Beyond earth: Designing root zone environments for reduced gravity conditions

Fluid management in plant root zones is critical for long duration space missions including lunar- or Martian-based missions, but key aspects of design and delivery of fluids under these conditions are poorly understood due to limited experimental opportunities. We review theoretical and experimental concepts for advancing understanding of fluidporous media interactions to improve design and management of plant-based life support systems for reduced gravity environments. In situ utilization of native lunar and Martian granular materials for plant-growth media requires reliable haracterization of media physical and hydraulic properties and processes. A key aspect is the enhanced effects of capillarity in reduced gravity resulting in an array of micro- and macroscale changes in fluid phase organization relative to conditions on Earth that may affect mass fluxes to plant roots and potentially result in excess water and hypoxia. Increasing the medium particle diameter above 1 mm and narrowing the distribution of particles, and thus pore sizes, may counter reduced gravity effects. Approaches used in previous microgravity systems involving sensor-based active water management assuming prescribed optimal set points (i.e., water potential) may fail in reduced gravity due to dynamic pore space alterations arising from air- or liquid-phase entrapment and root growth in a restricted volume that may alter the porous medium characteristics on which water management is oft en based. For example, about a 10% reduction in volumetric pore space was observed following rice (Oryza sativa L.) root growth, which could change a well-aerated root zone into an anoxic environment if not accounted for. Numerical modeling of plant transpiration and irrigation using volumetrically controlled water content under different gravity environments revealed similar hydraulic responses in fine-textured porous media typically unsuitable for plant growth in greenhouses. Volumetric water content-based management of plant root environments appears to be a safer approach than other methods discussed here.