Hypobaria and Hypoxia Enhance Phytochemical Production of Lettuce Plants (Lactuca sativa) in NASA Advanced Life Support Systems (ALS)

Growing plants for long-term spaceflight environments is essential for providing nutritional, physiological, environmental and psychological well-being of the astronauts. Plants will supplement oxygen, scrub CO₂, purify water, and supply food.  In addition, plants provide phytochemicals (bioprotectants) that safeguard astronauts from the ionizing cosmic radiation exposure during long-term habitation. There are important engineering and crop production advantages in growing plants under hypobaric (reduced atmospheric pressure) conditions for extraterrestrial base environments. However, hypobaric conditions can lead to hypoxic-stress (low-oxygen), affecting plant photosynthesis and growth. A goal of the research was to enhance production of bioprotectants via hypobaria and hypoxia without reduction of plant biomass. The companion paper reports on plant gas exchange and growth, while this 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 (hypobaria) 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. In general, hypobaria in combination with hypoxia (during the final 3-d of production) enhanced the antioxidant activity (ORAC-value) of lettuce by stimulating the greatest synthesis of anthocyanins, total phenolics, chlorophyll a, chlorophyll b, and total carotenoids. While a 10-d exposure to hypoxia decreased biomass production (regardless of total atmospheric pressure) - there was no affect on chlorophyll or total carotenoid biosynthesis of ambient total pressure plants, while there was a reduction in hypobaric plants. Hypobaria enhanced chlorophyll and total carotenoids synthesis. The results show that bioprotectants can be increased by exposing hypobaric plants to hypoxia during the end of the production cycle, without loss of biomass.