Ethylene, Hypobaria, and Hypoxia: Influence on Plant Gas Exchange, Germination, Plant Growth, and Nutrition of Lettuce (Lactuca sativa) from Seed to Harvest

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 seeds were germinated and grown under variable total gas pressures of 25 kPa (hypobaria) and 101 kPa (ambient)] at partial pressures of O₂ of 12 kPa (hypoxia) or 21 kPa (normoxia) in a low pressure plant growth system (LPPG). Hypobaria and hypoxia had no effect on photosynthesis (C_A), except 25 kPa plants had a lower dark-period respiration (DPR). Hypobaria had no affect on germination and plant growth, however ethylene reduced plant gas exchange and growth (leaf area, specific leaf area, leaf, root, and total plant DM, and relative growth rate). The relative water content was unaffected by hypobaria, hypoxia or ethylene. The rate of photosynthesis showed a typical polynomial growth pattern with ethylene-scrubbed, but not when ethylene accumulated in chambers. Hypoxia decreased macroelements (N, P, K, Ca, and Mg), but not microelements. While total pressure and ethylene had no effect on macro- or microelements, ethylene reduced Mn.