At-Home STEM Activities: Plants and Gravity
In sci-fi space films and tv shows we often see plants grown for food, research, or décor, and plants are so common in our daily lives that it’s easy to assume that they would be our companions as we explore and live in space. But plants face their own challenges to survival in zero gravity.
Plants grow in so many challenging environments, we might think life in space wouldn’t present that many challenges. Photo by M. Zapletal
Plants need access to water, carbon dioxide, nutrients, and an energy source to grow- let's assume that all of these things are covered for a space journey: we have a hydroponic growing system set up (like the one in the Lunar Colony at the McAuliffe-Shepard Discovery Center) and the water in the system includes targeted fertilizer for our plants, there are many people and other animals spending time around the plants to respire the needed carbon dioxide, and we’ve either set up grow lights or we’re oriented to the Sun or another star so that the plants get hours of light each day. Would our plants grow?
Well, that’s the question- some varieties of lettuce, kale, cabbage, even zinnia flowers have already been cultivated for astronaut use on the International Space Station (ISS). But there are many species that haven’t been tested in that environment yet and growing in zero gravity creates a few problems for plants. Because fluids tend to form bubbles in zero gravity, a misplaced bubble could leave a plant parched or drowning, so you need some kind of growing medium that can ensure a steady level of access to water- on the ISS they often use clay.
These zinnias grew and bloomed on the ISS, but also had trouble fighting mold when over-watered in an enclosed environment. Photo courtesy of NASA
There are concerns that growing in zero gravity lessens a plant’s ability to fight off disease- plants grown in space experience more stress from oxidation and their immune systems may be less able to fight off viruses, bacteria, and other pathogens. Researchers are also exploring whether growing in zero gravity makes the lignin in plants less sturdy in the same way that human bones and muscles loss strength in space.
And then there is the question of what plants grown in space will look like- you may have seen seedlings started on a windowsill that lean toward the glass. While plants do use light as a cue for which direction to grow, they rely much more on gravity to help establish roots going down and the shoot going up- this is called gravitropism. Gravity helps distribute certain chemicals within the growing plant which guide growth- without that guidance, roots grow in every direction. To deal with this on the ISS, seeds are glued in a specific direction to the material they will grow on so that roots will point “down” and the shoot will go “up.”
Before spending 4 hours on its side in a dark closet
You can demonstrate the importance of gravity on the direction of plant growth if you have access to a seedling in a small pot.
Take a picture of the seedling at the start so that you have it for reference
Place the pot on its side in a dark closet (you want to remove any growth cues from light) for 4 hours
Compare the orientation of the top of the seedling before and after the time on its side- you'll notice that the stalk hasn’t moved, but the top of the plant is now curved toward the new ‘up’ side- this is because cells in different locations are growing at different rates in response to the new set up of chemical signals
After 4 hours, the top of the seedling is growing against gravity toward the new ‘up’
As astronauts and researchers on the ground work with more plant species under low- and zero-gravity conditions, we’ll have more effective strategies for growing plants in space for food, scientific experiments, green spaces, and many other options.
