- Describe phototropism and gravitropism
- xplain how to create conditions that support phototropism in the hydroponic system
Tropism is a natural phenomenon in which plants turn or grow in a direction in response to a trigger or stimulus. In plants, the most obvious tropism is phototropism in which plants sense light, trigger the redistribution of auxin, and then respond with growth that turns the seedling tip towards the light.
Auxin is a plant phytohormone that determines the response in plant growth. When auxin is concentrated in one area the plant triggers a growth response where cells will elongate allowing the stem to turn direction. It’s an amazing phenomenon!
But auxin plays a role in other plant processes as well including flowering, apical dominance, root development, and wound response. An increase or decrease in the concentration of auxin signals a response that mimics a rudimentary nervous system.
Gravitropism is another tropism where auxin plays a role. Gravity influences plants in two ways, one positive and one negative. Roots will grow towards gravity and the stem away from it. Auxin accumulates in both the upper part (stem and leaves) as well as the bottom half (roots) of the plant. The growth is directed by the auxin concentration. Positive gravitropism is the development of the root system and negative gravitropism is the directional growth of the stem and upper plant structure.
For growers using hydroponics in a vertical setup, the battle between phototropism and gravitropism plays out every day. We use lights to prompt horizontal growth from the vertical plane and yet the roots still respond to gravity.
It was Charles Darwin that first proved the existence of the directional gravitropisms but our understanding of the full involvement of auxin in the various plant processes continues to evolve. It’s intriguing to consider that something as simple as a seed can evolve and advance into something so complex.
Tools & Materials
- Seed starting mix
Introduction (10 minutes)
Begin the lesson by sowing seeds in a seed starting medium. This can be done a few days before the lesson with the students. When the seedlings have germinated and are an inch or more in height, introduce the lesson by defining “tropism” and asking students if they can name any forces in nature that might influence or turn plants in one direction or another.
Activity (20 minutes)
When the seedlings have germinated and are at least an inch in height, move the light to one side of the flat. Discuss as a class the response expected overnight. Will the plants move toward the light or does phototropism take more time? Repeat the experiment the following day by moving the light to the other side. Do the plants respond by turning back towards the light?
Discussion (10 minutes)
Phototropism is the response plants have to light. Gravitropism is the response to what influence? How can we observe the existence of gravitropism? Do these two tropism work in concert or do they compete at times?
Assessment (5 minutes)
Use the following questions to assess the Students before and after the lesson. Tally the responses of the group in the Assessment Tracking Log for comparison:
- By a show of hands, how many of you can describe how light influence the direction plants grow?
- Now that you know how phototropism works, how many of you think you can describe the process to someone else?
This lesson, and all other lessons on this website, are intended for use by teachers in the classroom. These lessons are protected by US and International copyright laws. Reproduction or distribution of lesson content, supporting materials, or digital creative is prohibited with written permission from Modern Steader LLC.