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Chuanjiu He, Fred Davies*, Ronald Lacey, and Que Ngo

Elevated levels of ethylene occur in enclosed crop production systems and in space-flight environments—leading to adverse plant growth and sterility. There are engineering advantages in growing plants at 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 gas exchange and ethylene evolution of lettuce (Lactuca sativa L. cv. Buttercrunch). Lettuce was grown under variable total gas pressures [50 and 101 kPa (ambient)]. The six chambered, modular low plant growth (LPPG) system has a Rosemount industrial process gas chromatograph (GC) for determining gas concentrations of oxygen (O2), carbon dioxide (CO2) and nitrogen (N). With the LPPG system, changes in CO2 can be tracked during the light and dark periods on a whole canopy basis, and transpirate collected as a measurement of transpiration. During short growth periods of up to seven days, growth was comparable between low and ambient pressure. However, there was a tendency for leaf tip burn under ambient pressure, in part because of higher ethylene levels. Tip burn increased under high light (600 vs. 300 μmol·m-1·s-1) and high CO2 (600 vs. 100 Pa). The CO2 assimilation rate and dark respiration tended to be higher under ambient conditions. High humidity (100%) reduced CO2 assimilation rate compared to 70% RH. Ethylene was increased by high light (600 vs. 300 μmol·m-1·s-1) and high CO2 (600 vs. 100 Pa). Ethylene was higher under ambient than low pressure. Enhanced plant growth under low pressure may be attributed to reduced ethylene production and decreased dark respiration (lower night consumption of metabolites).