Hypobaria, hypoxia, and light affect gas exchange
and the CO2 compensation and saturation points
of lettuce (Lactuca sativa)1

by Chuanjiu He, Fred T. Davies, Jr., and Ronald E. Lacey

Abstract: 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 (p02) (hypoxia) under low and high light
irradiance on carbon dioxide (C02) assimilation (CA), dark-period respiration (DPR), and the C02 compensation and C02
saturation points of lettuce (Laetuea sativa L. 'Buttercrunch'). Plants were grown under variable total gas pressures [25
and 101 kPa (ambient)] at 6, 12, or 21 kPa p02 (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) j.lmol·m-2·s-1. While
hypobaria (25 kPa) had no effect on CA or the C02 compensation point, it reduced the DPR and the C02 saturation point,
and increased the CA/DPR ratio. Hypoxia (6 kPa p02) and low light reduced CA, DPR, and the CA/DPR ratio. Hypoxia
decreased the C02 compensation point regardless of total pressure. Hypoxia also decreased the the C02 saturation point of
ambient-pressure plants, but had no effect on hypobaric plants. While low light reduced the CO2 saturation point, it increased
the C02 compensation point, compared with high-light plants. The results show that hypobaric conditions of
25 kPa do not adversely affect gas exchange compared with ambient-pressure plants, and may be advantageous during hypoxic