NRC Research Associate, Kennedy Space Center, FL, USA.
Studies were undertaken to evaluate the use of light-emitting diodes (LEDs) as an alternative light source for plants in space flight. We investigated the effects of narrow spectrum LEDs on photosynthesis and carbohydrate metabolism in wheat leaves (Triticum aestivum L. cv. Superdwarf). Plants were grown under red LEDs (660 nm) and compared to plants grown under daylight fluorescent (white light), red LEDs + 3% blue fluorescent light (BL), and red LEDs + 15% BL. Compared to white light, leaf photosynthetic CO2 uptake rates were 75% lower in plants under red LEDs, 45% lower under red LEDs + 3% BL, and 37% lower under red LEDs + 15% blue light. During vegetative growth, leaf starch concentrations were two-fold greater under the red LEDs than white light, but at pre-anthesis and during grain development, leaf starch concentrations were similar between the light treatments. Compared to white light, plants under red LEDs had higher activity levels of ADP-glucose pyrophosphorylase, a regulatory enzyme in starch synthesis, at all developmental stages. Sucrose concentrations were 25% lower in leaves under red LEDs compared to white light during vegetative growth, pre-anthesis and grain development. The activities of two regulatory enzymes in sucrose synthesis, fructose-1,6-bisphosphatase and sucrose phosphate synthase, were 50% lower under red LEDs. Plants grown under red LEDs + 3% BL and red LEDs + 15% BL had carbohydrate and enzyme activity levels less than or similar to plants under white light. In summary, the wheat plants grown under red LEDs had lower rates of photosynthesis and differences in carbohydrate levels compared to white light-grown plants. These alterations in leaf starch and sucrose concentrations may be a function of spectral quality-dependent changes in the activity of rate-limiting enzymes in starch and sucrose synthesis.

Keywords:

• Light
• Photosynthesis
• Plant Leaves
• Plants
• Space Flight
• Starch
• Triticum
• NASA Center KSC
• NASA Discipline Life Support Systems
• NASA Discipline Number 61-20
• NASA Program Advanced Life Support

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Growth and photosynthesis of Chinese cabbage plants grown under light-emitting diode-based light

 Abstract  We compared growth and the content of sugar, protein, and photosynthetic pigments, as well as chlorophyll fluorescence parameters in 15- and 27-day-old Chinese cabbage (Brassica chinensis L.) plants grown under a high-pressure sodium (HPS) lamps or a light source built on the basis of red (650 nm) and blue (470 nm) light-emitting diodes (LEDs) with a red to blue photon ratio of 7: 1. One group of plants was grown at a photosynthetic photon flux (PPF) level of 391 ± 24 μ mol/(m2 s) (normal level); the other, at a PPF level of 107 ± 9 μ mol/(m2 s) (low light). Plants of the third group were firstly grown at the low light and then (on the 12th day) transferred to the normal level. When grown at the normal PPF level, the plants grown under LEDs didn’t differ from plants grown under HPS lamps in shoot fresh weight, but they showed a lower root fresh and dry weights and the lower content of total sugar and sugar reserves in the leaves. No differences in the pigment content and photosystem II quantum yield were found; however, a higher Chl a/b ratio in plants grown under LEDs indicates a different proportion of functional complexes in thylakoid membranes. The response to low light conditions was mostly the same in plants grown under HPS lamps and LEDs; however, LED plants showed a lower growth rate and a higher nonphotochemical fluorescence quenching. In the case of the altered PPF level during growth, the plant photosynthetic apparatus adapted to new conditions of illumination within three days. Plants grown under HPS lamps at a constant normal PPF level and those transferred to the normal PPF level on the 12th day, on the 27th day didn’t differ in shoot fresh weight, but in plants grown under LEDs, the differences were considerable. Our results show that LED-based light sources can be used for plant growing. At the same time, some specific properties of plant photosynthesis and growth under these conditions of illumination were found.

Key words  Brassica chinensis - high-pressure sodium lamp - light-emitting diodes - light quality - light intensity - growth - photosynthesis - productivity 

What do plants need to grow?

By definition, a plant is a living thing that produces its own food through photosynthesis. This process uses carbon dioxide and water. Trapping light from the Sun, plants are able to change sunlight’s energy into useable chemical energy. Not only is chemical energy produced, but oxygen is a by-product of photosynthesis. Plants are essential to the balance of life on Earth … and to life, as we know it, on other planets.

Plants may play an important part in NASA’s Vision for Space Exploration. When human beings establish permanent bases on the moon or Mars, they will continue to need food, water, and air, but because those items aren’t widely available on other planets, keeping a ready supply on hand will be a challenge.
"We operate our space missions somewhat like a picnic right now," says Dr. Bill Knott, chief scientist for biological programs at the Biomedical Office at NASA Kennedy Space Center (KSC). "We pack up everything we need, use it while we’re away, and then bring home the trash. When we spend extended periods in space, we need to be able to produce, reuse, and recycle what we need without such a great reliance on Earth."

"The solution to extended life support," Knott says, "is simpler than you might think. Rather than packing the Orbiter full of precooked meals and other vital supplies, some day soon the cargo might be full of seedlings and gardening supplies."

"People have several basic needs: oxygen, water, food, and waste removal. "Growing plants in a closed system takes care of all those concerns," Knott says.

After all, Earth is a closed system, even though we can’t see the walls of the system. When human beings consume food, it’s oxidized and carbon dioxide (CO2) is given off as waste. Plants use the CO2, along with water and minerals, to generate new food and give off more oxygen.

"Plants also help with other human conditions," Knott says. Plants can take wastewater and filter out products and organisms that are harmful to people, leaving clean water in their place. They also provide an aesthetically pleasing and emotionally comforting atmosphere. "Astronauts enjoy working with plants," he says. "It brings a bit of home along with them into space."

Scientists at NASA’s Ames Research Center, Johnson Space Center, and Kennedy Space Center have been working out the details of growing plants in space to help human beings live independently of the Earth. The Advanced Life Systems (ALS) program has produced encouraging results.

"We've been pleased with the findings so far," says Ray Wheeler, plant physiologist at KSC. "Plants are a viable option for life support. They’re reliable and predictable if their environmental needs are carefully controlled; the plants themselves rarely malfunction. Most problems we encounter are because of a poorly functioning pump or a computer glitch, not because of a plant problem. This news is all very encouraging."

Some fundamental scientific studies are being conducted with plants on the Space Shuttle and the International Space Station (ISS) because a space environment helps show the effects of low gravity on plant growth. But these studies are typically too short to thoroughly explore the full extent of crop growth and development for the ALS program.

"ALS will need to follow a plant’s entire growth cycle to assess the crop yields," Wheeler says. "Even when we choose plants with short life cycles to optimize growth and production, such as salad crops like lettuce, it takes a while for the entire cycle to be complete. Small plantings could make a significant contribution to diversifying the crew diet, even on the ISS, but a system with plants would be more economically feasible for a space colony or long-duration trip to Mars."

The plants chosen would be ones that produce the most edible materials with the least waste, grow the quickest, and are the easiest to maintain. We may not need soil to grow these plants because hydroponics methods have been used successfully for years, but even hydroponics has its limits in space.

"You can’t have open containers of water to grow plants in," says Knott. "In microgravity, water wouldn't stay where you put it. Instead, astronauts would likely use water-soaked materials, including cloth strips, to provide the moisture to the plants."

Light is another significant factor in growing plants in space. On the Space Shuttle, there are extended periods of darkness, and in a space colony there could be even longer times of darkness. Artificial lighting requires thousands of watts of electricity to grow plants, and even natural light needs to be collected, stored, and channeled efficiently. Scientists are experimenting with light-emitting diodes as a form of light that is energy efficient as well as inexpensive to use.

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