Effect of Hypobaria and Hypoxia On Photosynthesis, Dark-Period Respiration and Growth of Lettuce Plants (Lactuca sativa) in NASA Advanced Life Support Systems (ALS)

There are important engineering, safety, and materials handling advantages in growing plants under subambient conditions for extraterrestrial base and long-term spaceflight environments. We are studying the potential for producing plants under hypobaric (reduced atmospheric pressure) conditions, and manipulating hypoxic stress (low oxygen) to increase bioprotectant production in lettuce plants. Hypobaric conditions can lead to hypoxia, thus limiting plant photosynthesis and growth. Plants provide nutritional and functional (secondary metabolites – bioprotectants) needed to maintain good health – particularly important for astronauts exposed to ionizing cosmic radiation during long-term space exploration and habitation. A goal of this research was to enhance production of plant bioprotectants via hypobaria and hypoxia without reduction of plant biomass. This paper describes plant gas exchange and plant biomass production, while the companion paper elucidates the enhanced production of plant bioprotectants. Twenty-one day-old seedling lettuce plants (Lactuca sativa L. cv. Red Sails) were grown under variable total gas pressures [25 (hyporia) and 101 kPa (ambient)] during 10-d studies with partial pressures of oxygen (pO₂) at: 101/ 21 (ambient), 101/6 (hypoxia), 25/12, and 25/6 (hypoxia) pO₂; two other treatments included exposing plants to 101/21 or 25/12 for 7-d, then to hypoxia (101/6 or 25/6) during the final 3-d of production. A 10-d exposure of ambient and hypobaric lettuce plants to hypoxia (6 pO₂) reduced carbon dioxide (CO₂) assimilation (CA), dark-period respiration (DPR), the ratio of CA/DPR (a measurement of photosynthetic efficiency), and subsequent biomass production. However, gas exchange and growth were comparable between non-hypoxic plants and plants exposed to hypoxia during the final 3-d of the production cycle. Hypobaria had no adverse effect on plant gas exchange or growth. The results also show that plants can be exposed to hypoxia during the end of the production cycle without adverse effects to gas exchange and growth.