Hypobaria, Hypoxia and Light Affect Gas Exchange, and the CO2 Compensation and Saturation Points of Lettuce (Lactuca sativa)

There are important engineering and crop production advantages in growing plants under hypobaric (reduced atmospheric pressure) conditions for extraterrestrial base or spaceflight environments. The objectives of this research were to determine the influence of hypobaria and reduced partial pressure of oxygen (pO₂) [hypoxia] under low and high light irradiance on carbon dioxide (CO₂) assimilation (CA), dark-period respiration (DPR), and the CO2 compensation and CO₂ saturation points of lettuce (Lactuca sativa L. cv. Buttercrunch). Plants were grown under variable total gas pressures [25 and 101 kPa (ambient)] at 6, 12 or 21 kPa pO₂ (approximately the partial pressure in air at normal pressure). Light irradiance at canopy level of the low pressure plant growth system (LPPG) was at 240 (low) or 600 (high) μmol m^-2s^-1. While hypobaria (25 kPa) had no effect on CA or the CO2 compensation point, it reduced the DPR and the CO₂ saturation point, and increased the CA/DPR ratio. Hypoxia (6 kPa pO₂) and low light reduced CA, DPR and the CA/DPR ratio. Hypoxia decreased the CO₂ compensation point. Hypoxia also decreased the CO₂ saturation point of ambient pressure plants, but had no effect on hypobaric plants. While low light reduced the CO₂ saturation point, it increased the CO₂ compensation point, compared with high light plants. The results show that hypobaric conditions of 25 kPa do not adversely affect gas exchange compared to ambient pressure plants, and may be advantageous during hypoxic stress.