Hypobaria and hypoxia affects growth and phytochemical contents of lettuce Nihal C. Rajapakse a, *, Chuanjiu He b , Luis Cisneros-Zevallos b , Fred T. Davies Jr. b a Department of Horticulture, Clemson University, Clemson, SC 29631, United States b Department of Horticulture, Texas A&M University, College Station, TX 77843, United States 1. Introduction The ability to produce plants under suboptimal conditions (i.e. low light, low pressure, low oxygen, low temperature) is of importance to space exploration programs to reduce engineering and material handling costs. The potential for plant growth under low pressure (hypobaria) and low oxygen (hypoxia) environments is being investigated at Texas A&M University as a part of a space exploration program. Without O 2 supplementation, hypobaric environments can lead to hypoxia, therefore, a potential limitation to plant growth under hypobaric conditions. Previous research has shown that seed germination of lettuce and wheat was not adversely affected by low pressure (50 kPa) and that seedlings grown under low pressure had greater shoot and root growth than those grown under ambient pressure (101 kPa) (He et al., 2003). Hypoxia increased ethylene production both under hypobaric and ambient pressure conditions (He et al., 2003). They also reported that growth of ‘Buttercrunch’ lettuce was comparable under 50 and 101 kPa total pressure and that low pressure reduced the occurrence of tip burn compared to ambient pressure (He et al., 2006). Hypobaria reduced ethylene accumulation, which resulted in better growth of seedlings than under ambient pressure (He et al., 2006). In another study, He et al. (2007) reported that biomass accumulation of ‘Buttercrunch’ lettuce was not affected by hypobaria (101 kPa vs. 25 kPa) but hypoxia (6 kPa oxygen partial pressure [pO 2 ]) reduced biomass regardless of the atmospheric pressure. There was no significant difference in biomass accumu- lation, photosynthesis or dark period respiration rate of lettuce plants grown under 21 or 12 kPa pO 2 . These results show that food crops can be grown successfully under certain levels of hypobaric and/or hypoxic conditions. Plants provide both nutritional (i.e. carbohydrates, proteins and lipids) and functional (i.e. secondary metabolites) phytochemicals needed to maintain good health. Epidemiological studies have reestablished the ancient wisdom that consumption of plant based foods reduces the risk of obesity and various chronic diseases such as cancer and diabetes (Fahey and Stephenson, 1999; Wargovich, 2000). Functional phytochemicals act as antioxidants by neutraliz- ing harmful free radicals thus, protecting plant’s cellular materials including DNA, proteins and lipids from oxidative damage and preventing the damage to vital functions. Functional phytochem- icals are thought to act similar manner in human bodies thus, protecting cellular materials and preventing the onset of various ailments. A diet rich in natural antioxidants and anti-carcinogenic compounds is highly desirable for a healthy life, particularly for those who are constantly exposed to oxidative environments, such as in long-term space exploration missions where astronauts are constantly exposed to high levels of ionizing cosmic radiation or workers in industrial zones that results in oxidative stresses and increased rate of carcinogenesis. Scientia Horticulturae 122 (2009) 171–178 ARTICLE INFO Article history: Received 16 January 2009 Received in revised form 16 April 2009 Accepted 6 May 2009 Keywords: Functional phytochemicals Controlled environments Low pressure Low oxygen ABSTRACT The primary objective of this research was to investigate how low pressure (hypobaria) and low oxygen (hypoxia) affect functional phytochemicals and the nutritional quality of ‘Red Sails’ lettuce (Lactuca sativa L.). Plants were grown under two levels of total gas pressure (reduced or ambient (25 or 101 kPa, respectively)) at three levels of O 2 partial pressures (low, medium or ambient (6, 12 or 21 kPa, respectively)). Hypoxia effects on nutritional and functional phytochemicals were more pronounced than hypobaria effects. Regardless of the total pressure, hypoxia, in general, enhanced leaf anthocyanin levels, enhanced total phenolic compounds, enhanced carbohydrate concentration and enhanced free radical scavenging capacity of lettuce but reduced leaf mineral concentration. Hypoxia increased the ethylene production of plants but ethylene accumulation was not the sole reason for enhanced anthocyanin production in plants grown under hypoxia. Our results suggest that low oxygen stress induces the production of protective phytochemicals and the free radical scavenging potential in lettuce, which may in turn enhance the functional value. However, further human intervention studies are needed to confirm if enhanced phytochemicals in plants have significant impact in human body. Published by Elsevier B.V. * Corresponding author. Tel.: +1 864 656 4970; fax: +1 864 656 4960. E-mail address: nrjpks@clemson.edu (N.C. Rajapakse). Contents lists available at ScienceDirect Scientia Horticulturae journal homepage: www.elsevier.com/locate/scihorti 0304-4238/$ – see front matter . Published by Elsevier B.V. doi:10.1016/j.scienta.2009.05.002