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There are engineering and payload advantages in growing plants under hypobaric (reduced atmospheric pressure) conditions in biomass production for extraterrestrial base or spaceflight environments. Objectives of this research were to characterize the influence of hypobaria on growth, gas exchange, and ethylene evolution of lettuce ( Lactuca sativa  L. cv. Buttercrunch). Elevated levels of the plant hormone, ethylene, occur in enclosed crop production systems and in space-flight environments—leading to adverse plant growth and sterility. Lettuce plants were grown under variable total gas pressures [25 (low) or 101 kPa (ambient)]. During short growth periods of up to 10 days, growth was comparable between low and ambient pressure plants. Regardless of total pressure, plant growth was reduced at 6 kPa pO 2  compared to 12 and 21 kPa pO 2 . At 6 kPa pO 2  there was greater growth reduction and stress with ambient (101 kPa) than low (25kPa) pressure plants. Plants at 25/12 kPa pO 2  had comparable CO 2  assimilation and a 25% lower dark-period respiration than 101/21 kPa pO 2  (ambient) plants. Greater efficiency of CO 2  assimilation/dark-period respiration occurred with low pressure plants at 6 kPa pO 2 . Low pressure plants had a reduced CO 2  saturation point (100 Pa CO 2 ) compared with ambient (150 Pa CO2). Low pO2 lowered CO2 compensation points for both 25 and 101 kPa plants, i.e., likely due to reduced O 2  competing with CO 2  for Rubisco. Ethylene was 70% less under low than ambient pressure. High ethylene decreased CO 2  assimilation rate of 101/12 kPa O 2  plants. The higher dark-period respiration rates (higher night consumption of metabolites) of ambient pressure plants could lead to greater growth (biomass production) of low pressure plants during longer crop production cycles.

hortscience

(147) Effect of Hypobaria, Oxygen, and Carbon Dioxide on Gas Exchange, Ethylene Evolution, and Growth of Lettuce Plants for NASA Advanced Life Support Systems