Lettuce, spinach, radishes, strawberries, and even Central American shrubs grown under a combination of green, blue, and red LEDs could play an important role in the future of space exploration and alternative energy.
Purdue’s Gioia Massa, from left, Cary Mitchell, and Judith Santini found that a particular type of strawberry seems to meet NASA guidelines for foods that could be grown in space. (Purdue Agricultural Communication photo/Tom Campbell)
But cultivating plants at near zero-gravity and without the help of the sun is certainly not the same as growing corn in Iowa or potatoes in Idaho.
"A major challenge to growing plants in space will be controlling and supplying sufficient quantity and quality of light," the U.S. National Aeronautics and Space Administration (NASA) said in a 2003 Research and Technology brief. "Conventional lighting technologies would prohibit growing plants on a large scale in space due to low electric power conversion efficiencies."
Put another way, conventional agricultural lighting solutions, which are sometimes called sun lamps or artificial-sunlight lamps, devour energy and give off far too much heat to be effective in space applications.
Identical lettuce plants under arrays of red (NASA)
"LEDs are a promising electric light source even for space-based plant growth chambers and bioregenerative advanced life support because of their small mass and volume, solid state construction, safety, and longevity," wrote Duong Tan Nhut and Nguyen Ba Nam of the Vietnam-based Tay Nguyen Institute of Biology in their January 2010 paper.
In addition to being energy efficient and relatively cool, LEDs can provide plants with the proper quality of light, too.
"LEDs can illuminate near the peak light absorption regions of chlorophyll while producing virtually no near-infrared radiation (which is not [used] in photosynthesis)," NASA said.
LEDs work where "conventional" lighting fails
The challenge for LEDs is not whether or not they can help plants grow in extraterrestrial greenhouses or farms but rather which combination of LEDs – red, blue, and green – work best with which plant species and cultivars.
According to Nhut and Nam, when only one color of LED is used to induce plant growth, there can be significant imbalances such as thin and elongated petioles or stems, yellow or otherwise discolored leaves, and even poor rhizogenesis (root growth). But some plants grown under a combination of blue and red LEDs showed normal growth.
Earlier NASA experiments with salad-type plants also used red and blue LEDs in combination to generate relatively good results. Specifically, the LED arrays in the NASA tests included red LEDs with peak wavelengths of 664, 666, 676, and 688 nanometers (nm) along with a row or rows of blue LEDs emitting a wavelength of 474 nm.
Interestingly, a study from the Universal Society for Horticultural Science also used LEDs with peak wavelengths from 460-688 nm when it researched the potential for narrow band lighting in earthbound greenhouses.
Green LEDs, which are mentioned in more detail below, help make space plants appear more edible and green.
LED array used on the International Space Station (NASA Marshall Space Flight Center photo)
LED testing is the next step for the Seascape variety of strawberry. In May, Purdue researchers Cary Mitchell and Gioia Massa identified the Seascape strawberry, which requires little light and only minor maintenance, as a good candidate for the space program’s first sweet-tasting crop.
The Seascape strawberry requires 10 hours per day or less of light and meets NASA’s mass and waste requirements since the berries are small relative to other fruit and almost entirely consumable. The Seascape variety also tends to grow fewer, larger berries, which is important in space cultivation since there are no insects or gentle breezes to spread pollen. Rather, astronauts will have to manually pollinate the plants like the Seascape, so fewer individual berries are actually better as long as the yield remains the same.
Biofuel producing Jatropha curcas
Another potential space crop is the Jatropha curcas, an oil producing plant. Researchers at the University of Florida and Zero Gravity, Inc. in Stevensville, Md. believe that growing J. curcas in microgravity could improve the plant’s cell structure and speed the production of new cultivars. These new and improved Jathropa plants could then be planted on earth and used to make biofuels more efficiently.
Green LEDs make plants more attractive, improve plant growth
NASA has also been studying the value of green LEDs for growing plants in orbit.
"The combination of red and blue light proved to be an effective lighting source for growing spinach, radishes, and lettuce. However, the addition of green light may promote increased plant growth since green light penetrates the plant canopy better than red or blue light. Leaves in the lower canopy could use the transmitted green light in photosynthesis. For a space mission, highly optimized lighting systems are necessary to conserve power and maximize plant growth, so supplemental green light needs to be evaluated," NASA said, adding that, "green light may also have additional psychological benefits for the crew. Plant leaves readily absorb red and blue light, so absorptance is high and reflectance is low. Therefore, even healthy plants grown under red and blue LEDs alone appear gray or black to humans. Hence the addition of green LEDs to red and blue LED arrays makes plants appear green to the crew."
About the Author
Armando Roggio is a technology and marketing professional with over a decade of experience in the electronics industry. Armando has written frequently about the lighting industry and is currently participating in an ongoing lighting research and publication project focused on LEDs. Armando has served in key roles at Micron Technology, worked as a consultant for Aptina Imaging, and consults with two startups.
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