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  • Author or Editor: Sheetal Rao x
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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 pO2 compared to 12 and 21 kPa pO2. At 6 kPa pO2 there was greater growth reduction and stress with ambient (101 kPa) than low (25kPa) pressure plants. Plants at 25/12 kPa pO2 had comparable CO2 assimilation and a 25% lower dark-period respiration than 101/21 kPa pO2 (ambient) plants. Greater efficiency of CO2 assimilation/dark-period respiration occurred with low pressure plants at 6 kPa pO2. Low pressure plants had a reduced CO2 saturation point (100 Pa CO2) 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 O2 competing with CO2 for Rubisco. Ethylene was 70% less under low than ambient pressure. High ethylene decreased CO2 assimilation rate of 101/12 kPa O2 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.

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The NASA Advanced Life Support (ALS) System for space habitation will likely operate under reduced atmospheric pressure (hypobaria). There are engineering, safety, and plant growth advantages in growing crops under low pressure. In closed production environments, such as ALS, excessive plant-generated ethylene may negatively impact plant growth. Growth of lettuce (Lactuca sativa) in the Low Pressure Plant Growth (LPPG) system was enhanced under low pressure (25kPa), due in part to decreased ethylene production. Under reduced pO2, ethylene production decreased under low as well as ambient conditions (He et al., 2003). During hypobaria, the expression of genes encoding ethylene biosynthesis enzymes, namely ACC synthase (ACS) and ACC oxidase (ACO), is not known. The primary objective of this research was to characterize the expression of ACS and ACO genes in response to hypobaria. Three-week-old Arabidopsis was used to determine the effects of hypobaria (25 kPa) and reduced O2 (12 kPa pO2) at the molecular level. Candidate gene expression was tested using quantitative real-time PCR at different times after treatment. Under low pressure, ACO1 expression is induced in the initial 12 hours of treatment, gradually decreasing with increased exposure. At 12 kPa pO2, ACO1 was induced under ambient conditions, suggesting that plants under low pressure may be more tolerant to hypoxic stress. The mechanism for enhanced growth of lettuce under hypobaric conditions will be studied further by analysis of the ACS and ACO gene families, and stress-responsive genes, namely late-embryogenesis abundant (LEA) proteins and dehydrins.

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