written by Sam Humphrey
As astrobotanists, we get a lot of questions about the environmental conditions we need to grow plants in space. When we grow plants in our backyards, we consider many factors: soil, nutrients, light, and water, but we don’t usually worry about our plants getting enough air. In this article, we take a closer look at this important piece of the puzzle—because in space, even air is more complicated than it seems.
In the mid 1770’s, scientist Joseph Priestley to leave his teaching job and become a Presbyterian minister. Surprisingly, becoming a minister began a chain of events that led to Priestley making one of the most important scientific discoveries of all time: the discovery of oxygen. To test the “superior quality of this air”, Priestley placed a mouse into a jar of oxygen, and was quite satisfied to find that the mouse survived far longer in oxygen than it would have survived in common air. He knew that this was an important discovery, but he probably never would have guessed that over two hundred years later, oxygen would be one of the most important factors for human life support in space.
Air composition—that is, the relative amounts of elements in the air—has huge implications for life on Earth and in spacecraft. In decades of astrobotany research, air composition has posed several challenging problems for growing plants in space.
Ethylene and VOCs
In your elementary school science class you were probably taught that the air is about 78% nitrogen, 21% oxygen, and 1% other gases. This is true, but that 1% “other” category is much more important than you might expect. For instance, that 1% contains carbon dioxide, which drives global climate change and feeds photosynthesis for every plant on Earth. Although many of these trace gases (like nitrogen oxides and ethylene) don’t typically harm plants when they are outdoors, they can drastically affect plants that are enclosed in spaceflight environments.
In most cases, the trace gas ethylene is useful for plant development: it acts as a plant hormone that promotes fruit ripening. You can even harness ethylene in your own home to control fruit ripeness! For example, if you leave a ripe banana next to an unripe one, the other will ripen faster because the ripe banana is releasing ethylene into the air. However, ethylene can be problematic in closed environments like space stations, where it can build up and damage growing plants. Aside from ripening fruits, ethylene also restricts plant growth and causes plants to drop their leaves—bad news if you’re trying to get crops to grow in space. Astrobotanists first faced this problem when they tried to grow wheat in the Svet growth chamber, on the space station Mir. They grew healthy wheat with plenty of seed heads, but there was just one problem—there were no seeds! After months of further testing they finally figured out that ethylene was to blame for the lack of seeds. Luckily, this problem was easy to solve in future growth chambers by adding ethylene filters to scrub the ethylene from the air.
Volatile organic compounds (VOCs) are another problematic air contaminant that can stunt plants in space. VOCs are emitted by plastic materials and by living things, including plants and astronauts. VOCs often build up in controlled environments, and can cause stunting, leaf curling, yellowing, lesions, and other symptoms that can reduce crop productivity. To keep our space plants healthy and happy, we use air filters to scrub the VOCs out of the cabin air.
Years after Joseph Priestley’s discovery of oxygen, he happened to stumble across photosynthesis. His mice didn’t survive when placed in an airtight chamber for a few hours, but he found that by adding a plant to the chamber, the mice survived much longer than he expected! Priestley learned three things from this experiment: 1. plants produce oxygen as a byproduct of photosynthesis; 2. the oxygen mixes into the air; and 3. the mouse inhales oxygen to survive. But in space, gases don’t mix the same way they do on Earth. On Earth, gravity-driven convection allows the molecules in the air to mix due to their relative heat or density. For example, heated gases rise and cooled gases fall. Denser gases move downward, and less-dense gases float upward. However, in microgravity environments like on the International Space Station, there is no “up” or “down”, so there is no gravity-driven convection. This means that an unmoving astronaut will develop a “bubble” of carbon dioxide around their head, and after a few minutes they’d need fresh air to breathe. To make up for the lack of gravity-driven convention we apply forced convection by mixing the air with fans, delivering fresh air to the astronauts. We do this for plants too, preventing that “bubble” of oxygen from developing around their leaves. Plants need carbon dioxide for photosynthesis, so if we allowed a “bubble” of oxygen to accumulate, the plants wouldn’t be able to photosynthesize, grow, or develop. This convection challenge is something that NASA scientists and engineers must keep in mind when they’re designing plant growth chambers for spaceflight. They must include enough fans in the growth chamber to mix the air for plants to grow.
Although we find this topic fascinating, we’re glad we don’t have to worry about our gardens on Earth getting enough fresh air to grow!
Astrobotany is still a relatively young field of science. In the past few decades astrobotanists have discovered some problems with ethylene, VOCs, and gas movement in space, and we’ve found ways to solve them with air filters and fans. However, we must remember that we’ll keep discovering new challenges as we move forward through the solar system. We study air composition in plant growth chambers to make sure we’re prepared to send plants on long duration missions to Mars and beyond.
- Joseph Priestley: The man who discovered Oxygen
- Earth’s Atmosphere: Composition, Climate & Weather
- Ethylene: Chemical Compound
- Growth Chamber Handbook: Chapter 5
- The biophysical limitations in physiological transport and exchange in plants grown in microgravity
- Farming in space: Environmental and biophysical concerns
- Plant mineral nutrition, gas exchange and photosynthesis in space: A review
- Ethylene Production throughout Growth and Development of Plants
- Plant growth during the greenhouse II experiment on the Mir orbital station