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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.

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.

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.