Phototropism and Gravitropism

Plants use stimuli to determine which direction to grow in.



©Russell Neches via flickr
A13.13 Lentil Seedlings growing toward light
Plants may not be able to see or hear, but they can still respond to the environment around them. They respond to a variety of stimuli, and they use these stimuli to guide how they grow. The two main stimuli plants respond to are light and gravity. Phototropism is how the plant responds to light, and gravitropism is how the plant responds to gravity. The stem of a plant will show positive phototropism and negative gravitropism. This means the stem grows toward light and away from the source of gravity. The roots of a plant will show negative phototropism and positive gravitropism. This means the roots grow away from light and toward the source of gravity.

  Problem-Solving Activity


Plants in Space


Over the past few years, there have been many experiments done to see if we can grow plants on space stations. It is very important that we are able to grow plants in space for longer trips, such as to Mars. There is not enough storage space to bring all the food needed, and it is beneficial to be able to eat fresh food. If we can grow food, it would make longer space trips easier.

One of the concerns with growing plants in space is how the roots know where to grow. We can simulate sunlight so the plants can still perform phototropism, but what about gravitropism? 


Write down what you think will happen to plant roots when we try to grow plants in space?

Click on the experiment tab to continue.

Watch this video titled “Space Station Live: Cultivating Plant Growth in Space” to learn about the experiments being performed around how roots grow without gravity.

Click on the analysis tab to complete the analysis questions.
 
©YouTube NASA Johnson    

Space Station Live:  Cultivating Plant Growth in Space

  1. What direction do roots grow in in space?

    It has been found roots still tend to grow away from the top of the plant (or down) in space.


  2. How does this result compare to your hypothesis?

    Your answer should be a variation of the following: My hypothesis was wrong. I believed the roots would not grow straight down but grow in a bunch of different directions without the pull of gravity to signal them. However, the results show the roots still grow down in space.


  3. How do scientists think roots know which way to grow in space?

    Scientists are still doing experimentations to figure out how roots know which way to grow. One theory is the roots use light (or lack of light) to know which way to grow. The roots grow from where there is light into the darkness.