When given a sufficient quantity and certain quality of light, plants can produce food, regenerate oxygen, and purify water for space inhabitants. Conventional lighting technologies would be prohibitive to growing plants on a large scale in space due to low electric power conversion efficiencies. Current lighting research for space-based plant culture is focused on innovative lighting technologies that demonstrate high electrical efficiency and reduced mass and volume. Accordingly, light-emitting diodes (LED’s) and microwave lamps are promising technologies being developed to efficiently generate photosynthetic radiation. LED’s can illuminate near the peak light absorption regions of chlorophyll while producing virtually no near-infrared radiation. The sulfur-microwave electrode-less high-intensity discharge (HID) lamp uses microwave energy to excite sulfur and argon, which produces a bright continuous broad-spectrum white light. Compared to conventional broad-spectrum sources, the microwave lamp is highly efficient and produces limited amounts of ultraviolet (UV) and infrared radiation. The work in the KSC Life Sciences Support Program in association with the Johnson Space Center (JSC) Advanced Life Support (ALS) Program gives insight into the feasibility of using LED’s and/or microwave lamps as innovative alternative light sources for plant biomass production.

Within the ALS Program, salad-type plants represent crops that could provide a portion of fresh food as well as psychological benefits to the crew aboard future space transportation vehicles. Laboratory data generated with salad-type crops in the presence of various lighting sources will provide important data for the modeling and development for future missions. Work was completed with Swiss chard, spinach, radish, and lettuce plants grown in the presence of different lighting sources for 28 days. Three lamp banks represented broad-spectrum white light sources (microwave, high-pressure sodium, and cool-white fluorescent). Past tests have also included separate LED arrays filled with a given peak wavelength of red (664, 666, 676, 688 nanometers) LED’s. Each LED array contained single rows of blue LED’s (474 nanometers) evenly distributed within the multiple rows of red LED’s. Current research has begun to measure the relative stoichiometry of photosystem I and II reaction centers in response to light quality. Laboratory instrumentation was incorporated, which monitors plant chlorophyll fluorescence, accessory pigment concentrations, and responses of photosynthetic rate to light and carbon dioxide levels.

Key accomplishments:

• 1999: Began experiments with salad-type plant growth with LED’s and microwave lamps.
• 2000: Completed initial salad-type plant growth studies with LED’s and microwave lamps. NASA NRA Solicitation 98-HEDS-01 grant to Dynamac Corporation extended through 2001.
• 2001: Began experiments with mixtures of salad species to compare the growth of multiple crops in a common environment/hydroponics system. Testing included evaluation of green LED’s as an aid to improve visual perception of leaf color.