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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.

Bioregenerative life-support systems proposed for long-duration space missions require an understanding of the physical processes that govern distribution and transport of fluids in particulate porous plant-growth media. Our objectives were to develop hardware and instrumentation to measure porous-medium water retention and hydraulic transport properties during parabolic-flight induced microgravity. Automated measurements complimented periodic manual operations in three separate experiments using porous ceramic aggregates and glass beads. The water content was adjusted in multiple steps in periods of 1.8g. Continuous hydraulic potential measurements provided information on water retention. The short duration of microgravity limited the occurrence of equilibrium potentials under partially saturated conditions. Measured pressure gradients under fixed flow rates were largely unaffected by gravity force in saturated cylindrical porous-medium-filled flow cells.

Fluid management in plant growth media under reduced gravity remains a challenge hindering integration of this critical component of advanced life support. A recent NASA-funded project diagnosed causes for limited progress in control of water and air in the root zone. Small finite volumes of porous media are used for plant growth in microgravity. Difficulties controlling water and air in the root zone likely result from an incomplete understanding of the system in 1g that is compounded by changes in water and air distribution due to microgravity. Key areas for future progress include engineered plant growth media for reduced gravity, improvements in sensing technology and better understanding of rhizosphere processes.

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.

Results from plant growth experiments utilizing particulate growth media during space flight revealed difficulties associated with providing reliable reproducible gaseous and water supply to plant roots. These limitations were attributed to insufficient understanding of liquid configuration and growth media transport processes in reduced gravity. The objective of this NASA-funded research program is to develop a framework for modeling and quantitative characterization of physical processes associated with flow of wetting and non-wetting phases in particulate plant growth media in microgravity. This paper provides an overview of research plans and current status of research activities. Characterization and modeling of substrate water retention and transport properties in microgravity is key to management and control of gas and liquid fluxes within plant root zones.