Gas Exchange, Carbon Dynamics and Energy Use Efficiency of Lettuce (Lactuca sativa) Under Ethylene and Hypobaric Environments for NASA Advanced Life Support (ALS) Systems

There are important engineering, safety, and materials handling advantages in growing plants under hypobaric (reduced atmospheric pressure) conditions for extraterrestrial base and long-term spaceflight environments. Ethylene accumulation is a problem under confined, space environments, leading to irregular growth. Lettuce (Lactuca sativa L. cv. Red Sails) was grown from seed to harvest for 32-d under variable total gas pressures of 25 kPa (hypobaria) and 101 kPa (ambient) at partial pressures of pO₂ of  12 kPa or 21 kPa under 400 μmol m^-2s^-1 light intensities at canopy level of low pressure plant growth system (LPPG). There were no differences in plant gas exchange between hypobaric and ambient pressure plants, however ethylene accumulation reduced gas exchange and plant growth. Ethylene exceeded 1000 nmol.mol^-1 (ppb), but was lowest with hypobaric plants.  The results show that lettuce gas exchange and growth was comparable between 101 and 25 kPa plants. The rate of photosynthesis showed a typical polynomial pattern with or without ethylene scrubbed from chambers. While photosynthesis (C_A) and dark-period respiration (DPR) were significantly reduced by ethylene, ethylene had no effect on the C_A/ DPR ratio. Net carbon gain increased during the light period and was highest with ethylene-scrubbed hypobaric and ambient pressure plants. There was also a polynomial response in carbon daily gain (CDG) of lettuce under hypobaric and ambient total pressure. The CDG increased at day-8, and greatly increased after day-15 day from seeding. Carbon use efficiency (CUE) showed a linear decrease with increasing plant age for both ambient pressure and hypobaric plants. Energy use efficiency (EUE) was detectable 8-days after seeding and had showed a polynominal response through the final day of harvest.