food in the spring and summer, the leaves green becaue of all the chlorophyll—but as the days get shorter and darker, the chlorophyll production slows down and the other pigments become more visible. This is why you see yellow and red leaves.
The same thing happened to your grass in the dark. Grass turning yellow can be explained this way: when the grass was no longer exposed to sunlight, chlorophyll production slowed and pigments like xanthophylls became visible. This loss of green is called chlorosis.
During photosynthesis, plants also absorb carbon dioxide while releasing oxygen. Plants still need to absorb some oxygen to help them break carbohydrates down into energy. At night, plants take in a little oxygen from the air—about a tenth of what they produce in the daytime. Over time, a plant in a place that’s completely airtight might have difficulty balancing carbon dioxide and oxygen in its environment. When you added yogurt containers with flaps and holes, this allowed oxygen to enter the container even though there was not a lot of light. Did you notice much of a difference in plant growth? Now, let’s think about what happened to your plants growing under the cellophane filters. Light has all of the colors of the rainbow in it, and plants need all of those colors to grow well. Plants without the full spectrum will grow in strange ways. Plants will grow tall and spindly in red light. Blue light controls leaf development, so plants will be short and stocky. Did this happen to your grass?
What would happen if you tried the same experiment with a different plant? Do some plants respond better than others to low or odd light conditions?
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