Plants harness energy from the Sun through the process of photosynthesis, which converts light energy into chemical energy. Plants then store this chemical energy within their molecular bonds as sugar. To fuel photosynthesis, plants need light, water, and carbon dioxide. Sunlight isn’t the only light that can fuel photosynthesis. Scientists have been experimenting with how to grow plants under “artificial” conditions in space using light-emitting diodes (LEDs) instead.
The chlorophyll within plants absorbs red and blue light. (Chlorophyll can’t absorb green light; that’s why green light passes through to our eyes when we look at plants.) Scientists have discovered that some plants need only red light for photosynthesis, so NASA is experimenting with using red LEDs to grow plants in a lab.
“Tropisms” are growth movements living organisms have toward certain external stimuli. Plants on Earth have three tropisms: phototropism, geotropism, and hydrotropism. These refer to plants’ responses to light, gravity, and water. In a space-based lab, each of these tropisms presents a challenge. The need for light can be solved with color-specific LEDs, but since gravity conditions in space are very different that they are on Earth, scientists are experimenting to figure out how to grow plants that can orient themselves in space. Blue light, though not crucial for photosynthesis, does seem to provoke plants to grow in specific directions and with specific morphology. Watering plants in space presents an engineering challenge that scientists are still solving. Here on Earth, water flows “down” so it’s easy for plants to “capture” water in the dips and crevices of their leaves. In fact, many plant adaptations evolved to help plants gather and hold water as it drops, flows, or falls. In the microgravity of space, though, water must be delivered to the plants so it gets continuously absorbed, or else it would just float away.
In their explorations of growing plants in space, scientists are paying special attention to plants with high antioxidant levels. Antioxidants are chemical substances that protect human cells against free radicals, which are potentially damaging molecules formed during the breakdown of certain foods in the digestion process, during exposure to radiation, and during other physical processes. Scientists believe free radicals play a role in cancer, heart disease, and other illnesses, so allowing astronauts to grow an especially healthy “space garden” could be beneficial during a long trip into space.
Classroom Activity: Plant Growth Light Test
This is a longer-term classroom activity that takes place over days and weeks rather than in one class session. With guidance from the teacher, students set up four trays of plants with 3 or 4 plants in each tray. Each tray is lit with a separate colored light, and all trays are kept under or in an enclosure to block natural sunlight. All plants receive water according to the same schedule and in the same amounts. Students should track and log all instances of watering.
Before beginning the experiment, the teacher and students come up with a set of plant growth metrics – height, girth, subjective appearance, etc. Using the measuring tool and the camera, students take periodic measurements of each plant. The class can periodically check in to review and assess the data, and, at the end of the experiment, students analyze their data, discuss their findings, and reflect on the results. Note: During the experiment, plants that obviously aren’t thriving can be “rescued” before they die and can be transferred to an area where they can receive natural light.
BLAIR: What do you actually do in the lab with plants?
DR. WHEELER: In this case, we’re doing preliminary setups of some plant growth approaches that we think might work in space, like on the International Space Station. Then we’ll do follow-up tests in controlled environment chambers, larger chambers where we can set the plants out and control the temperature and the light intensity. If you think about space, you have certain constraints. You have to contain water for example. How do you water a plant in weightlessness? It’s not a trivial issue. So, you have to be able to contain that and come up with techniques to provide water to the roots.
BLAIR: I noticed earlier you were changing the color of the light.
DR. WHEELER: In this case, we’re using LEDs, light-emitting diodes. As you know light-emitting diodes, LEDs, usually have very discreet colors. That is just intrinsic to their nature. We have used combinations of them to make different spectral composition comparisons.
BLAIR: You, actually, can grow a plant with only one color, red?
DR. WHEELER: Yes. You can. Red light is a good light source for photosynthesis. Chlorophyll absorbs it so it drives the photosynthetic reactions. But if you use red light only, the plants tend to be leggy. But if you add some blue light, they behave much more normally in terms of their morphology. They stay more compact and normal looking. Another thing is you need blue light to orient the plants toward the direction of light. That’s important in space because you don’t have a gravitational clue anymore for the plants to grow against the gravity. So you have to orient them with light. And you need blue light to do that.
One of the factors we are trying to incorporate into our testing and our strategies is to choose plants that are high in antioxidants. Those are compounds that can repair damage in your cell tissue, your DNA. If there’s radiation damage, for example, can we add fresh foods to the diet that could serve as the radiation counter measure? It’s a high-radiation environment. So the astronauts are exposed to this. Can we augment the diet with something that would give them a measure of protection in living in that environment? As the missions go farther, and stay longer, then, of course, if you can begin to expand these systems, now you can generate oxygen. You can remove and reduce the carbon dioxide. And you could couple wastewater treatment systems with these plants as well. You could do multiple life support functions as you begin to scale these up.
BLAIR: Well it’s funny, as important as I think plants are to the aesthetics of living, it seems there are lots of ways that strategically they can be used to help the astronauts.
DR. WHEELER: All the oxygen we’re breathing right now on the surface of the earth was generated through photosynthesis.
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