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Lunar-and Martian-based plant growth facilities pose novel challenges to design and management of porous medium-based root-zone environments. For example, to achieve similar equilibrium water content distribution using potting soil, a 10 cm tall root zone on earth needs to be 60 cm tall on the moon. We used analytical models to parameterize porous plant growth media for reduced gravity conditions. This approach is straight-forward because the equilibrium capillary potential scales linearly with gravity force. However, the highly non-linear water retention character is tied to particle size through the resulting pore-size distribution. Therefore interpreting the corresponding particle size and generating and evaluating the porous medium hydraulic properties remains a challenge. Soil physical principles can be applied to address the ultimate concern of controlling fluids (O2, H2O) within the plant root-zone in reduced gravity.

Management of water, air and nutrients in coarse-textured porous plant-growth substrates relies not only on the relative amounts of fluids but also on their distribution within porous media. Integration of plants in future life support systems for space exploration raises the question of how fluid distributions in porous plant-growth substrates are altered under reduced gravitational conditions. Central to addressing this issue is the behavior of the water retention characteristic (WRC). WRC encapsulates fluid-porous medium interactions and is key for control of water supply to plants. The hysteretic nature of WRC implies non-homogenous water distributions between its primary draining and wetting curves. During dynamic drainage and wetting cycles, considerable water content gradients develop at separations of only a few pore lengths.

Management of water content and nutrient status during space flight is a critical element for successful plant production systems. Our objectives were to determine if dual-probe heat-pulse (DPHP) sensors could improve water content determination accuracy over single-probe heat-pulse (SPHP) sensors, and to test a design using coupled heat-pulse and direct-current electrical conductivity sensors, paired as a 4-electrode array. The DPHP predicted water content correlated well with independently measured water contents based on a physically-derived one-point calibration model. SPHP water content prediction was comparable to the dual-probe sensors when using an empirical relationship. Pooled regression analysis showed that water content for both sensors was accurate with a root-mean square error of 0.02 cm3 cm−3. Electrical conductivity was measured in both saturated flow-through and static unsaturated measurements.

Liquid and gas exchange within a particulate plant-rooting medium is likely to be altered in a microgravity environment. A difference in gravitational force can result in significant offsets in control parameters developed on earth for optimum plant growth, due to the shift in hydrostatic water distribution. The experiment being developed will examine the effects of variable gravity on water distribution and gas diffusion. We are developing and testing an automated gas diffusion measurement system for use on the International Space Station (ISS). To allow comparison of μg and 1g conditions, gas diffusion cell designs were horizontally oriented to minimize gravitational effects using 1) a ‘thin rectangular profile’ cell and 2) a cylindrical cell design for flight. Electronic solenoid valves provide air and water flow control while pressure transducers measure water and substrate potential.

The ORZS flight experiment is designed to measure gas diffusion through plant growth substrates at varying water content levels in microgravity. This information is critical for proper water management and the prevention of root zone hypoxia during plant growth and advanced life support (ALS) biomass production experiments. Microgravity data that suggest enhanced hysteresis in water retention may alter the gas diffusion process, changing the optimum root zone moisture control set point in μg plant growth systems. Small gas diffusion cells are being evaluated as measurement systems for coarse-textured plant growth media at 1g and 0g. Design guidelines aim to minimize gravitational force while maintaining a representative porous medium. Substrate physical properties (e.g., water retention) pose additional complications for diffusion coefficient determination.

Lada, named for the ancient Russian Goddess of Spring, is a plant growth system developed jointly by the Space Dynamics Laboratory and the Institute of Biomedical Problems for long-term deployment on the International Space Station. Lada uses design features and technology similar to the Svet greenhouse on the Mir orbital outpost, and will be launched to ISS in June 02. It is scheduled to support its first crop (a leafy vegetable - Mizuna [Brassica rapa var. nipposinica]) in October 02. Lada consists of four major components (a control module, two vegetation modules and a water tank) and is designed to be deployed on a cabin wall. This deployment scheme was chosen to provide the crew therapeutic viewing and easy access to the plants. The two independently controlled vegetation modules allow comparisons between two vegetation or substrate treatments. The vegetation modules consist of three sub-modules, a light bank, the leaf chamber, and a root module.

During 1995, we tested instruments and attempted a seed-to-seed experiment with Super-Dwarf wheat in the Russian Space Station Mir. Utah instrumentation included four IR gas analyzers (CO2 and H2O vapor, calculate photosynthesis, respiration, and transpiration) and sensors for air and leaf (IR) temperatures, O2, pressure, and substrate moisture (16 probes). Shortly after planting on August 14, three of six fluorescent lamp sets failed; another failed later. Plastic bags, necessary to measure gas exchange, were removed. Hence, gases were measured only in the cabin atmosphere. Other failures led to manual watering, control of lights, and data transmission. The 57 plants were sampled five times plus final harvest at 90 d. Samples and some equipment (including hard drives) were returned to earth on STS-74 (Nov. 20). Plants were disoriented and completely vegetative. Maintaining substrate moisture was challenging, but the moisture probes functioned well.