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Radish (Raphanus sativus L.), green onion (Allium fistulosum L.), and lettuce (Lactuca sativa L.) are among several “salad” crop species suggested for use on the International Space Station (ISS) as a supplement to the crew’s diet. Among the more important factors affecting the crop yields will be the light intensity or photosynthetic photon flux (PPF) used to grow the plants. Radish (cv. Cherry Bomb), green onion (cv. Kinka), and lettuce (cv. Flandria) plants were grown for 35 days in growth chambers at 8.6, 17.2, and 26 mol m−2 d−1 (150, 300, or 450 μmol m−2 s−1 PPF, respectively) with a 16 hr photoperiod and cool-white fluorescent lamps and either 400 or 1200 μmol mol−1 CO2. Final (35-day) edible yields were taken for the treatments under ambient or supplemented CO2. Results showed a response of growth to incident PPF that indicated a strong influence of lighting on yields.

The first spaceflight opportunities for Advanced Life Support (ALS) Project testing with plants will likely occur with missions on vehicles in Low Earth Orbit, such as the International Space Station (ISS). In these settings, plant production systems would likely be small chambers with limited electrical power. Such systems are adequate for salad-type crops that provide moderate quantities of fresh, flavorful foods to supplement the crew diet. Successful operation of salad crop systems in the space environment requires extensive ground-based testing with horticultural methodologies that meet expected mission constraints. At Kennedy Space Center, cultivars of radish, onion, and lettuce are being compared for performance under these “flight-like” conditions.

In controlled environment plant growth chambers, electric lamps provide photons necessary to drive photosynthesis. In order to determine the most productive, energy efficient, and safest way of providing light to plants for a given application, new lighting technologies are being evaluated by various researchers. Light-emitting diodes (LEDs) represent an innovative lighting source with several appealing features specific for supporting plants whether on space-based transit vehicles or planetary life support systems. For this study, there was specific interest in Swiss chard (Beta vulgaris L. cv. ‘Ruby Red Rhubarb') because these plants are among “salad-type” species chosen for early mission testing on Space Station. Of particular interest, were the growth dynamics and gas exchange characteristics of Swiss chard grown under red LEDs at narrow wavebands, which give different ratios of blue quanta to far-red photons.

Light-emitting diodes (LEDs) represent an innovative artificial lighting source with several appealing features specific for supporting plants, whether on space-based transit vehicles or planetary life support systems. Appropriate combinations of red and blue LEDs have great potential for use as a light source to drive photosynthesis due to the ability to tailor irradiance output near the peak absorption regions of chlorophyll. This paper describes the importance of far-red radiation and blue light associated with narrow-spectrum LED light emission. In instances where plants were grown under lighting sources in which the ratio of blue light (400–500 nm) relative to far-red light (700–800 nm) was low, there was a distinct leaf stretching or broadening response. This photomorphogenic response sanctioned those canopies as a whole to reach earlier critical leaf area indexes (LAI) as opposed to plants grown under lighting regimes with higher blue:far-red ratios.

A primary challenge for supporting plants in space is to provide as much photosynthetically active radiation (PAR) as possible, while conserving electrical power. Light-emitting diodes (LEDs) and microwave lamps are innovative artificial lighting technologies with several appealing features for supporting plant growth in controlled environments. Because of their rugged design, small mass and volume, and narrow spectral output, red and blue LEDs are particularly suited for outfitting plant growth hardware in spaceflight systems. The sulfur-microwave electrode-less high-intensity discharge (HID) produces a bright broad-spectrum visible light at a higher electrical conversion efficiency than conventional light sources. Experiments compared the performance and productivity of spinach (Spinacia oleracea L.) grown under conventional lighting sources (high-pressure sodium and cool-white fluorescent lamps) with microwave lamps and various wavelengths of red LEDs.

Under certain nutrient solution management practices in hydroponic systems, sweetpotato [Ipomoea batatas (L.) Lam.] plants can exhibit excessive shoot growth and reduced storage root yield. An experiment was conducted which compared sweetpotato production in nutrient film technique (NFT) systems either with daily nutrient solution replenishment + real-time pH control or with nutrient solution replenishment 3-times per week + periodic pH adjustment. Results showed that replenishment of nutrient solution on a daily basis produced excessive foliage growth with very little storage root production. Nutrient solution replenishment 3-times per week produced manageable vine growth and respectable storage root yields.

Nitrate concentration in spinach and lettuce is known to be influenced by light quantity. The enzyme nitrate reductase is regulated by phytochrome in some species, and in the presence of light, electrons that reduce nitrite to ammonium come from photosynthetic electron transport. It was hypothesized that light quality as well as light quantity may be used to manipulate nitrate concentration in spinach. To test this, narrow-band wavelength light-emitting diode (LED) sources (670 nm and 735 nm peak emission) were utilized in combination with cool white fluorescent (CWF) lamps. Nitrate concentration was compared in spinach seedlings grown for four weeks under CWF, followed by one of three 5-day pre-harvest light treatments. The three different light quality regimes were 1) CWF, 2) CWF + RED (670 nm) LED, and 3) CWF + FR (735 nm LED).

Procedures were developed for a future experiment to measure wheat (Triticum aestivum L.) photosynthesis in microgravity. Specific attention was given to growing and maintaining vigorous wheat plants relative to the challenging conditions predicted in microgravity. These ground-based tests included comparisons of different rooting media, media, wicking materials, and nutrient delivery system pressures. To facilitate seed germination in microgravity, several clinostat tests were conducted to characterize the importance of initial seed orientation. Following establishment of a vigorous crop canopy, photosynthesis rates were measured and found to be affected by mutual plant shading within the growth chambers.

Because of mass and power constraints in spacecraft, plant growth units designed for spaceflight have limited volume and low photosynthetic photon flux (PPF). Sufficient lighting and nutrient delivery are basic challenges to the success of supporting long-term plant growth in space. At the Kennedy Space Center, plant lighting and nutrient delivery hardware currently under NASA-sponsored development are being evaluated to define some of the fundamental issues associated with producing different fresh salad crops. Lettuce crops performed well under all nutrient delivery systems and lighting sources tested. Spinach and radish yields were lower in the presence of zeoponic media (using an ASTROCULTURE™ root tray) relative to plant grown in conventional NFT systems. Within each nutrient delivery system, yields of salad crops under red LEDs + blue light were similar to those crops grown under conventional white light.

A reliable nutrient delivery system is essential for long-term cultivation of plants in space. At the Kennedy Space Center, a series of ground-based tests are being conducted to compare candidate plant nutrient delivery systems for space. To date, our major focus has concentrated on the Porous Tube Plant Nutrient Delivery System, the ASTROCULTURE™ System, and a zeoponic plant growth substrate. The merits of each system are based upon the performance of wheat supported over complete growth cycles. To varying degrees, each system supported wheat biomass production and showed distinct patterns for plant nutrient uptake and water use.