Abstract Growth of plants in a Controlled Ecological Life Support System (CELSS) may involve the use of hypobaric pressures enabling lower mass requirements for atmospheres and possible enhancement of crop productivity. A controlled environment plant growth chamber with hypobaric capability designed and built at Ames Research Center was used to determine if reduced pressures influence the rates of photosynthesis (Ps) and dark respiration (DR) of hydroponically grown lettuce plants. The chamber, referred to as a plant volatiles chamber (PVC), has a growing area of about 0.2 m 2, a total gas volume of about 0.7 m 3, and a leak rate at 50 kPa of < 0.1%/day. When the pressure in the chamber was reduced from ambient to 51 kPa, the rate of net Ps increased by 25% and the rate of DR decreased by 40%. The rate of Ps increased linearly with decreasing pressure. There was a greater effect of reduced pressure at 41 Pa CO 2 than at 81 Pa CO 2. This is consistent with reports showing greater inhibition of photorespiration (Pr) in reduced O 2 at low CO 2 concentrations. When the partial pressure of O 2 was held constant but the total pressure was varied between 51 and 101 kPa, the rate of CO 2 uptake was nearly constant, suggesting that low pressure enhancement of Ps may be mainly attributable to lowered partial pressure of O 2 and the accompanying reduction in Pr. The effects of lowered partial pressure of O 2 on Ps and DR could result in substantial increases in the rates of biomass production, enabling rapid throughput of crops or allowing flexibility in the use of mass and energy resources for a CELSS.